GB2593437A

GB2593437A - Surface layer

Info

Publication number: GB2593437A
Application number: GB2001989.9A
Authority: GB
Inventors: Hall Gregory
Original assignee: Automotive Fusion Ltd
Current assignee: Automotive Fusion Ltd
Priority date: 2020-02-13
Filing date: 2020-02-13
Publication date: 2021-09-29
Also published as: GB202001989D0; WO2021161288A1

Abstract

A surface layer 10 and method for selectively altering boundary layer adhesion of a fluid flowing, across the surface layer 10 is disclosed. The surface layer 10 comprising a fluid engagement layer 11 and plural surface elements 1 each comprising an actuator 4. Some or each actuator 4 is configured to be individually controlled by a controller (2, figure 1) to selectively generate or alter the height of a projection from the fluid engagement layer 11 or to selectively generate or alter the depth of a recess into the fluid engagement layer 11. Optionally, some or each actuator 4 comprise a sensor (240, figure 5). The controller (2, figure 1) can be configured to receive data from the or each sensor (240, figure 5) and control the actuators 4 base on said data.

Description

SURFACE LAYER

This invention relates generally to a surface layer for selectively altering boundary layer adhesion of a fluid flowing, in use, across or over the surface layer. More specifically, although not exclusively, this invention relates to a surface layer for adaptively or actively altering adhesion of a boundary layer of fluid flowing, in use, across or over the surface layer.

The interaction between a fluid flow and a surface is known to affect the forces acting on the surface, to alter the flow of the fluid and/or to alter heat transfer between the fluid and the io structure underlying the surface. These effects are generated based on relative motion of a fluid and a surface which is contacting the fluid. Accordingly, the effects occur regardless of whether the fluid is flowing across a stationary structure, a structure is moving through a static fluid or both the fluid and the structure are moving (e.g. relative to one another).

In fluid dynamics, a boundary layer may be formed adjacent a surface across which a fluid is flowing. The boundary layer typically comprises a different velocity and/or direction of flow than does the bulk body of the fluid flowing across the surface (e.g. where the bulk body of fluid is spaced from the surface by the boundary layer). Boundary layers may be laminar or turbulent. In particular, boundary layers may begin as laminar flow at the leading edge of a surface and may then transition into turbulent flow as the boundary layer travels across or along the surface.

Advantageously, bulk fluid flow has a relatively higher velocity relative to a surface over which it is flowing when a boundary layer is provided against that surface than in the absence of a boundary layer. The presence of a boundary layer reduces the drag acting on the surface from the fluid flowing thereacross. In vehicles this is particularly beneficial as the presence of a boundary layer can result in relatively greater velocity of the vehicle and/or relatively reduced energy consumption for achieving the same velocity. For this reason, the surfaces of vehicles can be designed to be more streamlined, so that fluid flow, in use, relative to the surfaces of the vehicle is more likely to promote and maintain boundary layer formation. The presence of a boundary layer reduces the drag acting on the vehicle from the fluid through which the vehicle is moving.

A specific surface topography can be used to optimise fluid flow, in use, relative to that surface. An example of this is the dimples on a golf ball. The dimples promote the adhesion of a boundary layer to the surface of the golf ball, which thereby has relatively reduced drag as it moves through the air. Without wishing to be bound by any particular theory it is believed that the dimples generate localised vortexes which are substantially parallel to the surface at their generation location. These vortexes generate a relatively thin layer of turbulent flow which exists under a laminar layer of the boundary layer. This turbulent layer acts to aid in adhering the laminar layer to the surface for a relatively greater duration and/or distance (for the leading edge) than would otherwise be the case. The boundary layer is thereby maintained over a greater proportion of the surface than would otherwise be the case, hence reducing the drag experienced by the surface (e.g. of the golf ball).

Alternatively, plural protrusions can be provided on a surface to alter the generation, extent and/or adhesion of a boundary layer to that surface, in use. Examples of such protrusions are the ridges which are sometimes provided on aircraft wings.

The relative roughness of a surface may also influence the amount of drag acting in the boundary layer adjacent the surface from the fluid, as well as the transition between laminar and turbulent flow of the boundary layer.

An alternative method of altering the boundary layer on a surface is by directly changing the characteristics of the fluid flowing across the surface. This may be by accomplished using electrical power to create a plasma layer on the surface.

As will be appreciated, fluid flow relative to a surface may not be constant in direction, velocity and/or magnitude, e.g. during relative motion of the fluid and the surface. For example, an automobile may speed up and slow down and may turn around corners, resulting in relatively rapid changes in the velocity, direction and/or mass flow rate of fluid relative to surfaces of the automobile. Provision of dimples into or projections from the surfaces may be suitable in one circumstance to promote boundary layer adhesion but not in other circumstances. It may also be advantageous to be able to retrofit aids to improve aerodynamics to pre-existing structures (e.g. existing vehicles).

Typically, vehicles (for example) encounter fluid flow over their upper surface (e.g. across the roof) and also over their lower surface (e.g. across the underside). A boundary layer may be formed on both surfaces. Once the two boundary layers have separated they will typically connect with one another at a location downstream of the vehicle. The volume of fluid (e.g. air) encompassed by these connecting boundary layerflows is known as a separation bubble. When this separation bubble is relatively small the boundary layer flows have connected relatively close to the downstream end of the vehicle. Such a connection of boundary layer flows close to a vehicle is believed to generate a drag force on the vehicle, and thereby reduce its forward velocity and/or increase the energy required to cause the vehicle to travel relatively forward (with respect to the drag force). It would therefore be beneficial to at least partially mitigate this effect.

It is therefore a first non-exclusive object of the invention to provide a surface layer which overcomes or at least partially mitigates one or more of the above problems. It is a further non-exclusive object of the invention to provide a surface layer which more effectively adheres boundary layers to said surface layer, e.g. in use. It is a further non-exclusive object of the invention to provide a surface layer which can selectively promote separation of a boundary layer from the surface layer, e.g. at a desired location and/or angle.

Accordingly, a first aspect of the invention provides a surface layer for selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer and one or more actuators, wherein the, one, some or each of the actuators is configured or arranged to be controlled (e.g. individually controlled), in use, to selectively generate local turbulence on or adjacent the fluid engagement layer.

A further aspect of the invention provides a surface layer for selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer with one or more apertures therethrough and one or more actuators configured or arranged to selectively generate, in use, local turbulence above and/or adjacent the or each aperture.

In embodiments, the, one, some or each actuator may be configured or arranged to also function as a sensor. For example, the, one, some or each actuator may be configured or arranged to sense one or more characteristic of a fluid flowing across or over the surface layer, in use. In embodiments, there may be plural actuators. Where there are plural actuators, some or each may be configured to be independently controlled (e.g. by a controller), in use.

A further aspect of the invention provides a surface layer for selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer with one or more apertures therethrough and one or more actuators configured or arranged to selectively generate, in use, local turbulence (e.g. vortices) above and/or adjacent the, one, some or each aperture, wherein at least a portion of the fluid engagement layer about or bounding (e.g. about the circumference of) the, one, some or each aperture is flexible and where the surface layer further comprises a means for selectively altering the size and/or shape of the, one, some or each aperture.

Advantageously, the generation of local turbulence (e.g. vortices) above the apertures alters the adhesion of a boundary layer of fluid flow over the surface layer to the fluid engagement layer. In this way it is possible to encourage the boundary layer to remain adhered to the fluid engagement layer for a relatively greater distance across that layer than would otherwise be the case, absent the local turbulence. Additionally, it is possible to encourage separation of the boundary layer of fluid flow from the fluid engagement layer at a relatively increased angle and/or at a relatively earlier point along the fluid engagement layer than would otherwise be the case (absent the local turbulence). Additionally, by altering the size and/or shape of one or more io of the apertures the properties of the generated turbulence can be altered. For example, the direction, magnitude and shape of the vortices can be altered. As will be appreciated, the boundary layer of a fluid flowing across the surface layer will be effected by the velocity, direction, viscosity, pressure, etc of the fluid flow. Altering the properties of the generated local turbulence (e.g. vortices) allows selective matching to the properties of the fluid flow across the surface layer.

The invention is unique in the way that it has the ability to analyse and respond to the flow of fluid and the changes that disturb the flow and make it turbulent and detach. It has the ability to alter the vibration frequency and directional wave manipulation in such a way to control the flow similar to how Dolphin skin operates and alter the reynolds number. This ability to respond to changing fluid flow directions and control and manipulate that flow to regulate the boundary layer delamination is critical in maintaining a strong boundary layer adhesion and reducing drag over the surface. Reduced drag and greater adhesion means faster fluid flow and thus greater down force can be achieved by apparatus such as spoilers and diffusers and general body shapes as a whole. Ultimately reversing the wing topology and creating greater down force. In contradiction to this the same analysis and control actuation mechanism can be used to purposely disturb the air flow to create breaking forces or alternatively to be applied over the greater part of the vehicle and alter flow over the entire vehicle in straight line conditions and reduce overall drag and rolling forces by applying reduced down force loads. This acts to not only reduce wind resistance but by providing lift reduces rolling resistance forces. The actuation of the individual cell is done in order to appropriately generate a vibration and actuated movement that generates the required coordinated vortices and thin layer turbulence to control the delamination and flow distribution. The result is an interactive sensing and reactive system that reacts in real time to variations in fluid flow conditions and controls and manipulates the boundary conditions and TS waves.

In embodiments, a minority or majority of the fluid engagement layer is flexible. In embodiments, all of the fluid engagement layer is flexible. For example, the fluid engagement layer (or a portion or potions thereof) may be formed from a flexible material. The fluid engagement layer (or a portion or portions thereof) may be formed from rubber, or the like.

The, one, some or each actuator may be configured or arranged to be controlled, in use, (e.g. by a controller) to selectively generate local turbulence above and/or adjacent the, one, some or each aperture.

io In embodiments, the fluid engagement layer may comprise a fluid engagement surface. The fluid engagement surface may comprise a major surface of the fluid engagement layer. The fluid engagement layer may comprise a major surface obverse the fluid engagement surface.

In some embodiments, the, one, some or each actuator may be configured or arranged to generate or force or direct a flow (e.g. a jet) of fluid into and/or out of the, one, some or each aperture, in use. In some embodiments, the, one, some or each actuator may be configured or arranged to generate or direct or force a flow (e.g. a jet) of fluid into and/or out of the, one, some or each aperture to beyond the fluid engagement surface (where provided), in use.

In embodiments, the surface layer comprises plural apertures through the fluid engagement layer. The surface layer may comprise plural actuators. Each of the plural actuators may be configured or arranged to selectively generate, in use, local turbulence above and/or adjacent one of the plural apertures.

There may be a single actuator and plural apertures. There may be plural actuators and plural apertures. There may be as many actuators as apertures. Where there are the same number of actuators and apertures, each actuator may be positioned adjacent or below a different aperture. The number of plural actuators may be less than the number of plural apertures. Where there are fewer actuators than there are apertures then one or more of the actuators may be configured or arranged to selectively generate or direct or force a flow (e.g. a jet) of fluid through more than one of the apertures.

The, one, some or each aperture through the fluid engagement layer may comprise or provide or at least partially define a nozzle.

Where there are plural apertures they may be arranged in an array, e.g. a regular or irregular array. The array may be or comprise a grid, for example of rows and/or columns of apertures.

The rows may be evenly spaced or unevenly spaced. The columns may be evenly space or unevenly spaced.

The, one, some or each aperture may have a starting or relaxed shape or condition, e.g. in plan.

In embodiments, the starting or relaxed shape or condition may be polygonal or circular or ovaloid or irregular. The, one, some or each aperture may have a deformed or final shape or condition, e.g. in plan. The means for selectively altering the size and/or shape of the, one, some or each aperture may be configured to alter the, one, some or each aperture from the starting or relaxed shape or condition, in plan, to the deformed or final shape or condition. The, one, some or each aperture may have a starting or relaxed size, e.g. in plan. The, one, some or each aperture may have a deformed or final size, e.g. in plan. The means for selectively altering the size and/or shape of the, one, some or each aperture may be configured to alter the, one, some or each aperture from the starting or relaxed size, in plan, to the deformed or final size. The starting or relaxed size may be larger than the deformed or final size. The starting or relaxed size may be smaller than the deformed or final size.

The means for selectively altering the size and/or shape of the, one, some or each aperture may be or comprise one or more shape actuator. The shape actuator or actuators may be arranged or configured to be controlled, in use, (e.g. by a controller) to selectively alter the size and/or shape of the, one, some or each aperture. In embodiments, there may be one or more shape actuators for each aperture. In embodiments, there may be plural shape actuators for each aperture. The, some or each shape actuator may be located adjacent an aperture (e.g. the circumference of an aperture). The, one, some or each shape actuator may be on the fluid engagement layer (e.g. on a fluid engagement surface thereof, where provided). In embodiments, the one, some or each shape actuator may be at least partially (e.g. fully) embedding in the fluid engagement layer.

The shape actuator or actuators may comprise or be at least partially formed from shape memory alloy material. The shape actuator or actuators may comprise one or more shape memory alloy wires. The shape actuator or actuators may comprise or be at least partially formed from electroactive polymers (EAP). The shape actuator or actuators may be configured to be heated and/or cooled and/or supplied with electricity, in use. For example, the shape actuator or actuators may comprise one or more contacts or connections for connection to a means of heating and/or cooling and/or supplying electricity to the shape actuator or actuators.

The means of heating and/or cooling the shape actuator or actuators may comprise a heating or cooling source and/or an electrical source.

In embodiments, plural shape actuators may be provided. The plural shape actuators may be arranged or positioned to at least partially (e.g. entirely) surround the, one, some or each. In embodiments, each of the apertures may be at least partially (e.g. entirely) and individually surrounded by one or more shape actuators. Where the shape actuators comprise one or more shape memory alloy wires one or more wires may be disposed on a first side of an aperture and one or more wires may be disposed on a second side of the aperture. The shape memory wires may be disposed such that one or more wire crosses another one or more of the wires.

The surface layer may further comprise one or more diaphragms. The or each diaphragm may io be arranged or positioned to seal (or substantially seal) one side of the, one, some or each of the apertures through the fluid engagement layer. The or each diaphragm may be arranged or configured to at least partially define or provide a flow volume for providing or receiving fluid flow, in use. The, one, some or each actuator may be arranged or configured to bias the, one, some or each diaphragm, in use. Biasing of the diaphragm may relatively reduce or increase a flow volume (e.g. at least partially defined by the, one, some or each diaphragm). In embodiments, there may be a diaphragm for each aperture. Where there are plural apertures and diaphragms, each diaphragm may at least partially define a flow volume below an aperture. The or each diaphragm may be flexible.

A further aspect of the invention provides a surface layer for selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer and plural surface elements each comprising an actuator, wherein some or each actuator is configured or arranged to be individually controlled, in use, by a controller to selectively generate or alter the height of a projection from the fluid engagement layer and/or to selectively generate or alter the depth of a recess into the fluid engagement layer (for example, so as to generate vortices or turbulence).

Advantageously, the topography of the fluid engagement layer of the surface layer according to the invention can be selectively and/or dynamically adjusted to the conditions of fluid flow relative to the surface layer and, thereby, provide a more efficient adhesion or separation of a boundary layer (as required). Additionally, the location of the separation of the boundary layer may be precisely controlled. Furthermore, the angle at which the boundary layer separates from the surface layer (and/or a structure or vehicle to which the surface layer is attached, in use) may be controlled and may be greater than would be the case absent the surface layer.

Vehicles, for example, with the surface layer thereto attached thereby have relatively reduced drag coefficient, in use.

In embodiments, the fluid engagement layer may comprise plural surface portions. In some embodiments, one, some or each of the surface elements may comprise a part of the fluid engagement layer, for example a surface portion. In embodiments, each of the plural surface elements may comprise a surface portion.

In some embodiments, generating or altering the height of the projection from the fluid engagement layer and/or generating or altering the depth of the recess into the fluid engagement layer may comprise moving at least a portion of the fluid engagement layer in a direction substantially perpendicular to a plane across (e.g. defined by) the outer surface of the io surface layer at the location of the surface element or actuator. Where each of the surface elements comprise a surface portion, generating or altering the height of the projection from the fluid engagement layer and/or generating or altering the depth of the recess into the fluid engagement layer may comprise moving at least a portion of the fluid engagement layer in a direction substantially perpendicular to a plane across (e.g. defined by) the outer surface of the surface layer at the location of the surface portion.

According to a further embodiment of the invention there is provided a surface layer for selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer may comprise plural surface elements each comprising a surface portion and an actuator configured or arranged to be individually controlled, in use, by a controller to move the surface portion or a part thereof in a direction substantially perpendicular to a plane across (e.g. defined by) the outer surface of the surface layer at the location of the surface portion.

The or each actuator may be configured or arranged to be individually controlled, in use. The or each actuator may be configured or arranged to be individually controlled, in use, by a controller to move the surface portion (where provided) or a part thereof The or each actuator may be configured or arranged to be individually controlled, in use, to bias a diaphragm (where provided). The or each actuator may be configured or arranged to bias a fluid (e.g. air) against a or the surface element, e.g. in use. The or each actuator may comprise or at least partially define an air spring.

The, one, some or each actuator may comprise a piezoelectric actuator (for example a cantilevered piezoelectric actuator). The actuator of one, some or each surface element (where provided) may comprise a piezoelectric actuator (for example a cantilevered piezoelectric actuator). The piezoelectric actuator may comprise or have a disc shape. Alternatively, the piezoelectric actuator may have a different shape. The, one, some or each actuator may comprise a shape memory alloy material. The actuator of each surface element may comprise a shape memory alloy material. The shape memory alloy material may comprise a wire or a coil. The, one, some or each actuator may comprise or be at least partially formed from electroactive polymers (EAP). The, one, some or each actuator may comprise an electromagnetic system. For example, the electromagnetic system may utilize induction induced vibration. The actuator of one, some or each surface element may be operable, in use, to move the surface portion of that surface element linearly, e.g. in a linear direction. The actuator of one, some or each surface element may be operable, in use, to move its surface portion reciprocally, and/or cyclically, e.g. in a linear direction. The actuator of one, some or each surface io element may be actuated, in use, to expand and/or contract (e.g. at least partially), for example reciprocally and/or cyclically. The actuator of one, some or each surface element may be operable, in use, to move its surface portion reciprocally and/or cyclically, e.g. in a linear direction. In embodiments, the surface portion may comprise a projection (e.g. a hair shaped protrusion). The actuator may be configured to at least partially retract and/or extend the protrusion, in use, e.g. cyclically or reciprocally or the like. The, one, some or each actuator may be operable, in use, to generate, force or direct a flow (e.g. a jet) of air into and out of the, one, some or each aperture (where provided) at regular or irregular intervals. The period of these intervals may have a frequency greater than once a second, for example greater than 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500 or 3000 times per second.

The actuator of one, some or each surface element may comprise one or more contact (e.g. electrical contact), for example which may be configured or arranged to enable or allow for individual control of the, some or each actuator, in use.

The plural surface elements may be arranged in an array, e.g. a regular or irregular array. The array may be or comprise a grid, for example of rows and/or columns of surface elements. A first surface element of the plural surface elements may be adjacent (e.g. directly) a second surface element of the plural surface elements. In embodiments, each of the surface elements of the plural surface elements may be adjacent (e.g. directly) one or more other surface elements of the plural surface elements. Alternatively, one or more of the plural surface elements may be spaced from the other surface elements of the plural surface elements.

In embodiments, the surface portions may have any suitable shape, e.g. when seen in plan.

The surface portions may be or have a regular or irregular shape, e.g. when seen in plan. The surface portions may have or be a polygonal shape, e.g. when seen in plan. For example, the surface portions may have or be square, rectangular, triangular, pentagonal, hexagonal or the like, e.g. when seen in plan. The plural surface elements may comprise one or more surface elements having surface portions with a first shape and one or more surface elements having surface portions with a second shape, e.g. where the first shape is different to the second shape. The plural surface elements may comprise one or more surface elements having surface portions with a first size and one or more surface elements having surface portions with a second size, e.g. where the first size is different to the second size. In embodiments, the shape of all of the surface portions may be the same (for example and the size of one or more surface portion may be different from one or more other surface portion).

io The surface layer may comprise one or more sensors, e.g. for sensing a flow of fluid, in use, across the surface layer. One of the surface elements (where provided) may comprise one of or the sensor. The one or more sensors may be provided in and/or on the fluid engagement layer. The, one, some or each of the actuators may comprise a sensor. The actuator of one, some or each of the surface elements may comprise a sensor. The, some or each sensor may be configured or configurable to sense and/or measure a flow of fluid (e.g. one or more property of that fluid), in use, across the surface layer. In embodiments, the sensor or sensors may be configured or configurable to transmit a measurement of a flow of fluid across the surface layer, e.g. to a controller.

The surface layer may comprise a flexible cover, for example extending over one or more or each of the plural surface elements. Alternatively, one or more of the plural surface elements (e.g. the surface portions of the surface elements) may define or provide the or an outer surface of the surface layer, e.g. when the surface layer is in use. For example, one or more of the plural surface elements (e.g. the surface portions of the surface elements) may be arranged or configured to be in contact with fluid flowing across the surface layer (e.g. in use). One or more of the plural surface elements may be flexible (e.g. formed from a flexible material), for example at least partially. In embodiments the fluid engagement layer (e.g. at least a portion thereof) may be flexible, for example formed of a flexible material.

The surface layer may comprise a surface deforming means (e.g. device). The surface deforming means may be arranged or configured to at least partially deform the surface of one or more surface element (where present) and/or of the surface layer. The surface deforming means may comprise an actuator, for example which may comprise shape memory material or an electroactive polymer. The surface deforming means may be arranged to be controlled, in use, by the or a controller. The surface deforming means may be arranged to be controlled to generate a projection and/or a depression in the surface of a surface element (where provided) and/or the surface layer. The surface deforming means may have arranged or configured (e.g. shaped) to generate a goosebump' shaped projection from and/or depression into the surface of a surface element and/or the surface layer, in use.

In embodiments, the surface layer may comprise a base. The base may comprise a unitary or contiguous structure. Alternatively, the base may comprise a plurality of base portions (e.g. which may be separate or separable, for example in use). Each of the plural surface elements may comprise a base portion. Alternatively, each base portion may correspond to a plurality of surface portions. Each of the surface elements may comprise a surface portion and an actuator located or positioned between the surface portion and the base or base portion (e.g. the base portion of the surface element). The, some or each sensor may be located between the surface layer of a surface element and the base or base portion (e.g. of the surface layer or the surface element).

In embodiments, the surface layer may comprise a main layer. The main layer may comprise plural recesses or apertures. Each of the actuators of the plural apertures may be located in a recess or aperture of the main layer.

The actuator of one, some or each of the plural surface elements may be configured to be controlled, in use, to move at least a part of the fluid engagement layer (for example the surface portion of the surface element, or a part thereof), e.g. toward and away from the base of that surface element. The actuator of one, some or each of the plural surface elements may be configured or arranged to be controlled to move at least a part of the fluid engagement layer (for example the surface portion of the surface element, or a part thereof) toward and away from the base at a frequency greater than once a second, for example greater than 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500 or 3000 times per second.

The surface portion (where provided) of one, some or each of the plural surface elements may be moveable between a first position or condition and a second position or condition. The surface portion may be in the first position or condition when the actuator is not or has not been actuated. The first position or condition may represent or result from a neutral or rest position or condition (e.g. of the actuator). The first position or position may be substantially coplanar or aligned with adjacent or near-by (e.g. directly adjacent) surface portions when the respective actuators of those surface portions are not or have not been actuated. In the second position or condition the surface portion or part thereof may project or extend beyond the first position or condition, e.g. on the or an opposite side of the surface portion to that of the actuator. Alternatively, in the second position or condition the surface portion or part thereof may be rebated relative to or below the first position, e.g. on the same side of the surface portion as the actuator.

VVhere the surface layer comprises a base and or base portions, in the first position the surface portion of one, some or each of the plural surface elements may be spaced from the base (or a base portion) by a first distance. In the second position the surface portion of one, some or each of the plural surface elements may be spaced from the base (or a base portion) by a second distance. The second distance may be less than or greater than the first distance.

io In some embodiments, the surface portion of one, some or each of the plural surface elements may be moveable between a first position, a second position and a third position. The surface portion (where provided) may be in the first position when the actuator is not or has not been actuated. The first position may represent or result from a neutral or rest position or condition (e.g. of the actuator). The first position may be substantially coplanar with adjacent or near-by (e.g. directly adjacent) surface portions when the respective actuators of those surface portions are not or have not been actuated. In the second position the surface portion or part thereof may project or extend beyond the first position, e.g. on the or an opposite side of the surface portion to that of the actuator. In the third position the surface portion or part thereof may be rebated relative to or below the first position, e.g. on the same side of the surface portion as the actuator. Where the surface layer comprises a base and or base portions, in the first position the surface portion of one, some or each of the plural surface elements may be spaced from the base (or a base portion) by a first distance. In the second position the surface portion of one, some or each of the plural surface elements may be spaced from the base (or a base portion) by a second distance. In the third position the surface portion of one, some or each of the plural surface elements may be spaced from the base (or a base portion) by a third distance.

The second distance may be greater than both the first and third distances. The third distance may be less than both the first and second distances.

In embodiments, the surface layer may be for attachment to a site of use, for example to the surface of an object or vehicle. The surface layer may be configured or arranged to be attached to a site of use (e.g. a surface of an object or vehicle). In embodiments, the surface layer may be attached to a site of use (e.g. a surface of an object or vehicle). In embodiments, the surface layer may be at least partially integrally formed with a site of use (e.g. a surface of an object or vehicle).

According to a further aspect of the invention there is provided an apparatus for selectively altering boundary layer adhesion, the apparatus comprising a controller and a surface layer as described herein.

The controller may be configured to control individual actuation of one or more of the actuators, for example to control individual actuation of each of the actuators.

The controller may be configured to control the actuators via multiplexing, e.g. via a plurality of switches.

Using low cost multiplexing or high frequency processing methods the cell is able to act as both sensor and actuator. If the cell is able to use the same actuator as a sensor then the actuating and sampling frequencies are alternated as required. This rapid reaction control feedback mechanism enables rapid response to alternating fluid flow conditions, thus manipulating the flow and controlling those flow conditions.

The controller may be configured to receive, in use, sensor data from the or each sensor. Where the cover layer comprises plural sensors the controller may be configured to receive, in use, sensor data from an individual sensor of the plural sensors. The controller may be configured to receive, in use, sensor data from an individual sensor of the plural sensors via multiplexing, e.g. via a plurality of switches.

As used herein, the term 'multiplexing' is used to describe a means of controlling the actuators and/or of receiving sensor data from the sensor or sensors. Multiplexing may be frequency based or time based. Multiplexing may comprise a means of control similar to that of controlling a multiplexed display, for example a multiplexed dot-matrix display. In embodiments, the surface layer may comprise an array of columns and rows of surface elements. All of the actuators in each column may be electrically connected or connectable to one another. All of the actuators in each row may be electrically connected or connectable to one another. Each of the rows and/or each of the columns may comprise an electrical contact. The controller may be configured to control electrical supply and/or send a control signal via each of the electrical contacts, for example in order to individually control one or more of the actuators.

The controller may be configured to control the actuators, in use, based on received sensor data from the or each sensor. The controller may be configured to control an individual actuator based on received sensor data from one or more sensor. The controller may be configured to control a first group of actuators based on sensor data from one or more sensors. The controller may be configured to control a second group of actuators based on sensor data from one or more other sensors. The controller may be configured to control each actuator individually based on received sensor data from one or more sensor.

The sensors may be configured to detect changes in flow conditions (e.g. of fluid flowing over the surface layer, in use). For example, the sensors may be configured to feed back voltage changes around an asymmetric sensor orientation in the anchor of a sensor. This can provide a torque orientation by a varied voltage dependent on the direction of bend. A similar orientation of sensors in a rose or similar configuration can detect further directional flow feedback through io processing the directional pull of torque from the changing flow and pressures that the changing boundary flow produces. This analysis then feeds into a control algorithm that will produce a change in the actuation frequency and or amplitude in a coordinated way that the surface elements can produce a local or global actuation in a set pattern that can be directional or even wave like. At such high actuation frequencies waves can travel at high required speeds and manipulate flow responding to TS waves.

The controller may be configured to determine one or more of a direction, a velocity, a mass flow rate, a pressure and a Tollmien-Schlichting wave magnitude (e.g. frequency and/or amplitude) of a flow of fluid across the surface layer, in use, for example based on sensor data zo received from the or each sensor.

The controller may be configured to determine the magnitude of a torque exerted on the surface layer and/or on a structure or body to which the surface layer is attached, due to the flow of a fluid across the surface layer, in use, for example based on sensor data received from the or each sensor.

The controller may be configured to control, in use, the actuator of a surface element to move at least a portion of the fluid engagement layer (e.g. the surface portion of the surface element or a part thereof), toward and/or away from the base (where the surface layer or surface element comprises a base). The controller may be configured to control the actuator of a surface element to move at least a portion of the fluid engagement layer (e.g. the surface portion of the surface element or a part thereof) toward and away from the base at a frequency greater than once a second, for example greater than 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500 or 3000 times per second. The controller may be configured to control the actuator of a surface element or the actuators of plural surface elements at a different frequency to the actuator of another surface element or the actuators of other surface elements. The controller may be configured to control the actuator a surface element or the actuators of plural surface elements at a varying frequency, e.g. varying over time. The controller may be configured to control the actuator of a surface element or the actuators of plural surface elements to displace to a varying displacement distance (e.g. relative to maximum displacement), e.g. varying over time. The controller may be configured to control the actuator of a surface element or the actuators of plural surface elements to vary the frequency of vortices creation.

The controller may be configured to control the actuator of a first surface element to move a first portion of the fluid engagement layer (e.g. the surface portion of that first surface element or a part of that surface portion) toward the base at a first time, for example and to control the actuator of a second surface element to move a second portion of the fluid engagement layer (e.g. the surface portion of the second surface element or a part of that surface portion) toward the base at a second time (e.g. wherein the second time is after, before or simultaneous with the first time). The first and second surface elements may be adjacent or near one another.

The controller may be configured to control the actuator of a or the first surface element to move a or the first portion of the fluid engagement layer (e.g. the surface portion of that first surface element or a part of that surface portion) away from the base at a third time, for example and to control the actuator of a or the second surface element to move a or the second portion of the fluid engagement layer (e.g. the surface portion of the second surface element or a part of that surface portion) away from the base at a fourth time (e.g. wherein the fourth time is after, before or simultaneous with the third time). The first and second surface elements may be adjacent or near one another.

The controller may be configured to control the actuators of a plurality of surface elements to move a first set of portions of the fluid engagement layer (e.g. the surface portions of the surface elements or parts thereof) away from (or toward) the base at a fifth time, for example and to control the actuator of a plurality of different surface elements to move a second set of portions of the fluid engagement layer (e.g. the surface portions of the surface elements or parts thereof) away from (or toward) the base at a sixth time. The sixth time may be after, before or simultaneous with the fifth time.

The apparatus may further comprise a source of electricity. The apparatus may comprise a source of heating and/or cooling. The apparatus may comprise attachment means, for example for attaching the surface layer to a site of use (e.g. the surface of an object or vehicle).

Another aspect of the invention provides a series or array of modules comprising plural surface layers as described herein. In embodiments, the series or array of modules may comprise a first surface layer comprising a first array (e.g. regular or irregular array) of surface elements or apertures and a second surface layer comprising a second array (e.g. regular or irregular array) of surface elements or apertures. The first and second surface layers may be contiguous or unitary or may be at least partially separated or separable (e.g. connected or connectable). The first and second surface layers may contain any number of surface elements or apertures, e.g. the same number or a different number of surface elements or apertures to each other. The first surface layer may comprise surface elements or apertures having a different size and/or spacing io and/or number and/or shape than does the second surface layer. Alternatively, the first surface layer may comprise surface elements or apertures having the same size and/or spacing and/or number and/or shape as does the second surface layer.

The series or array of modules may comprise a controller as described herein. Alternatively, the series or array of modules may comprise plural controllers, for example a controller for each of the surface layers. Each controller may be configured to control operation of one of the surface layers. In embodiments, there may be a master controller. The master controller may be configured or configurable to override one or more of the plural controllers (where provided).

Further mechanisms in the invention may control the surface topography of a surface layer or surface element. A shape memory alloy coil can be used to manipulate a flexible substrate in a surface layer or surface element, for example akin to the goosebumps' effect of skin. Additional structures could be embedded in the surface layer or surface element to act like small hairs on the surface and actuated by the shape memory alloy structures. Further topography changes and actuations may operate in boundary manipulation methods to break up the flow into micro turbulences and smaller more controllable flow pattems at the leading edge and down the main body of a surface to which a surface layer as described herein is attached, in use. Finally, the trailing edge of a surface layer may use serrations or similar structures to break up the turbulence at the trailing edge. Shape memory alloy or similar wire type extrusion can act like retractable hair type structures again much like human skin but with improved actuation so the structure can fully retract.

A further aspect of the invention provides a method of selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing across a surface layer, the method comprising: providing a surface layer comprising a fluid engagement layer and plural surface elements each comprising an actuator; and individually controlling the movement of each actuator to generate or alter the height of a projection from the fluid engagement layer and/or to generate or alter the depth of a recess into the fluid engagement layer.

A further aspect of the invention provides a method of selectively (e.g. adaptively) altering boundary layer adhesion of a fluid flowing across a surface layer, the method may comprise: providing a surface layer comprising plural surface elements each comprising a surface portion and an actuator; and individually controlling the movement of a surface portion in a direction substantially perpendicular to a plane across (e.g. defined by) the outer surface of the surface layer at the location of the surface portion.

A further aspect of the invention provides a structure comprising a surface layer or apparatus as described herein. The surface layer (e.g. the surface layer of the apparatus) may be positioned or located or attached on or to the structure, e.g. on or to a surface (for example an inner or outer surface) of the structure. In embodiments, the structure may be or comprise a vehicle or a part thereof or therefor, for example a land, sea or air vehicle or a part thereof or therefor. The vehicle may be or comprise an automobile, aeroplane, boat or the like.

For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. For example, the method may comprise any one or more features of the surface layer relevant to the apparatus and vice versa and/or the method may comprise any one or more features or steps relevant to one or more features of the surface layer or the apparatus.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of an apparatus for selectively altering boundary layer adhesion according to a first embodiment of the invention; Figure 2 is a cross-sectional side view of a surface element of the surface layer shown in Figure 1; Figure 3 is a cross-sectional side view of part of a row of surface elements of the surface layer shown in Figure 1; Figure 4 is a perspective view of an apparatus for selectively altering boundary layer adhesion according to another embodiment of the invention; Figure 5 is a cross-sectional side view of a surface element of a surface layer according to another embodiment of the invention; Figure 6 is a cross-sectional side view of part of a row of surface elements of a surface layer according to another embodiment of the invention; Figure 7 is a perspective view of an apparatus for selectively altering boundary layer io adhesion according to another embodiment of the invention; Figure 8 is a cross-sectional side view of part of the surface layer shown in Figure 8; Figure 9 is a perspective view of an apparatus for selectively altering boundary layer adhesion according to another embodiment of the invention; Figure 10 is a cross-sectional side view of part of the surface layer shown in Figure 9; Figure 11 is a partial close-up view of the surface layer shown in Figure 9; Figure 12 is a schematic view of an apparatus for selectively altering boundary layer adhesion according to another embodiment of the invention; Figure 13 is a perspective view of a modular arrangement of surface layers; Figures 14 to 21 are surface topography manipulation devices useful in surface layers according to the invention; Figure 22 is a plan view of an alternative sensor configuration; Figure 23 is a side view of an alternative actuator; Figure 24 is a perspective view of an alternative sensor configuration; Figure 25 is a cut-away side view of an alternative actuator arrangement; and Figure 26 is a perspective view of a surface layer or surface element having protrusions thereon.

Referring now to Figure 1, there is shown an apparatus 100 for selectively altering boundary layer adhesion, the apparatus 100 comprising a surface layer 10 and a controller 2.

The surface layer 10 comprises a fluid engagement layer 11 and a plurality of surface elements 1aa-1ff. The surface elements 1aa-1ff comprise the fluid engagement layer 11 in this embodiment (as will be explained below). The surface elements 1aa-1ffi are arranged in an array of columns and rows. Each surface element 1aa-111 is denoted by a reference numeral the first letter of which denotes the row and the second letter of which denotes the column. The controller 2 is in electrical communication with each surface element 1aa-1ff, in use, via wired connections (not shown).

Referring now to Figure 2, there is shown a single surface element 1 of the surface elements 1aa-1ff shown in Figure 1. The surface element 1 corresponds to any of the surface elements 1aa-1ff of the surface layer 10 shown in Figure 1.

The surface element 1 comprises a surface portion 3, an actuator 4 and a base portion 5. The surface portion 3 and the base portion 5 are formed from a plastic material, in this embodiment. In embodiments, however, the surface portion 3 and/or the base portion 5 may be formed from any other suitable material, for example a rubber material, a metal or the like.

The actuator 4 is located between the surface portion 3 and the base portion 5. The actuator 4 is connected at one of its ends to the surface portion 3. The actuator 4 is connected at the other of its ends to the base portion 5. The fluid engagement layer 11 comprises the surface portions 3 of the surface elements 1aa-1ff, in this embodiment. The surface element 1 comprises side walls 6, in this embodiment. The side walls 6 are attached to or at the periphery of the base portion 5. The side walls 6 are arranged such that the surface portion 3 can be moved therebetween, in use (as will be described below). The side walls 6 are sized and shaped to substantially seal against the surface portion 3 when the surface portion 3 is in a neutral position, e.g. when the surface portion 3 has not been moved by the actuator 4. The side walls 6 may be formed of the same material as the base portion 6 and/or the surface portion 3.

The surface portion 3 comprises a flow surface 31, against or over which fluid may flow, in use. The flow surface 31 comprises a first major surface of the surface portion 3 On this embodiment).

The surface portion 3 comprises a second major surface, opposed to the flow surface 31 The actuator 4 is attached or connected to the second major surface (or at least acts thereagainst, in use).

The actuator 4 is a piezoelectric actuator, in this embodiment. The actuator 4 is configured to move in response to an applied electrical current. In use, input of the electrical current causes the expand and/or contract relative to its neutral size and position (e.g. when an electrical current is not applied to the actuator). The actuator 4 is also configured to operate as a sensor, in this embodiment, as will be described in greater detail below.

The end of each of the rows and each of the columns of surface elements 1aa-1ff of the surface layer 10 comprises an electrical contact (not shown). An electrical contact is electrically connected to the actuator 4 of each of the surface elements 1aa-111 at the end of each of the rows. Additionally, the actuator 4 of each surface element 1 aa-1 ff in each row is electrically connected to the actuator s4 of the adjacent surface elements laa-lff. For example, the actuator 4 of surface element lbc is electrically connected to the actuators 4 of surface elements lac and lcc. Additionally, the actuator 4 of each surface element 1 aa-1 ff in each column is electrically connected to the actuators 4 of the adjacent surface elements 1aa-1ff. For example, the actuator 4 of surface element lbc is electrically connected to the actuators 4 of surface elements lbb and lbd.

The controller 2 is operably connected to each of the electrical contacts. The controller 2 is io configured to send control signals and receive sensor signals (e.g. in the form of electrical signals) individually to and from each of the actuators 4 of the surface elements laa-lff. The controller comprises a program for individually controlling the actuators 4 of the surface elements laa-lff, in use. The program is also configured to determine one or more characteristic of fluid flow, in use, across or over the surface layer 10 from sensor signals received from one, some or each of the actuators 4 of the surface elements laa-lff. The controller 2 is configured to individually control a specific actuator 4 by use of an electrical control signal having a specific frequency. Each actuator 4 is controllable using a different, individual frequency. In this way, a relatively reduced amount of wiring is required compared to individually connecting each actuator 4 to the controller 2.

Prior to use, the surface layer 10 is positioned and/or secured to a structure, for example to the surface of a vehicle. The controller 2 may be located or positioned in or on the structure (e.g. the vehicle).

In use (as described with reference to Figure 3) the controller 2 individually controls actuation of the actuators 4 of the surface elements laa-lff. Actuation of the piezoelectric material of the actuators 4 is accomplished by applying an electrical current thereto, as is known to one skilled in the art.

When an actuator 4 is not being actuated by the controller 2 (e.g. no electrical current is being applied thereto), the surface portion 3 is located in a first position, e.g. relative to the base portion 3. Surface elements laa and lac are shown in this first position in Figure 3. In the first position, the flow surface 31 of surface element 1 aa is substantially aligned with a plane P defined by the flow surface 31.

Actuation of the actuator 4 causes its respective surface portion 3 to move linearly away from the base portion 5 into a second position (as shown by surface element lab in Figure 3). In the second position the surface portion 3 is spaced by a greater distance from the base portion 5 than in the first position.

In the second position the flow surface 31 is offset from the plane P. In the second position the surface portion 3 of surface element lab projects relative to the first position. In the view shown in Figure 3, the surface portion 3 of surface element lab also projects relative to the surface portions 3 of the adjacent surface elements laa and lac.

In use, fluid flows over the flow surfaces 31 of the surface elements 1 in the surface layer 10.

io This may be due, for example, to the movement of a vehicle to which the surface layer 10 is attached through a fluid, such as air. A boundary layer forms in the fluid adjacent the flow surfaces 31 of the surface elements 1. The controller 2 controls the actuator 4 of one or more of the surface elements laa-lff to move the surface portion 3 thereof to the second position. In this way, localised projections on the surface layer 10 may be created. The actuator 4 of one or more of the surface elements 1aa-1 ff may be controlled to move between the first and second positions, e.g. cyclically. In this way, a turbulent layer may be generated in the boundary layer. Without wishing to be bound by any particular theory it is believed that this turbulent layer advantageously encourages adhesion of the boundary layer to the surface layer. Accordingly, the boundary layer will adhere to the surface layer (and hence to the structure or vehicle on which it is provided) for a greater distance and/or duration than would otherwise be the case.

Beneficially, the drag co-efficient of the structure or vehicle is thereby positively enhanced.

When a fluid flows, in use, over or across the surface layer 10, a pressure is generated thereagainst. The pressure is sensed by the actuator 4 of one or more of the surface elements 1 aa-1 ff (dependent on the location and size of the fluid flow). The pressure causes the piezoelectric material of the actuator 4 to at least partially deform, thereby generating an electrical current. This electrical current is transmitted as a sensor signal to the controller 2. The controller 2 is configured to determine one or more characteristic of the fluid flow, in use, across the surface layer 10. For example, by receiving sensor signals from plural actuators 4 the controller 2 can determine the direction of the flow of fluid (e.g. via triangulation). Additionally, the velocity, mass flow rate, and the like of the fluid flow can be determined by the controller 2.

The controller 2 processes received sensor signals from individual actuators 4 and determines an optimum topography of the surface layer 10. The optimum topography may comprise one or more of: * the location and/or number of surface elements to have their actuators actuated; * the rate at which the actuators surface elements (or some of them) are cycled to move their surface portions from the first to the second position and back again; and * the timing at which actuators of surface elements are actuated.

Sensor and control signals provided from and to the actuators 4 to and from the controller 2 may be delivered continuously or intermittently, during use. As such, the surface portions 3 can be actuated statically or dynamically. In static actuation one or more actuator 4 may be maintained in a specific position (e.g. the first or second position, or a location therebetween). In dynamic actuation one or more actuator 4 may be cyclically moved to or between the first io and second positions. The surface portions Scan also be actuated in a cascading fashion. For example, if a flow of fluid travels from left to right over the surface layer 10 as shown in Figure 3, the left-most surface element laa may be actuated first; the centre-left surface element lab actuated second; and the centre-right surface element lac actuated third.

Referring now to Figure 4, there is shown an apparatus 1100 for selectively altering boundary layer adhesion according to a further embodiment of the invention, wherein like features to those described in respect of the apparatus 100 shown in Figure 1 are designated by like references preceded by a t and will not be described further herein. In this embodiment a sensor 40 is provided on the surface layer 110. Although only one sensor 40 is shown in Figure 4, any number may be provided. Additionally or alternatively, in embodiments the sensor 40 (or sensors 40) may be provided adjacent or beside the surface layer 110.

The sensor 40 is provided to sense characteristics of fluid flow, in use, over or across the surface layer 110. The sensor 40 is operably connected to the controller 12 and is configured to transmit sensor data thereto. The sensor 40 (or some or each sensor 40) may be or comprise any suitable kind of sensor. For example, the sensor 40 may be or comprise a flow sensor, a temperature sensor or the like. In this embodiment the actuators (not shown) of the surface elements llaa-11ff are not configured to transmit sensor signals to the controller 12. However, in embodiments, one or more of the actuators of the surface elements 11 aa-1 1 ff may be configured as a sensor (in accordance with the description of the Figure 1 embodiment).

The measurements from the sensor/s 40 can be used (for example, in place of or alongside sensor signals from the actuators 4) to calculate, for example, flow rate, flow velocity, flow direction and/or Tollmien-Schlichting wave magnitude (e.g. frequency and/or amplitude), and the like. The sensor/s 40 are mounted or connected to or in the surface layer 110 via any suitable means (e.g a mechanical and/or adhesive connection). In embodiments the sensor 40 may project beyond the surface of the surface layer 110, e.g. locally to the sensor 40. Alternatively, however, the sensor 40 may be positioned to be flush or at least slightly recessed relative to the surface layer 110, e.g. locally to the sensor 40. In Figure 4 the sensor 40 is shown generally in the centre of the surface layer 10, but any number of sensors 40 could be mounted at any location on the surface layer 10. The sensor 40 is operably connected to the controller 12 by wires (not shown), in this embodiment. In embodiments, however, the sensor 40 may be operably connected to the controller 12 wirelessly.

Referring now to Figure 5, there is shown an apparatus 2100 for selectively altering boundary layer adhesion according to a further embodiment of the invention, wherein like features to those described in respect of the apparatus 100 shown in Figure 1 are designated by like references preceded by a '2' and will not be described further herein. In this embodiment a sensor 240 is mounted to the surface portion 23 on the opposite surface to the flow surface 231. Any number of the surface elements 21 of the surface layer 210 may be provided with a sensor 240 (for example, in embodiments, each of the surface elements 21 is provided with a sensor 240). The sensor 240 may be or comprise a sensor 40 similar to that described above with respect to the apparatus 1100 shown in Figure 4. In embodiments, the sensor 240 may be configured to measure or determine forces acting against or upon the surface portion 23. For example, the sensor 240 may be or comprise an accelerometer. The controller (not shown) may be configured to determine one or more characteristic of fluid flow, in use, over or across the surface layer 210 from received sensor data from the sensor 240 (or sensors 240, where plural thereof are provided). Although the sensor 240 is shown attached to the surface of the surface portion 23 obverse to the flow surface 231 this need not be the case. Instead, the sensor 240 may be attached to or at any suitable location. In embodiments, the sensor may communicate (e.g. fluidly) with a fluid flow, in use, across the surface layer 210. The sensor 240 may be provided (e.g. at least partially) in or on the surface portion 23, for example in an aperture or recess thereof (not shown). The sensor 240 is operably connected to the controller via wires, in this embodiment (though in embodiments it may be connected wirelessly).

Referring now to Figure 6, there is shown an apparatus 3100 for selectively altering boundary layer adhesion according to a further embodiment of the invention, wherein like features to those described in respect of the apparatus 100 shown in Figure 3 are designated by like references preceded by a '3' and will not be described further herein. In this embodiment the surface layer 310 comprises a flexible cover 8. In embodiments, the flexible cover 8 may comprise or be comprised as part of the fluid engagement layer 311.

The flexible cover 8 is attached (e.g. adhered or otherwise bonded or mechanically attached) to the flow surface 31 of each surface portion 3, in this embodiment. In embodiments, however, the flexible cover 8 may be attached to only some of the surface portions 33. The flexible cover 8 is formed from a plastic material, for example from PVC or the like. The flexible cover 8 beneficially mitigates against the ingress of air and/or debris into the surface elements 31aa31ff. Additionally, provision of the flexible cover 8 also creates a smoother transition between recesses and projections over the surface layer 10. The flexible cover 8 is attached to some or each flow surface 33 in a region at or near to the centre, such that a portion of the flexible cover 8 at or adjacent the edge of each surface portion 33 can separate from the flow surface 331 if the adjacent surface portion 33 is at a different position or moving toward a different position (e.g. the first or second position). Advantageously, the flexible cover 8 is sufficiently flexible to io conform to the surface layer 10 shape when the actuators 35 have been actuated to move the surface portions 33 into the second position (e.g. as shown by surface element Slab in Figure 6).

Referring now to Figures 7 and 8, there is shown an apparatus 4100 for selectively altering boundary layer adhesion according to a further embodiment of the invention, wherein like features to those described in respect of the apparatus 100 shown in Figures 1 to 3 are designated by like references preceded by a '4' and will not be described further herein. The apparatus 4100 differs from that shown in Figures 1 to 3 by way of including a base 45 which extends across the entire surface layer 410. Additionally, the apparatus 4100 differs from that shown in Figures 1 to 3 by way of having fewer surface elements 41aa-41cc. Additionally, the apparatus 4100 differs from that shown in Figures 1 to 3 by way of the surface elements 41aa41cc not being provided directly adjacent one another. Instead, the surface layer 410 comprises a main layer 46 provided with apertures within which the surface elements 41aa-41cc are provided. The apertures and the surface portions 43 of the surface elements 41aa-41cc are shaped and sized to closely correspond to one another. In embodiments, the base 45 and main layer 46 may comprise the same material and/or may be unitary or separate components. The fluid engagement layer 411 is comprised of the main layer 46 and the surface portions 43, in this embodiment. The main layer 46 is formed from a plastic material, in this embodiment. In embodiments, however, the main layer 46 may be formed from any suitable material, for example a rubber material, a metal or the like.

In use, the apparatus 4100 functions similarly to the way in which the apparatus 100 shown in Figures 1 to 3 functions. However, in the apparatus 4100 the surface portions 43 are spaced by a lesser distance from the base 45 in the second position than in the first position (in contrast to the apparatus 100 shown in Figures 1 to 3, in which the distance is greater in the second position than in the first position). According, in the second position the surface portion 43 of surface element 41ca is rebated relative to the first position, e.g. relative to plane P (for example as defined by the flow surface at the location of the surface element 41ca).

Referring now to Figures 9 to 11, there is shown an apparatus 5100 for selectively altering boundary layer adhesion according to a further embodiment of the invention. The apparatus 5100 comprises a surface layer 510 and a controller 52. The surface layer 510 comprises a fluid engagement layer 56 comprising plural apertures 51aa-51cc therethrough. As can be seen in Figure 10, each aperture 51aa comprises a chamber 512. Each aperture 51aa comprises a nozzle 513. The chamber 512 is suitably shaped and sized to generate jets of fluid in use, as will be described further below. In embodiments, the aperture 51aa (e.g. the nozzle 513 and/or the chamber 512) may be sized and/or shaped differently to as shown in Figure 10, as will be appreciated by one skilled in the art. Any suitable number and/or arrangement of apertures 51aa-51cc may be provided.

The fluid engagement layer 56 is formed of a flexible material in this embodiment, e.g. rubber.

The fluid engagement layer 56 comprises a first major surface 56a which is a fluid engagement surface 56a. The fluid engagement layer 56 comprises a second major surface 56b, obverse to the first major surface 56a. An optional flexible diaphragm 57 is positioned on the second major surface 56b of the fluid engagement layer 56. The flexible diaphragm 57 covers the chamber 512 of the aperture 51aa. The flexible diaphragm 57 is sealed about its periphery to the fluid engagement layer 56. An actuator 58 is positioned to bias the diaphragm 57 in the directions of double-ended arrow A, in use. In this embodiment, the actuator 58 is a piezoelectric actuator 58. In this embodiment, the piezoelectric actuator 58 is a disc shaped piezoelectric actuator, although a cantilever or column piezoelectric actuator (for example affixed to a wall or base of the surface layer) could be used instead. In embodiments, however, the actuator 58 may be of any suitable type (for example an electroactive polymer or electro-magnetic actuator) and/or may have any suitable shape or configuration. Electrical contacts 59 are provided to the actuator 58. In embodiments, the flexible diaphragm 57 is omitted and a disc shaped piezoelectric actuator is used in its place. The disc shaped piezoelectric actuator flexes to deform, in use, similarly to the flexible diaphragm.

A shape actuator 53 surrounds the aperture 51aa in this embodiment. However, in embodiments, the shape actuator 53 may not surround the aperture (for example, the shape actuator 53 may be positioned about a portion of the aperture 51aa, e.g. a major or minor portion of the circumference thereof). The shape actuator 53 is configured or arranged to alter, in use, the shape of the nozzle 513 of the aperture 51aa, in embodiments. The shape actuator 53 is disposed adjacent the nozzle 513 of the aperture 51aa, in this embodiment. In this embodiment, the shape actuator 53 is embedded in the fluid engagement layer 56. However, in embodiments, the shape actuator 53 may be on (e.g. at least partially) the fluid engagement layer 56. In this embodiment, the shape actuator 53 is formed of shape memory alloy material. The shape actuator 53 is provided as plural shape memory alloy wires 53a, in this embodiment (as shown in Figure 11). The shape memory alloy wires 53a are disposed about the aperture 51aa. Some of the shape memory alloy wires 53a overlap with other of the shape memory alloy wires 53a (although in embodiments this need not be the case). As will be appreciated, although only a single aperture 51aa has been described above the same structure is repeated for each of the apertures 51aa-51cc of the apparatus 5100 shown in Figure 9. An actuator 58 and diaphragm 57 is provided for each of the apertures 51aa-51cc.

In use, the controller 52 actuates the piezoelectric actuator 58 to flex in the directions of double-headed arrowA. The piezoelectric actuator 58 thereby biases the diaphragm 57 in the directions of double-headed arrow A. The volume of fluid (e.g. air) in the chamber 512 is relatively reduced or increased by this biasing of the diaphragm 57. In this way a jet of air is driven out of and then drawn into the aperture 51aa, e.g. through the nozzle 513 thereof. These alternating flows of fluid (e.g. jets) generate vortices at the fluid engagement surface 56a of the fluid engagement layer 56, about the apertures 51aa-51cc. These vortices constitute turbulence. The turbulence acts to alter the adhesion of a boundary layer of fluid flowing over the surface layer 510 (as described previously).

The piezoelectric actuator 58 also acts as a sensor for measuring the characteristics of fluid flow across the surface layer 510. In between actuations of the piezoelectric actuator 58 the venturi effect of fluid flow across the surface layer 510 creates small deflections in the piezoelectric actuator 58. Sensor data corresponding to these deflections is transmitted to the controller 52 (e.g. via the electrical contacts 59). The sensor data is received and processed by the controller 52 in a similar manner as described with respect to other embodiments of the invention. The controller 52 then determines which piezoelectric actuators 58 to actuate, at what frequency and at what magnitude (in response to determined characteristics of the fluid flow across the surface layer 510).

The memory shape alloy wires 53a are actuated in use by the controller 52 to alter the shape and/or size of the aperture 51aa (e.g. of the nozzle 513 of the aperture 51aa). The shape memory alloy 'Ares 53a are controlled to elongate or retract to thereby stretch or compress the flexible fluid engagement layer 56 about the aperture 51aa. In this way, the shape and/or size of the aperture 51aa (e.g. of the nozzle 513 thereof) can be dynamically altered. The controller 52 determines the appropriate shape and/or size of the aperture 51aa (e.g. the nozzle 513 thereof) based on the determined characteristic(s) of the fluid flow across the surface layer 510. Altering the size and/or shape of the aperture 51aa (e.g. the nozzle 513 thereof) advantageously allows vortices to be generated of different shapes, sizes and intensities. Additionally, altering the size and/or shape of the aperture 51aa (e.g. the nozzle 513 thereof) has a similar effect to that of a musical instrument. Additionally, altering the size and/or shape of the aperture 51aa (e.g. the nozzle 513 thereof) can alter the resonance of the piezoelectric actuator. This is achieved by relatively restricting or relaxing the flow of air through the aperture 51aa (e.g. through the nozzle 513 thereof). Air flow velocity and other characteristics and acoustics can be altered, in this way. Such acoustic control can advantageously control noise generated by the io jets emitted from the aperture. Similar or different alterations may be made to some or each of the apertures 51aa-51cc.

Referring now to Figure 12, there is shown an apparatus 6100 for selectively altering boundary layer adhesion according to a further embodiment of the invention. The apparatus 6100 comprises plural surface layers 610a, 610b, 610c each having their own controller 62a, 62b, 62c. Additionally, a central controller 60 is provided, for controlling the controllers 62a, 62b, 62c. The plural surface layers 610a, 610b, 610c may be attached, in use, to the surfaces of a vehicle (not shown). For example, the first surface layer 610a may be attached to the surface of a diffuser. The second surface layer 610b may be attached to the surface of a main floor. The third surface layer 610c may be attached to the surface of a spoiler. Each of the surface layers 610a, 610b, 610c may be the same (e.g. have the same construction). Alternatively, one or more surface layer 610a, 610b, 610c may be different to the others. The surface layers 610a, 610b, 610c may be in accordance with any of the embodiments described herein.

In use, the individual controllers 62a, 62b, 62c may each individually control each of the surface layers 610a, 610b, 610c. Alternatively, the central controller 60 may be configured to override the control of one or more of the controllers 62a, 62b, 62c. The central controller 60 may, for example, may actuate all of the actuators to purposefully disrupt fluid flow over each of the surface layers 610a, 610b, 610c (for example during breaking of a vehicle to which the surface layers 610a, 610b, 610c are attached). Alternatively, fine control could be provided by the central controller 60 to produce desired alterations in down force and/or drag (for example at different parts of the vehicle).

In embodiments, the first surface layer 610a may be attached near to or at the leading edge of a surface. The second surface layer 610b may be attached near to or at the trailing edge of a surface. The third surface layer 610c may be attached at a location spaced from both the trailing and leading edges of a surface. The central controller 60 (and/or the individual controllers 62a, 62b, 62c) may be programmed to operate the surface layers 610a, 610b, 610c differently dependent on their point of attachment to a surface. The programming may be different for surface layers attached or intended to be attached at a position spaced from a leading and/or trailing edge of a surface (with respect to surface layers attached or intended to be attached at the leading or trailing edge of a surface). In embodiments, controller 62a for a surface layer 610a attached or intended to be attached to the leading edge of a surface can be programmed to control the surface layer 610a differently than for a surface layer 610b attached or intended to be attached to the trailing edge of a surface. In embodiments, the controller 62a, 62b, 62c of a surface layer may be programmed to control actuators and/or surface elements relatively io nearer to a trailing edge and/or leading edge of a surface to which the surface layer is attached or intended to be attached differently from the remaining actuators and/or surface elements of the surface layer.

The surface layer 20a of any embodiment described herein may be or comprise a module in a series or array of modules 20 of surface layers (for example as shown in Figure 13). For example, one or more additional surface layers 20a (which may be the same or different to the first surface layer in one or more respect) may be provided adjacent and/or substantially coplanar to a first surface layer 20a. In embodiments, each surface layer 20a may comprise the same or a different number and/or size and/or shape and/or arrangement of surface elements or apertures. The series or array of modules 20 may be attached to the surface of a spoiler 22 of vehicle. In embodiments, the series or array of modules 20 may be attached to the surface of a different object or vehicle. Alternatively, the series or array of modules 20 may be for attachment to the surface of an object or vehicle.

Referring now to Figures 14 to 21, there are shown surface topography manipulation devices which can be combined or utilized with any of the surface layers described herein.

Referring to Figures 14 and 15, there is shown a surface layer 710 comprising an aperture 71 therethrough. The surface layer 710 may be similar to the surface layer 510 shown in Figures 9 to 11 (although a simplified view is provided here). Surrounding the aperture 71 there is provided a radial fibre 72. In use, the radial fibre 72 is configured to relatively expand or contract (e.g. through control via a controller, not shown). Expansion of the radial fibre 72 causes the formation of a protrusion 74 about the aperture 71 (as shown in Figure 15). The radial fibre 72 may have any suitable cross-section, for example square, circular, oval, or the like. Different cross-sections have different vortices generating effects. In embodiments, the radial fibre 72 may be formed of a shape memory alloy material or an electroactive polymer. Alternatively, a shape memory alloy material coil 73 may be positioned about the aperture 71 (as in the surface layer 710a shown in Figure 16). Again, activation of the coil 73 results in formation of the protrusion 74 shown in Figure 15. It will be appreciated that such a protrusion 74 mimics that of a 'goose bump on skin. Although Figures 14 to 16 are described with respect to a surface layer 710, 710a comprising an aperture 71 (or plural apertures) this need not be the case and, instead, the radial fibre 72 and/or the shape memory alloy material coil 73 could be provided on a surface element of a surface layer and/or in any other suitable location of any surface layer described herein.

Referring now to Figures 17 and 18 there is shown a surface layer 810 comprising plural shape memory alloy wires 811 thereon. The shape memory alloy wires 811 are positioned in a grid formation. A first set of shape memory alloy wires 811a extend generally parallel to one another. A second set of shape memory alloy wires 811b extend generally parallel to one another. The first set of wires 811a crosses the second set of wires 811b. The first and second sets of wires 811a, 811b are generally perpendicular to one another. In use, the first or second set of wires 811a, 811b can be actuated (e.g. by a controller, not shown) to generate a ridged surface (as shown in Figure 18). The direction of the ridges depends on which set of wires 811a, 811b has been actuated. As will be appreciated by one skilled in the art, the selective generation of ridges on a surface layer mimics the behaviour of shark skin. Actuation of the wires 811 may be controlled (e.g. by a controller) in response to read sensor data relating to fluid flow over the surface layer (as described herein with respect to other embodiments of the invention). The configuration of the wires 811 may give variable shapes. Additionally or alternatively, the applied voltage may provide variable shapes.

Referring now to Figures 19 to 21, there is shown a surface layer 910 comprising a protrusion 911 attached to a shape memory alloy material actuator 912. Prior to activation, the protrusion 911 sits below or flush with the local surface of the surface layer 910 (as shown in Figure 20). When the actuator 912 is actuated (e.g. by a controller, not shown) the protrusion 911 (which may be or comprise a fibre) is extended to project beyond the local surface of the surface layer 910 (as shown in Figure 21). As will be appreciated, the opposite is also possible. For example, in a non-actuated state the protrusion 911 my project beyond the local surface of the surface layer 910 and in an actuated state the protrusion 911 may be flush with or below the local surface of the surface layer 910.

Referring now to Figures 22 to 25, there are shown a series of modified actuators and sensors which can be used as well as or in place of any of the actuators or sensors in any of the embodiments of surface layers and apparatus described herein Referring to Figure 22, there is shown a rose formation R to enable directional calculation of fluid flow across a surface layer. The rose formation R comprises strain gauges G disposed at equally spaced locations about a central point C. Deflection of the central point C causes varied strain on each of the strain gauges G. Reading of the output of the strain gauges G allows a controller (not shown) to determine the direction of fluid flow which has cause the deflection of or in the central point C. In embodiments, the central point C may be one or more surface element of a surface layer. As will be appreciated, other configurations than a rose formation may be used.

io Referring to Figure 23, there is shown an alternative actuator 80. The actuator 80 comprises a lever arm 80a connected to a base portion 80b. The lever arm 80a (for example and the base portion 80b) may be formed of a shape memory alloy material. The top of the lever arm 80a is attached to a surface element SE. The base portion 80b is attached to a base B of a surface layer (for example as described herein). In use, activation of the actuator 80 causes the lever arm 80a to flex relatively toward and/or away from the base portion 80b. In this way, the surface element SE is moved toward or away from the base portion B. As will be appreciated, the actuator 80 shown in Figure 23 can also function as a sensor, if so desired.

Referring to Figure 24, there is shown a cylinder or column piezoelectric actuator or sensor 90.

zo The actuator or sensor 90 is attached at one end to a surface element SE. The actuator or sensor 90 is attached at its other end to a support SU. The surface of the support SU to which the actuator or sensor 90 is attached is non-parallel to the surface element SE. In this embodiment, the top of the support SU is sloped to form an ellipse. In this way, the cylinder or column piezoelectric actuator or sensor 90 extends in a direction which is not perpendicular to the surface element SE (e.g. to a major plane thereof). Advantageously, when a fluid flow across the surface element SE acts to deflect or bend the cylinder or column piezoelectric actuator or sensor 90 the direction of the bend provides feedback of the direction of torque of the fluid flow. This is due to the length of the piezoelectric actuator or sensor being relatively different at its front side (for example) relative to at its back side (for example). The voltage generated by the piezoelectric actuator or sensor is dependent on the length thereof Referring to Figure 25, there is shown a cantilever actuator 95. The cantilever actuator 95 is fixed at one end to a fixed structure, for example a wall W (internal or external) of a surface layer. The cantilever actuator 95 is fixed at its other end to a surface element SE.

Advantageously, the cantilever actuator 95 is formed from memory shape alloy or piezoelectric material. Activation of the cantilever actuator 95, in use, causes flexing of the actuator 95. In this way the surface element SE is caused to move in a direction generally perpendicular to the wall W (for example in the directions of double-ended arrow C). An optional skin or cover layer SK is provided over the surface element SE. An optional electromagnetic tuning fork element TF may be provided between the ends of the cantilever actuator 95. The tuning fork element TF can be actuated, in use, to selectively bias against the cantilever actuator 95. In this way the frequency and/or amplitude by which the cantilever actuator 95 displaces the surface element SE may be altered.

Referring now to Figure 26, there is shown a surface layer or surface element 98 having plural protrusions 99 toward or adjacent one edge thereof The surface layer or surface element 98 is io intended for use at a trailing edge of a surface of an object or vehicle. The plural protrusions 99 act to alter or control turbulent fluid flow from the trailing edge in a manner similar to that provided by owl wing protrusions. The plural protrusions 99 can be fixed. Alternatively, the plural protrusions 99 can be at least partially retractable (for example, entirely retractable). Alternatively, the plural protrusions 99 can be formed of a material which expands or contracts upon the application of heat and/or electricity (for example shape memory alloy or electroactive polymers or the like). The plural protrusions 99 shown in Figure 26 could be utilized with any of the surface layers or surface elements described herein It will be appreciated by those skilled in the art that any one or more feature from any of the above-described embodiments may be combined with any of the other features from any of the other above-described embodiments. For example, any of the embodiments may comprise second positions in which the surface portions are spaced by a relatively lesser distance from the base than in the first position. Additionally or alternatively, any of the embodiments may comprise a flexible cover, for example as shown in Figure 6. Additionally or alternatively, the base portions of any of the Figure 1 to 6 embodiments may be replaced by the base 45 shown in the Figure 7 embodiment, or vice versa.

It will also be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. For example, the surface layer 10 is shown as having 36 surface elements 1 in Figures 1 and 4 but this need not be the case and, instead, any suitable number, size, shape and layout of surface elements may be provided. Additionally or alternatively, the surface layers are shown as being flat and square in shape, however this need not be the case and, instead, the surface layers may have any suitable shape and/or may be curved or at least partially curved or otherwise be non-flat (for example comprise one or more portions which are not co-planar or flat).

Additionally or alternatively, the side walls (where provided) may be omitted. The side walls may be flexible and move with the surface portions (e.g. attached to the periphery of the surface portions). Additionally or alternatively, gaps may be provided between the peripheries of adjacent surface elements in any of the above embodiments (for example as shown in respect of the embodiment shown in Figures 7 and 8).

Additionally or alternatively, in embodiments the base portions may not be provided, and alternatively the piezoelectric actuators 4 may be attached or attachable directly to the or a structure or vehicle (e.g. a surface thereof).

io Additionally or alternatively, in embodiments the piezoelectric actuators or sensors may be replaced or supplement by electromagnetic systems and induction i.e a tuning fork vibration. Analysis can be generated by induction to measure the torques and stress/strains induced on the cell using similar techniques to the piezo. The tuning fork model works in a similar way to piezo in that in the presence of exterior forces acting on it the induction driver can become the sensor as current variation will induce a small generation capability from the fork. The system of the electronic tuning fork and induction system form a type of servo system. Such systems are used in reliable watch designs to replace piezo mechanisms. Driving frequencies of 700hz and higher can be driven by the induction unit.

Additionally or alternatively, although the actuators in the above-described embodiments are described as being piezoelectric actuators this need not be the case and, instead, one or more (e.g. each) of the actuators may be a memory shape alloy or a different type of suitable actuator. Additionally or alternatively, each surface element may comprise more than one actuator, for example two, three, four or more. The plural actuators may be disposed toward or at the periphery of the surface portion, for example configured to move the surface portion at plural points near, at or about its periphery.

Additionally or alternatively, in embodiments the fluid engagement layer may be unitary, e.g. the surface portions may be joined. Where this is the case the fluid engagement layer is at least partially flexible.

Additionally or alternatively, the depth of the generated recesses or projections may be tailored to optimise boundary layer adhesion or separation based on characteristics of a flow, in use, of fluid across the surface layer. In embodiments, the controller may be configured to calculate an optimum second position (e.g. the distance from the base, where present), for example based on received sensor data. Additionally or alternatively, although the surface elements are shown as forming a planar or substantially planar fluid engagement layer when in the first position this need not be the case. Instead, one or more of the surface elements may be located at a different distance from the base (where provided) than one or more other of the surface elements, in the first position. Accordingly, one or more of the surface elements may be relatively recessed or projected relative to the other surface elements when in the first position (e.g. in a neutral or non-actuated position). The fluid engagement layer may thereby comprise one or more projections or recesses prior to actuation of the actuators. Actuation of the actuators may relatively increase or decrease the height of the one or more projections and/or may relatively increase or decrease the depth of the one or more recesses.

It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

CLAIMSA surface layer for selectively altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer and plural surface elements each comprising an actuator, wherein some or each actuator is configured or arranged to be individually controlled, in use, by a controller to selectively generate or alter the height of a projection from the fluid engagement layer and/or to selectively generate or alter the depth of a recess into the fluid engagement layer.
Surface layer according to claim 1, wherein the actuator of each surface element comprises a piezoelectric actuator.
3. Surface layer according to claim 1 or 2, wherein the plural surface elements are arranged in an array, e.g. a regular or irregular array.
Surface layer according to any preceding claim, comprising one or more sensors for sensing a flow of fluid, in use, across the surface layer.
5. Surface layer according to claim 4, wherein one of the surface elements comprises one of or the sensor.
Surface layer according to claim 5, wherein the actuator of one, some or each of the surface elements comprises a sensor.
Surface layer according to any preceding claim, comprising a flexible cover extending over the surface elements.
8. Surface layer according to any preceding claim, wherein the surface layer comprises a base.
Surface layer according to claim 8, wherein the actuator of one, some or each of the plural surface elements is configured to be controlled, in use, to move at least a part of the fluid engagement layer toward and away from the base.
10. A surface layer for selectively altering boundary layer adhesion of a fluid flowing, in use, across the surface layer, the surface layer comprising a fluid engagement layer with one or more apertures therethrough and one or more actuators configured or arranged to selectively generate, in use, local turbulence above and/or adjacent the, one, some or each aperture, wherein at least a portion of the fluid engagement layer about or bounding the, one, some or each aperture is flexible and where the surface layer further comprises a means for selectively altering the size and/or shape of the, one, some or each aperture.
11 An apparatus for selectively altering boundary layer adhesion, the apparatus comprising a controller and a surface layer according to any preceding claim.
12. Apparatus according to claim 11, wherein the controller is configured to control individual actuation of one or more of the actuators, for example to control individual actuation of each of the actuators.
13. Apparatus according to claim 12, wherein the controller is configured to control the actuators via multiplexing.
14. Apparatus according to any of claims 10 to 13 when dependent on any of claims 4 to 6, wherein the controller is configured to receive, in use, sensor data from the or each sensor.
15. Apparatus according to claim 14, wherein the apparatus comprises plural sensors and the controller is configured to receive, in use, sensor data from an individual sensor of the plural sensors.
16. Apparatus according to claim 15, wherein the controller is configured to receive, in use, sensor data from an individual sensor of the plural sensors via multiplexing.
17. Apparatus according to any of claims 14 to 16, wherein the controller is configured to control the actuators, in use, based on received sensor data from the or each sensor.
18. Apparatus according to any of claims 14 to 17, wherein the controller is configured to determine one or more of a direction, a velocity, a mass flow rate, a pressure and a Tollmien-Schlichting wave magnitude (e.g. frequency and/or amplitude) of a flow of fluid across the surface layer, in use, based on sensor data received from the or each sensor.
19. Apparatus according to any of claims 14 to 18, wherein the controller is configured to determine the magnitude of a torque exerted on the surface layer and/or on a structure to which the surface layer is attached, due to the flow of a fluid across the surface layer, in use, based on sensor data received from the or each sensor.
20. Apparatus according to any of claims 11 to 19 when dependent on claim 8, wherein the controller is configured to control, in use, the actuator of a surface element to move at least a portion of the fluid engagement layer toward and away from the base.
21 Apparatus according to claim 20, wherein the controller is configured to control the actuator of a surface element to move at least a portion of the fluid engagement layer toward and away from the base at a frequency greater than once a second, for example greater than 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500 or 3000 times per second.
22. Apparatus according to any of claims 20 or 21, wherein the controller is configured to control the actuator of a first surface element to move a first portion of the fluid engagement layer toward the base at a first time and to control the actuator of a second surface element to move a second portion of the fluid engagement layer toward the base at a second time, wherein the second time is after the first time.
23. Apparatus according to any of claims 20,21 or 22, wherein the controller is configured to control the actuator of a or the first surface element to move a or the first portion of the fluid engagement layer away from the base at a third time and to control the actuator of a or the second surface element to move a or the second portion of the fluid engagement layer away from the base at a fourth time, wherein the fourth time is after the third time.
24. Apparatus according to claim 22 or 23, wherein the first and second surface elements are adjacent or near one another.
25. A method of selectively altering boundary layer adhesion of a fluid flowing across a surface layer, the method comprising: providing a surface layer comprising a fluid engagement layer and plural surface elements each comprising an actuator; and individually controlling the movement of each actuator to generate or alter the height of a projection from the fluid engagement layer and/or to generate or alter the depth of a recess into the fluid engagement layer.