EP1514086A2

EP1514086A2 - Method and device for determining the aerodynamic wall shear stresses on the surface of a body, around which air flows

Info

Publication number: EP1514086A2
Application number: EP03732239A
Authority: EP
Inventors: Bernd Schulze; Ole Trinks
Original assignee: EADS Deutschland GmbH
Current assignee: Airbus Defence and Space GmbH
Priority date: 2002-06-07
Filing date: 2003-06-03
Publication date: 2005-03-16
Also published as: DE10225616B4; AU2003239764A8; WO2003104769A2; AU2003239764A1; DE10225616A1; WO2003104769A3

Abstract

The invention relates to a method and a device for determining the aerodynamic wall shear stress in a measuring zone (1a) on the surface of a body (K), around which a flow medium flows. A measuring layer (S) is arranged on said surface in the vicinity of the measuring zone (1a), said measuring layer being thermally transparent in a first IR-wavelength range and absorbing heat in a second IR wavelength range, in order to radiometrically measure the temperature (TW) on the surface of the measuring layer (S) and the temperature (Ts) on the surface (F1) that is covered by the measuring layer (S), using an infra-red camera measuring system (2), whereby the measuring layer (S) is in addition sensitive to pressure.

Description

Method and device for determining the aerodynamic wall shear stresses on the surface of a flowed body

The invention relates to a method and a device for determining the aerodynamic wall shear stresses on the surface of a flowed body.

Methods and devices for determining wall shear stresses on the surface of a flowed body are known from the general prior art, in which these are determined optically by means of oil painting methods or PSP layers, the luminophore being “washed out” of the layer during the experiment. The disadvantage of these methods and devices is that these methods only allow results by visualizing the wall shear stresses and are also not continuously applicable, which means that the wind tunnel has to be shut down and the model has to be recoated after the measurement for a specific flow condition and a certain model configuration (angle of attack of the flow around the body, etc.) has been carried out.

From DE 41 34 313 C2 an infrared measuring method for non-contact, non-reactive temperature determination on a surface-side subzone of a body through which heat is flowed by means of heat conduction is known, in which the infrared radiation emanating from the body surface is measured and from this taking into account the emission ratio Surface temperature is determined. The body is spectrally selective in the surface zone, i.e. radiation-absorbing in a first infrared wavelength range and radiation-transparent in a second infrared wavelength range that is delimited therefrom, but radiation-emitting on the inside of the partial zone. The one originating from the outer surface of the subzone

Radiation energy in the first and in the second wavelength range is measured separately. The temperature on the outer surface of the sub-zone is determined according to in accordance with the proportion of energy in the first and the temperature on the inner surface of the partial zone is determined in accordance with the proportion of energy in the second wavelength range. The local heat flow density is determined via the thickness of the surface-forming subzone.

DE 41 34313 C2 discloses an infrared measuring method for contactless, non-reactive temperature determination on a surface-side subzone of a body through which heat flows through heat conduction. The infrared radiation emanating from the body surface is measured and the surface temperature is determined from this, taking into account the emission ratio. The heat-flowed body is spectrally selective in the surface-side subzone in a first infrared wavelength range and is radiation-transparent in a second infrared wavelength range separated therefrom, but is radiation-emitting on the inside of the subzone. The radiation energy emanating from the outer surface of the partial zone is measured separately in the first and in the second wavelength range and the temperature on the outer surface of the partial zone is determined in accordance with the energy component in the first and the temperature on the inner surface of the partial zone in accordance with the energy component in the second wavelength range.

A pressure-sensitive coating is known from WO 96/41142 A1, which is applied to a surface on which a pressure distribution is to be measured.

A measuring device for determining the surface friction on a body that is exposed to a flow is known from US Pat. No. 5,963,310. With this measuring device, two-dimensional wall shear stress vectors can be determined based on a three-dimensional flow.

From US 5438 879 A a method is known with which the surface tension in size and direction can be determined on a surface. A pressure-sensitive liquid crystal layer is used, which with white Light is illuminated. The color of the liquid crystal is determined at a certain angle by means of a video camera. The size and direction of the shear stress is determined from this color information.

DE 35 14 801 C2 discloses a method for determining the wall shear stress on models and other flow-around bodies. A viscous, transparent liquid is applied in a layer to the surface of the body at the measuring point and the change in thickness and / or surface inclination of the layer which occurs as a result of the flow conditions is detected with a light beam. The light beam is directed from the inside of the flow-around body in the direction of the surface of the body and into the layer of viscous liquid, and the reflected light beam is caught inside the flow-around body and then further processed with regard to interference phenomena or impact point.

It is the object of the invention to provide a method and a device for determining wall shear stresses on the surface of a flow-around body, in which or in which the wall shear stress distribution on the surface of a flow-around body is reversibly and areally determined by means of contactless or non-reactive measurement techniques.

In the case of reversible and areal methods and devices, the wall shear stress measurements for various measuring points and measurement series are carried out continuously and across the board. In particular, this means that the wall shear stress measurements are carried out for different measuring points and series of measurements without having to shut down the wind tunnel between individual measuring points in order to carry out applications on the model.

According to the invention, infrared thermography (IRT) and optical pressure measurement technology (PSP) are used simultaneously or quasi-simultaneously in order to determine pressure and wall shear stress distributions across the board on scaled aircraft models in a wind tunnel test. This means that and friction-related quantitative measurement data without instrumentation effort for use cases or projects and used for the further development of aerodynamic calculation methods.

The combination of infrared thermography (IRT) and optical pressure measurement (PSP) methods makes it possible to continuously determine wall shear stresses during the test using the Reynold analogy of similarity. Furthermore, the PSP print image can be corrected with the temperature information (IR camera). According to the invention, a combination of both measurement techniques (IR and PSP) is provided via the Reynold similarity analogy. According to the invention, the wall shear stress is determined indirectly via a heat flow from the model to the fluid and vice versa. Using the inventive measurement method, it is possible to determine the local speed on the model in addition to the local heat flow and to determine the wall shear stress using three-dimensional wind tunnel models using an extended Reynold analogy.

The invention is described below with reference to the attached figures, which show:

Figure 1 is a functional representation of the inventive device and

2 shows a section through a region of the body near the surface with a measuring layer.

According to the invention, physical state variables, in particular the wall shear stress and the heat flow or the temperature distribution on a surface 1 or a zone 1 a of a body K around which flows, are determined. The body K is provided in the area of the measuring zone 1 a with a measuring layer S arranged on the surface 1, which the application according to the invention of both the intended pressure measuring method and the infrared Thermography process allowed. In a uy coordinate system (FIG. 2), in which the coordinate y is transverse to the direction of flow and is perpendicular to the surface 1 and shows u in the direction of flow, there are two reference surfaces F1, F2 spaced apart from one another in the y direction. The first reference surface F1 is identical to the surface of the body itself, while the second reference surface F2 forms the surface of the measuring layer S, in each case in the region of the measuring zone 1a. The first reference surface F1 can also be referred to as the substrate surface and the second reference surface F2 as the wall surface.

According to the invention, an infrared measuring system 2 is used to measure the heat flow into the surface 1 of the body, i.e. to be determined by the measuring layer at a local point in a zone 1a or the distribution of the heat flow over the surface 1 or F1 or F2. The distribution of the heat flow over the surface 1 in zone 1 a is determined by determining the temperature distribution over the first surface or wall surface F1 and the second reference surface or measuring layer surface F2. According to the invention, the heat flow distribution or temperature distributions and surface pressure distribution can take place in one measurement process with the same measurement configuration. The measurement and determination of the values of the heat flow distribution or

Temperature distributions and the surface pressure distribution can be carried out by means of a processor unit 4 and a synchronization unit 6 at the same time, i.e. simultaneously, or quasi-simultaneously or in directly or almost successive iteration steps, the time period for the determination of these values in one must lie within a time frame in which a predetermined accuracy of the determined values is ensured.

The heat flow is determined by means of an infrared measuring system 2. The surface pressure distribution on the surface 1 of the body being flowed around is detected by means of a PSP system 3 or a pressure measuring system. Via a processor unit 4 from the simultaneously determined heat flow or temperature Distribution and surface pressure distribution, the wall shear stress distribution on surface 1 of the flowed body is determined.

The device according to the invention shown in FIG. 1 for determining aerodynamic wall shear stress distribution by determining the heat flow distribution or temperature distribution and surface pressure distribution on the surface 1 of a body with a flow shows the pressure measuring system 3 for determining the surface pressure distribution on the surface 1 of the flowed body and the infrared measuring system (IR system) 2 for determining the heat flow distribution or temperature distribution on the surface 1 of the flowed body. The IR system 2 has a filter device 5 with which the temperature distribution in at least two predetermined, different spectral emission ranges can be detected. Furthermore, FIG. 1 shows the synchronization unit 6 for the temporal control of the data acquisition, to determine the instantaneous distribution of the physical variables to be determined, pressure or temperature, in a predetermined time interval.

The measuring method according to the invention for determining wall shear stresses is a simultaneous, multispectral measuring method which works in the visible and infrared wavelength range. For the determination of the wall shear stresses, the local heat flow on the surface of the flow-around model is used with the aid of a two-channel infrared camera system which is adapted to the absorptive and the transmissive area of layer S, which forms a pressure-sensitive coating. This enables the temperature distributions on and under the layer to be measured and the heat flow to be determined taking into account the layer thickness and thermal conductivity of the material. The local wall shear stress is then determined from the heat flow or the heat transfer coefficient and the local pressure coefficient or the local velocity using the Reynolds analogy. This takes advantage of the fact that the measuring layer S is suitable both for the application of the pressure measuring method and for the application of the IR measuring method for determining the temperature distributions on the surfaces F1, F2 or the corresponding heat flow distribution. According to the invention, the local heat flows in the surface layer are detected by infrared thermography and the corresponding surface pressures by means of the spectroscopic pressure measurement technique using a camera-supported method in the same measurement process (ie with the same measurement arrangement). The two measurement parameters are linked via the so-called Reynold analogy to the aerodynamic wall shear stress.

The measurement parameters are used to calculate the aerodynamic wall shear stresses by means of the processor unit 4

- coating thickness d

Surface temperature T _w

Substrate temperature T _s , ie the temperature at the first reference surface,

Static pressure P,

of the flow model. The relationships to the calculation are shown below:

du xw = μ - (1) dy

with μ as the dynamic viscosity. With q _w as the heat flow at a location on surface 1 or surfaces F1, F2 and λβ as the thermal conductivity of the gas used as the disturbance medium and the gas temperature T:

q _w = - λG ~ (2) dy It follows from (2) and (3)

* " ^« ^ "*" ^~ DT 0 ⁾

The symbol c _PG designates the specific heat capacity of the gas used as the flow medium.

Without friction results in

you _ " ^• <?

1τ τ _x -τ (5)

Here denote:

Pr the Prandtl number, u _δ the speed at the outer edge of the boundary layer, u _w the speed at surface 1 or the second reference surface

F2, - _δ the temperature at the outer edge of the boundary layer,

T _{w is} the temperature at the surface 1 or the second reference surface F2.

With friction taking into account that

u = 0

is follows

you u _f

(7) dT T _e -T _w

and thus

Pr u _s

(8th)

^{"W '~} c _PG ^{q *} (T _e -T _w )

where q _w = ^ (τ _w -T _s ) (9). Taking into account

K- (10)

^~ (τ _e -τ _w )

surrendered

^T rw ">" ^Pr • hh _ww --uu _δδ (11) c ' _P P _G G

Since the measuring method only works with a non-adiabatic model surface (T _w ≠ T _e ), so T _e is not available from the measurement, the heat transfer coefficient is calculated as follows:

The speed at the boundary layer edge is determined from the optical pressure measurement on the model surface and the inflow conditions with an isentropic change of state. Because of dp

(13) dn

is P _w «P, and thus

(15)

1 + M

= M _δ - k -R -T _δ (16)

To link the heat flow and wall shear stress, the so-called Reynolds analog factor s must be taken into account, which, however, strictly speaking only applies to the flat plate or slightly curved surfaces:

Here denote:

s the Reynold analog factor,

U _{g is} the speed at the outer edge of the boundary layer, which over the

Pressure measurement method is determined, h _{w is} the heat transfer coefficient according to equation (12), which is due to the

Infrared measurement is determined. Using the proposed new measuring method, it is now possible to determine the local speed on the model in addition to the local heat flow and to determine the wall shear stresses using 3-dimensional wind tunnel models using an extended Reynold analogy.

A prerequisite for the indirect measurement of the wall shear stress is a heat flow from the model to the fluid or vice versa. With this measuring method, the heat flow is the indicator via which the wall shear stresses are measured indirectly. At adiabatic wall temperatures, the wall shear stresses cannot be determined because there is no heat flow.

The measuring layer S can be formed from different materials. Silicon or polyethylene can be considered as a component.

The measuring layer S is largely transparent in the visual radiation range (0.4 - 0.75 μm) and has radiation properties in the infrared range (0.75 to 14 μm) that are similar to the radiation properties of the pressure-sensitive layer S. It is crucial that the film is thermally transparent in an IR wavelength range and absorbs heat in an IR wavelength range. This property is used to radiometrically measure the temperature on the surface of the film (T _w ) and on the area covered by the film (T _s ). As a result, the infrared measuring method provided according to the invention by means of the system 2 can measure the infrared radiation emanating from the body surface and the surface temperature can be determined from this taking into account the emission ratio. Since the body is spectrally selective in the surface-side sub-zone, ie radiation-absorbing in a first infrared wavelength range and radiation-transparent in a second infrared wavelength range that is delimited therefrom, but radiation-emitting on the inside of the sub-zone that of the outer surface of the Radiation energy emanating from the sub-zone can be measured in the first and in the second wavelength range. The temperature on the outer surface of the partial zone is determined in accordance with the energy component in the first and the temperature on the inner surface of the partial zone is determined in accordance with the energy component in the second wavelength range. Knowing the two

Temperature measurements and the thickness of the surface-forming sub-zone as well as the material properties - here the thermal conductivity λ - can be used to calculate the locally existing heat flow q _w or its distribution.

In an alternative embodiment of the invention, instead of the pressure measurement system 3 and the infrared measurement system 2, only a one-channel infrared camera is used together with a pressure measurement-sensitive layer S, which is locally covered with a maximum radiant IR color. The temperature difference which determines the heat flow and which is determined by means of the processor unit then results from the IR radiation of the almost location-identical limit position of transparent pressure-sensitive layer S and emitting IR color. At least two different filters can be used on the camera, so that the rays in the visual radiation range (0.4 - 0.75 μm), for which the measuring layer S is largely transparent, and the radiation in the infrared range (0.75 up to 14 μm) can be detected with the first filter together with the rays of the pressure-sensitive layer S with the second filter.

In this way it is also possible, together with the pressures from the optical pressure measurement, to check the proposed measurement method using largely conventional means.

Claims

claims

1. Method for determining the wall shear stress on the surface (1) of a flowed body,

characterized ,

that the heat flow into the surface (1) of the flowed body is determined with the aid of infrared thermography methods (IRT, IR system 2);

that with the aid of a method of optical pressure measurement technology (PSP system 3) the surface pressure distribution on the surface (1) of the body around which flow is detected; and

that the wall shear stress distribution on the surface (1) of the flowed-around body is determined by means of a processor unit (4) from the heat flow and surface pressure distribution.

2. The method for determining the wall shear stress on the surface (1) of a flowed body according to claim 1, characterized in that the heat flow distribution and the surface pressure distribution are determined simultaneously or in successive iteration steps.

3. Device for determining the wall shear stress on the surface (1) of a flow-around body, with a PSP system (3) for determining the surface pressure distribution on the surface (1) of the flow-around body and an IR system (2) for determining the temperature distribution on the surface (1) of the flowed body, the IR system (2) using a filter device (5) to detect the temperature distribution in at least two specific, different spectral emission ranges,

d a d u r c h g e k e n n z e i c h n e t that

a synchronization unit (6) for timing the acquisition of the data of the PSP system (3) and the IR system (2) and

- A processor unit (4) for determining the instantaneous distribution of the local velocity of the fluid or wall shear stresses on the surface (1) of the body around which flow

are provided.

4. Device for determining the wall shear stress on the surface

(1) a flow around a body with a PSP system (3) and an IR system

(2) according to claim 2, characterized in that the determination of the surface pressure distribution on the surface (1) of the flowed body with the PSP system (3) and the determination of the temperature distribution on the surface (1) with the IR system ( 2) takes place simultaneously or in successive iteration steps.

5. Method for determining the aerodynamic wall shear stress in a measuring zone (1a) of the surface of a body (K) around which a flow medium flows, on which a measuring layer (S) is arranged in the region of the measuring zone (1a) and which is in a first IR wavelength range is thermally transparent and absorbs heat in a second IR wavelength range in order to use an infrared camera measuring system (2) to measure the temperature (T _w ) on the surface of the measuring layer (S) and the temperature (T _s ) on the surface (F1), the is covered by the measuring layer (S), to measure radiometrically, the measuring layer (S) also being sensitive to pressure, with the steps:

Determining the temperature (T _w ) on the surface of the measuring layer (S) and the temperature (T _s ) on the surface (F1) covered by the measuring layer (S),

Determining the distribution of the heat flow q _w over the measuring zone (1 a) on the basis of the two temperature measurements and the thickness of the measuring zone (1a) and the thermal conductivity λ _{M of} the measuring layer (S),

Determination of the surface pressure distribution and determination of the speed of the flow medium at the edge of the boundary layer using a pressure measurement camera system (3) using the pressure sensitivity of the measurement layer (S),

Determination of the aerodynamic wall shear stresses in the measuring zone (1a) using a calculation method based on Reynold's similarity analogy.