GB2513542A

GB2513542A - Device for improving aerodynamics of vehicles

Info

Publication number: GB2513542A
Application number: GB1302231.4A
Authority: GB
Inventors: Ashwyn Dhir
Original assignee: Swift Group Ltd
Current assignee: Swift Group Ltd
Priority date: 2013-02-08
Filing date: 2013-02-08
Publication date: 2014-11-05
Also published as: GB201302231D0

Abstract

A device to manipulate a fluid flow around a towed vehicle 200. The device comprises a deflecting member 830 and an attachment portion, wherein the attachment portion is configured to attach the deflecting member proximate a top surface 260 and/or the upper rear edge 226 of the towed vehicle. The deflecting member is configured to manipulate a position of a portion of the fluid flow such that the expansion of a wake behind the towed vehicle is controlled. The first being a tear drop shaped aerofoil (figure 3). The second being vanes which energise a boundary layer by the use of vortex creating vanes (figure 6 and 7) and the third being a separation trigger, in the form of a ridge, to separate the flow from the rear of the vehicle (figure 9). The first two are used to keep the flow of air attached to the caravan, while it goes around the top rear corner, the third separates the flow from the caravan at a desirable position, and in a way that reduces the wake and therefore the drag.

Description

DEVICE FOR IMPROVING AERODYNAMICS OF VEHICLES

FIELD OF INVENTION

The invention relates to apparatus for improving the aerodynamics of a vehicle. More particularly, the invention relates to apparatus for improving the aerodynamics of a towed vehicle, such as a caravan.

BACKGROUND OF THE INVENTION

Caravans are not particularly efficient when towed. This is due to their high weight relative to the towing vehicle, rolling resistance from tyres, and their large size and shape. The latter contributes to a high aerodynamic drag, which comprises, skin friction drag and principally the pressure drag. Consequently, the vehicle used to tow the caravan expends a large amount of fuel during towing.

Historically, aerodynamic studies of flow around caravans have been considered unnecessary since it is generally accepted that the wake from the towing vehicle encompasses the flow around the caravan. Therefore, it is considered more effective to attach components upstream of the rear of the towing vehicle, such as deflectors or diffusers to control the wake, rather than attempt to optimise the shape of the caravan.

An object of the present invention is to provide a caravan which overcomes one of the above or other problems. More specifically, an object of the present invention is to provide a towed vehicle which is more efficient to tow.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a device to manipulate a fluid flow around a towed vehicle, the device comprising: a deflecting member and an attachment portion, wherein the attachment portion is configured to attach the deflector member proximate a top and for upper rear edge of the towed vehicle, and the deflecting member is configured to manipulate a position of a portion of the fluid flow travelling on the top surface and f or upper rear edge of the vehicle such that the expansion of a wake behind the towed vehicle is controlled.

Preferably, the deflecting member comprises a first deflecting member which is configured to manipulate a portion of flow that travels over the top surface of the vehicle, such that it is forced to flow substantially along and I or at least partly towards the rear face of the towed vehicle. More preferably, the first deflecting member is configured to manipulate a portion of flow that travels over the top surface of the vehicle, such that it is forced to flow first at least partially around the upper rear edge of the towed vehicle. Preferably, the first deflecting member is configured to turn a portion of the flow travelling over the top of the vehicle such that it travels at least partially around the upper rear edge of the towed vehicle and downwards towards the ground.

Preferably, the towed vehicle is a caravan or a trailer.

By undertaking numerical modelling of the flow around a caravan the inventors have found that, contrary to conventional opinion, and at normal towing velocities, the wake of the flow from the tow vehicle is in general resolved before the flow reaches the caravan.

Accordingly, and contrary to the current technical prejudice, it has been found that substantial aerodynamic improvements can be made by optimising the shape of the caravan.

More particularly, it has been found that on a conventional caravan, the flow is in general attached to the top of the caravan. As this attached flow travels over the upper rear corner of the caravan the flow separates, such that on the rear face it is completely separated.

This is because the boundary layer detaches from the rear face due to an adverse pressure gradient, accordingly high velocity flow on the top of the caravan is unable to be turned around the sharp transition between the top and rear surfaces. Separation on the rear face is undesirable since it is responsible for a large wake, which in turn causes high pressure drag.

Accordingly, the invention solves the first objective by manipulating the flow around the upper rear edge of the caravan to thereby reduce the height of the wake behind the towed vehicle. The inventors have found that when using the device the typical reduction in drag for a conventional caravan is 2%.

Preferably, the flow is manipulated such that the position of flow separation proximate the upper rear edge of the towed vehicle occurs further downstream, wherein separation is defined as occurring when flow detaches from a surface of the towed vehicle due to an adverse pressure gradient. Preferably, the flow is manipulated such that the separation proximate the upper rear edge of the towed vehicle is delayed and for reduced.

Preferably, the flow is manipulated such that the volume of the wake is reduced behind the towed vehicle. Preferably, the flow is manipulated such that the length of the wake is reduced behind the towed vehicle. Preferably, the flow is manipulated such that the height of the wake is reduced behind the towed vehicle. The height is defined as extending normally from the ground to the extremity of the wake for a particular cross-sectional point of the wake, for instance, a point defined by the maximum height or of the wake. The length is defined as extending normally from the rear face of the towed vehicle to the extremity of the wake for a particular cross-sectional point of wake, for instance a point defined by the maximum length of the wake.

Preferably, the first deflecting member is arranged to transition a portion of the flow travelling along the top surface towards the rear surface through an angle of about 90° -450 relative to the free stream flow direction.

Preferably, the attachment portion is configured to attach the device to one or more of the top, rear and sides of the vehicle. Optionally, the attachment portion comprises a removable attachment comprising fixing means such as bolts. Optionally, the attachment portion is formed integrally with part of the vehicle, such as the body work or other structural member.

Preferably, the deflecting member is arranged to extend around at least part of the upper rear edge from a leading edge to a trailing edge.

More preferably, the first deflecting member comprises a control surface which is optionally shaped in the form of an aerofoil. wherein the aerofoil shape comprises a tear drop shape having a rounded leading edge, followed by a sharp trailing edge. Optionally, the control surface has a NACA defined cross section, such as NACA WXYZ, wherein the YZ parameter denotes the thickness to chord percentage, which may be about 0 -30%, and the WX parameter denotes the camber percentage and may be between about 0 -40%. Preferably, the aerofoil is cambered, wherein the camber line is curved such that it extends around at least part of the upper rear edge of the towed vehicle. Preferably, the curvature of an outer surface of the control surface is less than that at the adjacent point an inner surface of the control surface, more preferably, it is at least 10% less.

Alternatively, the control surface is shaped in the form of a flat plate. Preferably, the flat plate is curved such that it extends around at least part of the upper rear edge of the towed vehicle.

Preferably, the first control surface is inclined with respect to the free stream flow direction. Preferably, the inclination comprises and angle of attack with in the range of 0° to 60°. Preferably, the control surface has a chord line of about 5cm -70cm, more preferably, it is about 10cm.

Optionally, the control surface spans the width of the towed vehicle. Preferably, the control surface is substantially straight in the width direction.

Preferably, the first deflecting member is configured such that the leading edge of the control surface is a vertical distance of between 2 -30cm from an adjacent portion of the top surface of the towed vehicle, more preferably it is about 10cm. Preferably, the distance between the leading edge of the control surface and the adjacent portion of the top surface defines an inlet, the inlet being arranged to ingest a portion of flow that travels over the top of the towed vehicle.

Preferably, the first deflecting member is configured such that the trailing edge of the control surface is a horizontal distance of between 2 -30cm from an adjacent portion of the rear surface of the towed vehicle, more preferably it is about 10cm. Preferably, the distance between the trailing edge of the control surface and the adjacent portion of the rear surface defines an outlet, the outlet being arranged to outlet the flow ingested by the inlet.

Preferably, the outlet and inlet are arranged such that in angle of the outlet flow is about 90° _450 of that of in inlet flow, when measured in relation to the free stream direction.

Optionally, the cross sectional area of the outlet is substantially the same as the inlet.

Alternatively, the cross sectional area of the outlet is about 5 -20% greater than that of the inlet.

Preferably, the first deflecting member is configured such that, when positioned on the towed vehicle, a lower surface of the control surface and surface of the towed vehicle, which extends around the upper rear edge, defines duct for transmission of flow, which extends from the inlet to the outlet.

Preferably, the curvature of an inner surface of the control surface is the same as or less than that at the adjacent point on the rear edge of the vehicle, more preferably, it is at least 10% less.

Preferably, the first deflecting member is arranged to smoothly transition flow travelling proximate the top of the vehicle, to the rear face of the vehicle, such that a portion of the flow re-attaches to the rear of the vehicle.

Preferably, the first deflecting member comprises a plurality of walls which are arranged along the width of the deflecting member and are operable to reduce cross flow in a width direction.

Alternatively or additionally the deflecting member comprises a second deflecting member in the form of one or more flow boundary layer re-energising members which are arranged to extend into a boundary layer of the flow on the top surface of the vehicle, wherein the or each member is operable to re-energise the flow in the boundary layer, wherein the re-energised boundary layer is preferably thinner. Advantageously, the members re-energise the flow in the boundary layer such that subsequent separation on the top surface preferably proximate the upper rear edge occurs further down stream. The re-energising members preferably interact with the flow such that separation is delayed, which reduces the size of the wake and hence reduces the pressure drag.

Preferably, the flow re-energising members are operable to increase the energy in the boundary layer by at least 10%, preferably they are operably to reduce the thickness of the boundary layer by at least 10%.

Preferably, the or each re-energising member is positioned on the top surface, and preferably adjacent and I or upstream the upper rear edge. Preferably, the or each re-energising member is upstream of the upper rear edge by about 20-100cm. Preferably, the or each re-energising member extends in a horizontal direction from a first end to a second end, wherein the first end is positioned upstream in respect of the second end, and preferably the second end is adjacent the rear edge.

Preferably, the or each re-energising member comprises vortex generation means.

Preferably, the vortex generating means comprise a vane. Preferably, the vane is arranged to be substantially vertically aligned. Preferably, the vane is angled off-axis to the general direction of flow on the top of the vehicle, preferably by an angle of attack of about 2° -30°.

Preferably, the vanes are arranged in pairs, wherein for each pair the vanes are angled such that they are opposed to each other.

Preferably, the vane comprises a tip which is arranged to extend the furthest vertical distance from the top surface of the towed vehicle. Preferably, the tip extends from the top surface of the towed vehicle such that it is proximate the extremity of the boundary layer. More preferably, the tip is about 2cm -20cm from the top surface of the vehicle, more preferably it is about 10cm.

Preferably, the vanes are shaped in the form of triangles, which are preferably arranged with an acute angle at a first end, and preferably with an obtuse, preferably substantially 90° angle at a second end and preferably an acute angle at a tip. Preferably, the first and second end are separated by a horizontal distance of about 2cm -20cm, more preferably it is about 10cm.

Preferably, the re-energising members comprise an attachment portion configured to attach the re-energising members to the vehicle. Preferably, the attachment portion comprises a base of the or each re-energising member. Optionally, the attachment portion is removably attached to the top of the vehicle by fixing means, which may comprise bolts or other suitable means. Optionally, the attachment potion is formed integrally with the top surface of the vehicle.

Preferably, the first deflecting member is arranged such that it manipulates a portion of flow which is re-energised by the or each re-energising member of the second deflecting member. Advantageously, the re-energised flow is redirected by the first deflecting member around the upper rear edge and preferably onto the rear surface of the caravan. Since the re-energised flow is less likely to separate, the re-directed flow remains attached around the upper rear edge for longer than if it were not re-energised, to thereby reduce the wake behind the caravan.

Alternatively or additionally the deflecting member further comprises a third deflecting member in the form of one or more a separation triggering means, preferably in the form of a ridge. Optionally, the or each ridge is positioned on the upper surface of the first deflector portion. Optionally, the or each ridge is positioned in the proximity of the upper rear edge of the towed vehicle.

Preferably, the ridge is configured to trigger separation of flow travelling over it, preferably such that flow is prevented from accelerating around the curved surface of the upper edge of the towed vehicle / upper surface of the first deflector portion. It will be appreciated that flow that accelerates around the upper rear edge causes a low pressure region. Accordingly, by triggering separation at a particular point, this effect is mininiised.

Preferably, the ridge is configured to be positioned on the curved surface of the upper edge of the towed vehicle. Preferably the ridge comprises a curved upper surface and a substantially flat near face.

Preferably, the device is formed from one or more of the following materials: ABS, GRP, Nylon, ASA or similar.

Preferably, the device is operable at conventional vehicle velocities, such as 30 -120 KM/H.

According to a second aspect of the invention there is provided a towed vehicle comprising the device of one or more of the first, second and third deflecting members according to the first aspect of the invention.

According to a third aspect of the present invention there is provided a method of manipulating flow around a towed vehicle, the method comprising: interacting a deflecting member of a device with a portion of flow travelling over the top surface and / or proximate the upper rear edge of the towed vehicle such that the expansion of the wake behind the towed vehicle is controlled.

Preferably, the method includes a step of using a first deflecting member to turn a portion of the flow travelling over a top of the vehicle such that it travels at least partially around the upper rear edge of the towed vehicle and downwards towards the ground.

Optionally, the method includes a step of using a second deflecting member in the form of a flow re-energising member to re-energising the flow in a boundary layer on the top surface of the vehicle by means of flow re-energising members.

Optionally, the method includes a step of using a third deflecting member in the form of a separation triggering means, which is connected to a curved upper rear surface of the towed vehicle, to trigger separation of flow travelling over it such that a portion of flow is prevented from accelerating around the curved surface of the upper edge.

All of the features described herein may be combined with any of the above aspects, in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figure 1 shows a perspective view of a prior art caravan; Figure 2 shows a side cross-sectional view of fluid flow around a prior art caravan at a position central to the width of the caravan; Figure 3 shows a side view of a device to manipulate flow around a caravan according to an aspect of the invention; Figure 4 shows a front view of the device of figure 3; Figure 5 shows a side view of flow around the device of figure 3; Figure 6 shows a perspective view of a further device to manipulate flow around a caravan according to an aspect of the invention; Figure 7 shows a top view of the device of figure 6; Figure 8 shows a side view of flow around the device of figure 6; Figure 9 shows a side view of a further device to manipulate flow around a caravan according to an aspect of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Figure 1 shows a perspective view of a caravan 200. Herein, the caravan 200 is defined as having a front face 210 and rear face 220, both of which are generally arranged to have a plane which is substantially perpendicular to the direction of motion of the caravan 200 (although in this embodiment the front face is shown curved to improve the aerodynamics of the caravan) and extend substantially upwardly from the ground (not shown), wherein the front face 210 is positioned upstream with respect to the rear face 220 in relation to the flow over the caravan 200 when being towed.

The caravan 200 further comprises a left side face 230 (not shown) and right side face 240, both of which extend substantially upwards from the ground, and both of which extend from the front face 210 to the rear face 220.

The caravan 200 further comprises a bottom face 250 (not shown) and top face 260, both of which are substantially aligned to the ground, and both of which extend from the front face 210 to the rear face 220. The bottom face 250 is closest to the ground, and the top face 260 is positioned furthest from the ground.

In more detail, on one side of the front face 210 there is a left front edge 212, and on the other side there is a right front edge 214, both of which extend from a bottom to a top of the caravan 200. On one side of the rear face 220 there is a left rear edge 222 (not shown), and on the other side there is a right rear edge 224, both of which extend from a bottom to a top of the caravan 200. The left side face 230 and right side face 240 are positioned symmetrically about a centrally aligned plane 270 (not shown) of the caravan 200. The left side face 230 extends from the left front edge 212, to the left rear edge 222, and the right side face 240 extends from the right front edge 214, to right rear edge 224.

On the top of the front face 210 there is a top front edge 216, and on the bottom there is a bottom front edge 218, both of which extend in a direction which is substantially aligned to the ground and between the two side faces 230, 240. On the top of the rear face 220 there is a top rear edge 226, and on the bottom there is a bottom rear edge 228 (not shown), both of which extend in a direction which is substantially aligned to the ground and between the two side faces 230, 240. Hence the top face 260 extends from the top front edge 216 to the top rear edge 226, and the bottom face 250 extends from the bottom front edge 218 to the bottom rear edge 228.

The caravan 200 further comprises wheels 280. The wheels 280 are positioned between the bottom surface 250 and the ground and are located approximately mid-way between the front face 210 and rear face 220. In more detail the wheels 280 comprise a first wheel 282 (not shown) poisoned proximate the left side face 230, and a second wheel 284 positioned proximate the right side face 240. The wheels 282, 284 are connected by means of an axle 286 (not shown) and are arranged symmetrically about the central plane 270. The axle 286 protrudes from the bottom of the caravan 200 such that the wheels 280 extend partially into a wheel arch 288 of the bottom surface 250.

The caravan 200 yet further comprises a drawbar 290 for receiving a tow bar (not shown) of a towing vehicle, which may comprise a car or other suitable vehicle. The drawbar 290 is arranged to extend in the direction of towing from proximate the bottom front edge 218.

The caravan 200 yet further comprises a chassis 294 (not shown). The chassis 294 extends proximate the bottom face 250, and forms the main structural component of the caravan 294, to provide a rigid support for the wheels 280, drawbar 290, body panels which comprise the top, front, rear and side faces, and the framework 296 (not shown) that supports these body panels.

Figure 2 shows the flow around the bottom face 250, top face 260, and the rear face 220 of the caravan 200 of figure 1 when viewed from a direction normally into the side face 240 of the caravan. By undertaking numerical modelling of the flow around the caravan the inventors have found that, contrary to conventional opinion, and at normal towing velocities, the wake of the flow from the tow vehicle is in general resolved before the flow reaches the front of the caravan. As shown in figure 2 the resultant flow is substantially attached when traveling over the top face 260 of the caravan. However, due to a combination of the relatively high flow velocity over the top of the caravan, and the more or less perpendicular arrangement of the top and rear face, after the flow travels over the rear edge 226 it fully separates from the rear face 220. This contributes to a substantial wake behind the caravan, and therefore a high drag of the caravan.

Figures 3 -7 show various examples of a devices 800A, B, C to manipulate flow on the top face 260 and upper rear edge 226 of the caravan. It will be appreciated that these examples may be combined to achieve a synergistic effect or used on their own, as will be described in more detail below.

Initially considering the device 800A, as shown in figure 3, the device comprises a deflecting member 830 which is attached to the caravan by means of an attachment portion (not shown) 832 (not shown), which comprises a portion of the deflecting member 830 suitable for attaching the deflecting member 830 to the caravan 200. For instance, the attachment portion 832 may comprise a plurality of holes through the deflecting member 830 for receiving fixing means (not shown), such as bolts, to attach the deflecting member 830 to the caravan.

The attachment portion 832 may be configured to attach the deflecting member 830 to one or more of the top face 260, rear face 220 and side faces 240, 230 of the caravan. The attachment portion 832 may be removably attached to the caravan by the fixing means.

However, in other embodiments the attachment portion is formed integrally with part of the caravan, such as the body work or other structural member.

In this example the deflecting member 830 is shaped in the form of an aerofoil, wherein the aerofoil shape comprises a tear drop shape having a rounded leading edge 833, followed by a sharp trailing edge 834. The aerofoil is cambered, wherein the camber line is curved such that it extends around the upper rear edge 226 of the towed vehicle. Although, it will be appreciated that the deflecting member may be shaped in other forms, such as a flat plate, which extends around the upper rear edge 226.

The deflecting member 830 is positioned such that the leading edge 833 is closest to the top face 260 and the trailing edge 834 is closest to the rear face 220. The distance between the leading edge 833 and top face 220 defines and inlet 836, whereas the distance between the trailing edge 834 and rear face 220 defines and outlet 838. The outlet 838, is operable to expel the flow ingested by the inlet 836.

In this example the cord line is 10 -70cm and the distance between the leading edge 833 and top face 220 is 1 -10cm, and the distance between the trailing edge 834 and rear face 220 is 1 -10cm. The maximum thickness at the leading edge is about 5cm. In this embodiment the width of the deflecting member 830 is substantially the same as the width of the caravan. Accordingly, in this embodiment, the cross sectional area of the outlet 838 slightly greater than that of the inlet 834, however it will be appreciated that in other embodiments one of the outlets or inlets may have a greater size than the other.

The deflecting member 830 comprises an upper surface 840, and a lower surface 842, the lower surface being adjacent the surface of the upper rear edge 226. As shown in figures, the curvature of the upper surface 840 of the deflecting member 830 generally less than that at the lower surface 842. In a preferred embodiment, it is at least 10% less.

In the example embodiment the upper rear face 226 of the caravan has a radius of curvature of above 20cm. However, it will be appreciated that the degree of curvature will vary, and therefore that the exact shape of the deflecting member will be optimised for the particular caravan.

Figure 4 shows a front view of the deflecting member 830 on the top face 260 of the caravan. It can be seen that a plurality of walls 844 are arranged to extend between the lower surface 842 and a top surface 260 of the caravan. The walls comprises slats which are arranged inline with the length of the caravan. The walls 844 provide connection points for the attachment portion 832. In addition the walls 844 act to maintain the course of flow in a direction which his substantially perpendicular to the width of the caravan 200. Alternatively put the walls 844 act to prevent cross flow.

Figure 5 shows the flow around the deflecting member 830. Notably, a portion of the flow that travels along the top face is ingested by the inlet 834. This flow then travels through a duct defied by the lower surface 842 of the deflector portion and curved upper rear edge 226.

As can be seen, the flow is turned through about 30 -600 such that the flow at the outlet comprises a vertical component of direction and is partially directed towards the ground. In this way the expansion of the wake 30 behind the caravan is controlled, such that the point of separation 32 (as shown in both figures 2 and 6) occurs further around the curved surface of the upper rear edge. Alternatively put, the point of separation 32 is delayed. As when comparing figures 2 and 6, it can be seen that the device has the effect of the reducing the height of the wake 32.

Referring now to figure 6, the device 800B comprises deflecting members in the form of flow re-energising members. The re-energising members comprise vanes 810. It will be appreciated that four vanes 810 are shown in the exemplary embodiment, and that in other embodiments other quantities of vanes will be used.

In this embodiment the device 800B is shown proximate the rear edge 226, although in other embodiments the devices 800a may be positioned at any point on the top face 260.

The vanes 810 comprise upstanding substantially planar portions that extend into the boundary layer of the flow on the top face 260. In this embodiment the vanes are positioned such that they are substantially perpendicular to the top face 260, however it will be appreciated that they may also be angled relative to the top face 260.

The vanes 810 extend between a first end 812 and second end 814, with the first end 812 being positioned upstream of the second end 814 with respect to the global flow direction.

In this embodiment the second end 814 is shown to abut the rear edge 226, however it will be appreciated that the second end may be spaced away from the rear edge 226. In an exemplary embodiment it spaced away by a horizontal distance of about 10cm -20cm.

In this embodiment the vanes 810 are triangular in shape, however it will be appreciated that in other embodiments they may be other suitable shapes, such as a quadrilaterals with or without a parallelogram configuration.

Referring back to the present embodiment the triangular vanes 810 of have an acute angle at the first end 812, and with an obtuse, substantially 900 angle at the second end 814.

A tip 816, which is the highest vertical point of the vane, comprises an acute angle.

The tip 816 may extend to a vertical distance of 0.5cm -20cm from the top surface 260 of the vehicle to the tip 816. In this example the distance is about 10cm. The first end 812 and second 814 end may be separated by a horizontal distance of 2cm -20cm. In this example it is about 10cm.

The devices 800B further comprise an attachment portion 820, which in this embodiment is the flat base that extends between the first end 812 and second end 814. In this embodiment the attachment portion 820 is removably attachable to the top face 260. This may be achieved by fixing means (not shown), which comprise bolts that extend though part of the attachment portion 820 and into the top face 260, alternatively other suitable fixing means may be used. In another embodiment the attachment portion 820 is formed integrally with the top face 260.

Some of the vanes 810 may be aligned in the flow 811 direction from first 812 to second end 8154 and / or be angled, as the various embodiments of figure 4 show. In figure 4a some of the vanes 810a are angled with incidence a to the flow within the range of 2 to 40°, and others BlOb are aligned to the flow. In figure 4b the vanes BlOc, BlOd are arranged in pairs, with both being angled in the same direction an having an incidence a to the flow within the range of 2 to 20°. In figure 4c the vanes 810e, 810f are arranged in pairs, with the angles being opposed to each other, hence vane SlOe has an incidence a to the flow within the range of 2 to 40°, and vane 810f has an incidence a to the flow within the range of -2 to -40° By undertaking numerical studies of flow around the caravan 200 it has been found that the boundary layer on the top face 260 is approximately 0.5cm -5cm for conventional towing velocities of around 30-120 Km/h.

Accordingly, as shown in figure 8, by arranging the vanes 810 to extend into the boundary layer 818, the subsequent flow over them causes the generation of vortices 822 from their trailing edge. The vortices entrain high energy free stream flow into the boundary layer and increase mixing.

Advantageously, energy present within the boundary layer 818 is increased. In this way the position of separation on the curved upper rear edge of the caravan is delayed, such that expansion of the wake is controlled.

The device 800B may operate in conjunction with the device 800B, such that the device 800B re-energise the boundary layer of the flow entering or flow which has entered the inlet 836 of the device 800A. Advantageously, the re-energised flow is redirected by the deflecting member 830 onto the rear face 220. Since the re-energised flow is less likely to separate, the re-directed flow remains attached to the rear face 220 for longer, to thereby reduce the wake.

The devices 800A, B, C may be formed from one or more of the following materials: ABS, GFRP, Nylon, ASA or similar.

Referring now to figure 9, the device 800C comprises a deflecting member in the form separation triggering means 860. The separation triggering means comprise a ridge which is positioned on part of the curved surface of the upper edge 226. The ridge extends outwardly from the edge 226, and comprises a curved upper surface 862 and a substantially flat rear face 864, although it will be appreciated that other configurations of ridge may be used to achieve the same technical effect as discussed below.

The rear face of the ridge is between 2 -5cm long and the tip 866 is between 1 -4 cm away from the adjacent point of the edge 226. As shown in figure 9 flow which travels over the ridge is caused to separate from the surface of the caravan. In this way flow is prevented from accelerating from around the curved edge 226. It is desirable to prevent such acceleration, since the region of acceleration causes low pressure which results in increased drag.

Accordingly, the ridge provides a point of separation though allowing control of the wake behind the caravan.

The device 800C may be used in conjunction with either or both of the devices 800A, B. For instance, it may be positioned on the upper surface 840 of the device 800A.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS1. A device to manipulate a fluid flow around a towed vehicle, the device comprising: a deflecting member and an attachment portion, wherein the attachment portion is configured to attach the deflecting member proximate a top surface and / or upper rear edge of the towed vehicle, and the deflecting member is configured to manipulate a position of a portion of the fluid flow travelling on the top surface and I or upper rear edge of the vehicle such that the expansion of a wake behind the towed vehicle is controlled.
2. The device as claimed in claim 1, wherein the deflecting member comprises a first deflecting member which is configured to manipulate a portion of flow that travels over the top surface of the vehicle, such that it is forced to flow substantially along and I or at least partly towards the rear face of the towed vehicle.czj.. 20
3. The device as claimed in claim 2, wherein the first deflecting member is configured to turn a portion of the flow travelling over the top of the vehicle such that it travels at least partially around the upper rear edge of the towed vehicle and downwards towards the ground.

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4. . . . O 4. The device as claimed in any preceding claim, wherein the towed vehicle is a caravan or a trailer.

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5. The device as claimed in claims 2 -4, wherein the flow is manipulated such that the volume of the wake is reduced behind the towed vehicle.
6. The device as claimed in claims 2 -5, wherein the flow is manipulated such that the length of the wake is reduced behind the towed vehicle.
7. The device as claimed in claims 2 -6, wherein the flow is manipulated such that the height of the wake is reduced behind the towed vehicle.
8. The device as claimed in claims 2 -7, wherein the first deflecting member is arranged to transition a portion of the flow travelling along the top surface towards the rear surface through an angle of about 900_450 relative to the free stream flow direction.
9. The device as claimed in claims 2 -8, wherein the attachment portion is configured to attach the device to one or more of the top, rear and sides of the vehicle.
10. The device as claimed in claim 9, wherein the attachment portion comprises a removable attachment comprising fixing means such as bolts or is formed integrally with part of the vehicle.
11. The device as claimed in claims 2-10, wherein the deflecting member is arranged to extend around at least part of the upper rear edge from a leading edge to a trailing edge.
12. The device as claimed in claim 11, wherein the first deflecting member comprises a control surface which is shaped in the form of an aerofoil, wherein the aerofoil shape comprises a tear drop shape having a rounded leading edge, followed by a sharp trailing edge.
13. The device as claimed in claim 12, wherein the control surface has a NACA defined cross section, such as NACA WXYZ, wherein the YZ parameter denotes the thickness to chord percentage, which may be about 0 -30%. and the WX parameter denotes the camber percentage and may be between about 0-40%.
14 The device as claimed in claim 13, wherein the aerofoil is cambered, wherein the camber line is curved such that it extends around at least part of the upper rear edge of the towed vehicle.
15. The device as claimed in claims 11 -14, wherein the curvature of an outer surface of the control surface is less than that at the adjacent point an inner surface of the control surface.
16. The device as claimed in claims 2-11, wherein the control surface is shaped in the formofaplate.
17. The device as claimed in claim 16, wherein the flat plate is curved such that it extends around at least part of the upper rear edge of the towed vehicle.
18. The device as claimed in claims 11 -17, wherein the first control surface is inclined with respect to the free stream flow direction.
19. The device as claimed in claim 18, wherein the inclination comprises an angle of attack within the range of 00 to 60°.
20. The device as claimed in claims 11 -19, wherein the control surface has a chord line of about 5cm -70cm, or about 10cm.
21. The device as claimed in claims 2 -20, wherein the control surface spans the width of the towed vehicle, the control surface is substantially straight in the width direction.
22. The device as claimed in claims 2 -21, wherein the first deflecting member is configured such that the leading edge of the first control surface is a vertical distance of between 2 -30cm from an adjacent portion of the top surface of the towed vehicle, or about 10cm.
23. The device as claimed in claims 2 -22, wherein the first deflecting member is configured such that the trailing edge of the control surface is a horizontal distance of between 2-30cm from an adjacent portion of the rear surface of the towed vehicle, or about 10cm.
24. The device as claimed in claims 2 -23, wherein the distance between the leading edge of the control surface and the adjacent portion of the top surface defines an inlet, the inlet being arranged to ingest a portion of flow that travels over the top of the towed vehicle.
25. The device as claimed in claims 2-24, wherein the distance between the trailing edge of the control surface and the adjacent portion of the rear surface defines an outlet, the outlet being arranged to outlet the flow ingested by the inlet.
26. The device as claimed in claim 25, wherein the outlet and inlet are arranged such that in angle of the outlet flow is about 900 -450 of that of in inlet flow, when measured in relation to the free stream direction.
27. The device as claimed in claim 25-26, wherein the cross sectional area of the outlet is substantially the same as the inlet or the cross sectional area of the outlet is about 5-20% greater than that of the inlet.
28. The device as claimed in claims 25 -27, wherein the first deflecting member is configured such that, when positioned on the towed vehicle, a lower surface of the control surface and surface of the towed vehicle, which extends around the upper rear edge, defines duct for transmission of flow, which extends from the inlet to the outlet.
29. The device as claimed in claim 28, wherein the first deflecting member is configured to smoothly transition flow travelling proximate the top of the vehicle, to the rear face of the vehicle, such that a portion of the flow re-attaches to the rear of the vehicle.
30. The device as claimed in claims 2-28, wherein the first deflecting member comprises a plurality of walls which are arranged along the width of the deflecting member and are O operable to reduce cross flow in a width direction.
31. The device as claimed in any preceding claim, wherein the deflecting member 0 comprises a second deflecting member in the form of one or more flow boundary layer re-energising members which are configured to extend into a boundary layer of the flow on the top surface of the vehicle, wherein the or each re-energising member is operable to re-energise the flow in the boundary layer, wherein the re-energised boundary layer is of reduced thickness such that separation of flow from the vehicle occurs further downstream to thereby reduce the size of the wake.
32. The device as claimed in claim 31, wherein the flow re-energising members are operable to increase the energy in the boundary layer by at least 10% and I or they are operable to reduce the thickness of the boundary layer by at least 10%.
33. The device as claimed in claims 31 -32, wherein the or each re-energising member(s) are upstream of the upper rear edge by about 20-100cm, or are adjacent to the upper rear edge.
34. The device as claimed in claim 33, wherein the or each re-energising member extends in a horizontal direction from a first end to a second end, wherein the first end is operable to be upstream in respect of the second end.
35. The device as claimed in claims 31 -34, wherein the or each re-energising member comprises vortex generation means in the form of a vane which is configured to be substantially vertically aligned.
36. The device as claimed in claim 35, wherein the vane is angled off-axis to the general direction of flow on the top of the vehicle, by an angle of attack of about 2° -300.
37. The device as claimed in claims 35 -36, wherein the vane comprises a tip which is configured to extend the furthest vertical distance from the top surface of the towed vehicle such that it is proximate the extremity of the boundary layer.
38. The device as claimed in claim 37, wherein the tip is about 2cm -20cm from the top surface of the vehicle, oris about 10cm.
39. The device as claimed in claims 2 -30 and 31 -38, wherein the first deflecting member is arranged such that it manipulates a portion of flow which is re-energised by the or each re-energising member of the second deflecting member.
40. The device as claimed in any preceding claim, wherein the deflecting member further comprises a third deflecting member in the form of one or more a separation triggering means, in the form of a ridge.
41. The device as claimed in claim 40 and any of claims 2-30, wherein the or each ridge is positionable on the upper surface of the first deflector portion.
42. The device as claimed in claim 40, wherein the or each ridge is positionable in the proximity of the upper rear edge of the towed vehicle.
43. The device as claimed in claims 40 -42, wherein the ridge is configured to trigger separation of flow travelling over it, such that flow is prevented from accelerating around the 0 curved surface of the upper edge of the towed vehicle / upper surface of the first deflector portion.
44. The device as claimed in claims 40-43, wherein the ridge comprises a curved upper surface and a substantially flat rear face.
45. The device as claimed in any preceding claim, wherein the device is configured to manipulate the flow substantially at conventional vehicle velocities, of about 30-120 KM/H.
46. A towed vehicle comprising the device consisting of one or more of the deflecting members as claimed in any preceding claim.
47. A method of manipulating flow around a towed vehicle, the method comprising: interacting a deflecting member of a device with a portion of flow travelling over the top surface and / or proximate the upper rear edge of the towed vehicle such that the expansion of the wake behind the towed vehicle is controlled.
48. The method as claimed in claim 47, wherein the deflector member comprises a first deflecting member which is configured to turn a portion of the flow travelling over a top of the vehicle such that it travels at least partially around the upper rear edge of the towed vehicle and downwards towards the ground.
49. The method as claimed in claims 47 -48, wherein the deflector member further comprises a second deflecting member in the form of a flow re-energising member to re-energise the flow in a boundary layer on the top surface of the vehicle.
50. The method as claimed in claim 48, wherein the deflector member further comprises a third deflecting member which is in the form of a separation triggering means, which is connected to a curved upper rear surface of the towed vehicle, to trigger separation of flow travelling over it such that a portion of flow is prevented from accelerating around the curved surface of the upper edge. (4N