DE102007027773A1 - Measuring device for wall shear stress of flow guide surface pressurized by flow of fluid, has deformation element deformed by fluid flow, which is formed as flexible solid body and is arranged in area of flow guide surface - Google Patents

Abstract

The invention relates to a measuring device (1) for determining a wall shear stress of a flow guide surface (4) acted upon by a flow (2) of a fluid.
According to the invention, the flow guide surface (4) has a deformation layer (6) which is elastic and can be deformed elastically in the region of an outer surface (3) by application of pressure to the flow (2) and the wall shear stress caused thereby. In the area of the outer surface (3), marking elements (13) are provided which move as a result of the wall shear stress from a reference position x _REF to a deformed position x (marking element indicated here by 13 '). By means of a processing unit (24), the wall shear stress at different locations of the outer surface (3) can be determined from the displacements of the marking elements which have occurred.
The invention finds application for any body, which are exposed to flowing fluids, in particular wind tunnels with specimens or on land, in the air or in water moving bodies and vehicles.

Description

TECHNICAL FIELD OF THE INVENTION

The The invention relates to a measuring device for a wall shear stress of a from a current a fluid impinged flow guide. Farther The invention relates to a method for detecting a wall shear stress.

STATE OF THE ART

• – Die Wandschubspannung gibt Aufschluss über den Strömungszustand des Fluids und/oder Eigenschaften des den Körper umströmenden Fluids. Beispielsweise bedeutet – vereinfacht gesagt – eine Erhöhung der Strömungsgeschwindigkeit des Fluids eine Erhöhung der Wandschubspannung. Ebenfalls möglich ist, dass über abrupte Änderungen der Wandschubspannung ein Umschlagen einer laminaren Strömung zu einer turbulenten Strömung erkannt werden kann. Zusammengefasst kann somit über eine Wandschubspannung ein Rückschluss auf eine Strömung und/oder das Fluid ermöglicht werden, ohne dass ein Sensor die Strömung des Fluids selbst beeinflusst oder verfälscht. Hierbei kann die Wandschubspannung als integraler Wert für eine Fläche ermittelt werden, so dass eine integrale Beurteilung des Strömungszustandes in der Fläche ermöglicht ist, oder die Wandschubspannung wird an mehreren Punkten in einer Fläche ermittelt, so dass auch Veränderungen der Strömungsverhältnisse im Bereich der Fläche erfasst werden können.
• – Alternativ oder kumulativ kann über die Wandschubspannung der Einfluss der Strömung des Fluids auf den von dem Fluid umströmten Körper erfasst werden. Besonders eindrucksvoll kann dieses anhand der Biomedizin erläutert werden, wo die Wandschubspannung Aufschluss über die Beanspruchung der Venen und Arterien bei deren Durchfluss mit Blut gibt, weshalb die Wandschubspannung in diesem Bereich Gegenstand intensiver Forschungen ist. Entsprechend kann auch bei anderweitigen durch- oder umströmten Körpern, beispielsweise Fahrzeugen, Rohrleitungssystemen u. ä., über eine Erfassung von Wandschubspannungen ein Rückschluss über die Beanspruchungen von Strömungsleitflächen und ggf. deren Verschleiß gewonnen werden.
• – Schließlich ist es beispielsweise ebenfalls möglich, über eine Erfassung von Wandschubspannungen Erkenntnisse über Energieverluste, beispielsweise in einem Rohrleitungssystem für ein Fluid, zu gewinnen.

The determination of a wall shear stress of a body flowed around by a flow of a fluid can take place for different purposes, for example:

• - The wall shear stress provides information about the flow state of the fluid and / or properties of the body flowing around the fluid. For example, in simple terms, increasing the flow velocity of the fluid means increasing the wall shear stress. It is also possible that abrupt changes in the wall shear stress can detect a turning of a laminar flow to a turbulent flow. In summary, conclusions about a flow and / or the fluid can thus be made possible via a wall shear stress, without a sensor itself influencing or falsifying the flow of the fluid itself. Here, the wall shear stress can be determined as an integral value for a surface, so that an integral assessment of the flow state in the surface is possible, or the wall shear stress is determined at several points in a surface, so that changes in the flow conditions in the area of the surface are detected can.
• Alternatively or cumulatively, the influence of the flow of the fluid on the body circulated by the fluid can be detected via the wall shear stress. This can be explained particularly impressively with biomedicine, where the wall shear stress provides information on the load on the veins and arteries as they flow through blood, which is why the wall shear stress in this area is the subject of intensive research. Accordingly, in other through or flow around bodies, such as vehicles, piping u. Ä., About a detection of wall shear stresses a conclusion about the stresses of flow baffles and possibly their wear can be obtained.
• Finally, it is also possible, for example, to obtain information about energy losses, for example in a pipeline system for a fluid, by detecting wall shear stresses.

Usually be for a measurement of wall shear stresses thermoresistive surface sensors and sensor arrays containing a stationary or transient detection enable wall shear stresses used. Furthermore, in addition to conventional surface hot films also microfabricated hot films (so-called micro-electromechanical Systems or "MEMS") and surface hot wires used. The flush with the wall to be arranged hot wires are characterized by a particularly favorable thermodynamic arrangement of the sensor element and therefore allow one compared to classic hot movies improved dynamic signal resolution. You can continue with a thin one Polyimide film (KAPTON film) as a substrate flexible sensor arrays with which even three-dimensional curved contours instrumented can be. Also possible is that a determination of a wall shear stress using Pressure probes done, especially with so-called Preston tubes. Furthermore, it is known to visualize a wall shear stress thereby that one of the flow of one Fluid acted upon Strömungsleitfläche with a Gel layer is provided, due to the flow and in dependence is deformed by the wall shear stress. For known properties of the gel used in the gel layer may be from a measured deformation the wall shear stress can be determined. For this purpose, a speckle pattern interferometry be used.

For industrial Applications, such as aircraft, is in addition to the static and dynamic properties of surface sensors also a mechanical Robustness of the sensor used of great interest. Other assessment criteria for sensors, which are used to detect a wall shear stress are the dimensions of a sensor or sensor element which the spatial resolution the wall shear stress, the handling of the sensor, the dynamic behavior, the size of the interaction with the flow, which influences a noise component and any necessary amplification measures, the production and a temperature dependence. Problematic with the use of MEMS is that they are usually unable to in addition to the size of the wall shear stress as well to recognize their direction.

OBJECT OF THE INVENTION

The object of the present invention is to propose an alternative measuring device for detecting a wall shear stress of a flow guidance surface acted upon by a flow of a fluid. Furthermore, the present invention is based on the object, an alternative method for detecting a Wandschubspan to propose.

SOLUTION

The The object of the invention according to the invention with the features of the independent claim 1 solved. Further embodiments of the invention will become apparent accordingly the dependent claims 2-10. Furthermore, the object of the invention with a method according to the characteristics of the independent Patent claim 11 solved. Further embodiments of the method emerge from the features the dependent claims 12 and 13.

DESCRIPTION OF THE INVENTION

at The present invention is a measuring device for at least a wall shear stress of an acted upon by a flow of a fluid plane or arbitrarily curved, kinked or stepped flow control surface. in this connection it can be a discrete, local wall voltage, one over one Range averaged wall shear stress to several discrete or averaged Wall shear stresses or wall shear stress curves along any (in particular straight or curved) Coordinate axes or in space. It is possible that a wall shear stress recorded as absolute size or a wall shear stress change. Furthermore, alternatively or cumulative to the detection of an amount of wall shear stress or their change a direction of the wall shear stress using the present In the case of the fluid acting upon the flow-guiding surface can be any liquid, gas or Act mixture.

Under a flow guide according to the invention is any area understood, which with the flow The fluid may interact to produce a wall shear stress. For example, the flow guide surface is an outer surface of a in a fluid moving body, such as an aircraft and spacecraft, a motor vehicle, a flying object, or a stationary or moving body, past which the fluid is passed or through which the fluid is passed, in particular a tube, a channel or wind tunnel o. Ä.

In the measuring device according to the invention is provided a deformation element. The material properties and / or the structural design and dimensioning of the deformation element is such that it is elastically deformable by the flow of the fluid is. So that the deformation element be deformed by the flow of the fluid can, this is arranged in the flow guide surface. In the case, that with the measuring device, the wall shear stress of a anyway existing body detected in the area of its flow guide is to be formed preferably forms the deformation element of the fluid acted on the outer contour of the body after and is part of the flow control surface. For example, can a part of the outer surface of the Flow guide removed be and be replaced by the deformation element, wherein the outer contour the flow guide with the deformation element of the original Flow control without the deformation element corresponds. Unlike the one from the state the art known gel layer for visualization of the wall shear stress the deformation element is designed as a solid.

Farther includes the measuring device according to the invention at least one marking element. The absolute position or one relative position of such a marking element with respect to a another marking element or a reference position is of the elastic deformation of the deformation element dependent. Preferably the at least one marking element on or in the deformation element intended.

Of the The present invention is based on the finding that a Deformation of a partial region of the flow guide surface in the surface direction for elastic Formation of the flow guide a consequence the wall shear stress is. Thus, a direct connection between the wall shear stress and the deformation, the dependence yes according to the present material by the elasticity of the flow guide, here of the deformation element, and its structural design and Dimensions is affected. In the simplest case, the elasticity is a proportionality constant between the wall shear stress and the deformation. About the Choice of stiffness of the deformation element and / or the dimensioning of the same can thus be a kind of amplification factor for the caused deformation as a result of wall shear stress constructive be specified.

For converting the previously described deformation into a suitable measurement or evaluation signal, a detection element is furthermore provided according to the invention, by way of which a change in the position of a marking element or the relative position of two marking elements can be detected. Any such known means for detecting a path or a deformation can be used for such a detection member, wherein it is to be avoided that influencing the flow of the fluid and the geometry and the stress states of the flow guide surface via the detection member. For this reason, a non-contact sensing member is too zuzu gene.

By the solution according to the invention are stationary or unsteady, u. U. highly dynamic wall shear stresses and Wandschubspannungsänderungen measurable, with the wall shear stress discrete, as an integral value or continuously or spatially disbanded can be. For simplicity, in the claims and in the following description, partly to a single marker element or a few marking elements, a duplication of which Marking elements to increase the number of measuring points or areas is possible without the fundamental leave the inventive principle becomes.

• a) Im einfachsten Fall ruht das Erfassungsorgan relativ zu der Strömungsleitfläche und dem Verformungselement. Ist in diesem Fall eine Referenzposition eines Markierungselements vor Beaufschlagung mit der Strömung oder Veränderung einer Strömung bekannt sowie eine Position desselben Markierungselements nach Beaufschlagung des Markierungselements mit der Strömung oder Änderung der Strömung, so kann aus einem Vergleich der Referenzposition mit der Position auf die Wandschubspannung im Bereich des Markierungselements rückgeschlossen werden. Somit kann über eine Beobachtung einer Veränderung einer Position eines Markierungselements die Wandschubspannung im Bereich des Markierungselements detektiert werden. Doch selbst ohne Kenntnis einer Referenzposition für nicht mit einer Strömung beaufschlagten Strömungsleitfläche kann aus einer Veränderung der Position eines Markierungselements auf eine Wandschubspannungsänderung geschlossen werden.
• b) Für den Fall, dass sich die Strömungsleitfläche gegenüber einem Erfassungsorgan mit einer, u. U. unbekannten unveränderlichen Geschwindigkeit, bewegt, kann eine Veränderung der Position eines Markierungselements zwei unterschiedliche Ursachen haben: – einerseits kann die Veränderung der Position des Markierungselements durch die Relativbewegung der Strömungsleitfläche gegenüber dem Erfassungsorgan verursacht sein, ohne dass eine Änderung der Wandschubspannung gegeben ist, – während andererseits die Änderung der Position des Markierungselements teilweise oder ausschließlich durch eine Veränderung der Wandschubspannung bedingt sein kann.

The embodiment according to the invention comprises two basic basic variants:

• a) In the simplest case, the detection member rests relative to the flow guide and the deformation element. If, in this case, a reference position of a marking element before exposure to the flow or change of a flow is known, as well as a position of the same marking element after exposure of the marking element to the flow or change of the flow, the comparison of the reference position with the position on the wall shear stress in the region be deduced the marking element. Thus, by monitoring a change in a position of a marking element, the wall shear stress in the area of the marking element can be detected. However, even without knowledge of a reference position for flow guidance surface not acted upon by a flow, a change in the position of a marking element to a wall shear stress change can be concluded.
• b) In the event that the flow control surface with respect to a detection member with a u. A change in the position of a marking element can have two different causes: on the one hand, the change in the position of the marking element can be caused by the relative movement of the flow guide surface relative to the detection element, without a change in the wall shear stress, while on the other hand, the change in the position of the marking element may be partly or exclusively due to a change in the wall shear stress.

In In such a case, the evaluation of a sole change in position for the Marking element not readily to determine the two Unknown wall shear stress and relative motion can be used.

In In this case, it is advantageous if another marking element is provided, whose position is not the wall shear stresses is dependent in the region of the flow guide or only to a lesser extent. About this further marking element can then by the detecting member, the relative movement between the flow guide and Be detected. Alternatively or cumulatively for the Marking element or more such marking elements the Deformation due to the wall shear stress and thus the wall shear stress itself What is a change the relative position of the marking element (s) relative to the / n further marking element (s) is evaluated.

While that Deformation element may have any shape, for example a rod-shaped or annular Geometry, according to a further education the measuring device according to the invention the deformation element as a Strömungsleitfläche at least partially overlapping deformation layer educated. With such a deformation layer is the detection an integral wall shear stress in the region of the deformation layer or the detection of wall shear stress at several locations possible. For one Deformation layer of constant thickness becomes the contour of a not to be detected by measurement changed significantly. Furthermore, a deformation layer u. U. especially easy to generate, for example, by applying a film, by applying a liquid, hardening Material or lamination of the deformation layer. In the case of one Connection of a solid, separately manufactured deformation layer with the flow guide can before the production of the compound of the deformation layer with the flow guide the elastic properties of the deformation element metrologically be determined and over the manufacturing method of this layer are given constructive.

According to a further proposal of the invention, two groups of marking elements are arranged at different depths in the deformation layer, wherein the first group is, for example, marking elements whose change in position is largely dependent on the deformation of the deformation element, while the marking elements of second group than previously designated as further marking elements marking elements are formed whose change in position is dependent on a wall shear stress, at least to a reduced extent, but in any case by any relative movement between Strö mungleitfläche and detection element. Thus, in the deformation layer, the required elements can be arranged, which enable a reliable determination of the wall shear stresses even when a relative movement between the flow guide surface and the detection element occurs.

to Separation of the previously explained different effects on the one hand a deformation due to the Wall shear stress and on the other hand a change in position as a result of a relative movement, it has been found to be advantageous if the marker elements of the different groups in the Deformation layer as possible are arranged far apart in the thickness direction of the deformation layer, because of this for a given stiffness of the deformation layer a maximum gain of the position change can be achieved due to the wall shear stress. Accordingly, the first group of marking elements on the fluid facing outside the deformation layer, where large deformations due to the Shear stress occur. The second group of marking elements is arranged on the side facing away from the fluid of the deformation layer, where the deformations vanish due to the wall shear stress are small. This also includes embodiments, where the respective marking elements still up to a depth 10% of the deformation layer are embedded in these.

For an inventive embodiment are the marking elements as discrete, local marking elements formed, which are distributed in a regular matrix or randomly can. The area density The marking elements can be constant or different be.

For an alternative inventive design are the marking elements as global continuous marking elements educated. For example, a straight marking element insert Find. Acts on such a marker element wall shear stress transverse to the longitudinal extent the straight line, which in the direction of the longitudinal axis of the marking element a changeable one Owns size, leads the elastic deformation of the deformation element to that due the changeable Wall shear stress originally rectilinear marking element after deformation is curved. A comparison of the original Straight line shape with the deformed curve provides information about the Wall shear stress, although without knowledge of the original position of the marking element from the curve of the deformed marking element Information about one Gradient of the wall shear stress in the longitudinal direction of the marking element can be won.

During the Detection of deformation also, for example, via a suitable path detection with known displacement sensors, a deformation sensor or Transformation of the deformation over a mechanical connection or geared connection is possible, is for a special embodiment the invention, the detection member with an image sensing device formed, for example, a CCD camera, a C-MOS camera or a digital camera can be used. Such cameras provide at a relatively low cost a big or sufficient resolution ready and possible a simple subsequent image editing and evaluation over a computer-based processing unit. In the event that marking elements arranged in different layers in the deformation layer are arranged or at least partially in the interior of the deformation layer requires the detection of a position of such a marker by a detecting member that the marking element optically visible is. This also includes the formation of the deformation layer one opposite understood material permeable to any radiation with a for the radiation impermeable or less permeable Material for the marking element. This embodiment can be used when the deformation layer is illuminated by a radiation source is and image capture on the radiation source facing away Side of the deformation layer takes place. Accordingly, for the marking elements a stronger radiation reflective material can be used in the event that an image capture after reflection of the radiation of a radiation source, preferably on the same side as the radiation source. Advantageous it may still be, if marking elements and other marking elements the first and second group different reflection behavior and / or radiation transmission properties so that they differ from each other by the detecting member can be.

According to one special embodiment, the deformation layer is formed with a polymer material, which is easy and inexpensive available is easily applied to the flow guide, adhesive to the Adhere flow control surface can and / or the desired Radiation transmission properties or having radiation reflection properties.

In an alternative embodiment, the deformation layer is formed with microfibers. In this case, end regions of the microfibers can be connected to the flow guide surface, while the opposite end surfaces hi slightly in the flow neinragen. The microfibers deform as a result of the flow, similar to several hairs or grasses in the wind, with increased flow and thus increased wall shear stress is a greater deflection of the microfibers.

at a method according to the invention for metrological detection of a wall shear stress can several Marking elements stochastically be distributed over the flow guide. At a automated processing of an image of an image capture device to determine a change the position of marking elements is cross-correlated or an autocorrelation function for use with signals derived from the at least a picture has been taken.

advantageous Further developments of the invention will become apparent from the claims, the Description and the drawings. The in the introduction to the description advantages of features and combinations of several Features are merely exemplary and may be alternative or cumulative come into effect, without the benefits of mandatory embodiments of the invention must be achieved. Other features are the drawings - in particular the illustrated Geometries and the relative dimensions of several components to each other as well as their relative arrangement and operative connection - can be seen. The combination of features of different embodiments the invention or features of different claims is also different from the ones chosen The antecedents of the claims possible and is hereby stimulated. This also applies to such features as in separate drawings are shown or in their description to be named. These features can be combined with features of different claims. As well can in the claims listed Features for more embodiments the invention omitted.

BRIEF DESCRIPTION OF THE FIGURES

in the The invention is described below with reference to the figures preferred embodiments further explained and described.

1 shows a measuring device in a schematic representation.

2 shows an exemplary image of an image sensing device of a measuring device according to the invention before admission of a flow guide with deformation element and marking elements of different groups.

3 shows an exemplary image of an image sensing device of a measuring device according to the invention under the action of a flow guide with deformation element and marking elements of different groups.

DESCRIPTION OF THE FIGURES

1 shows a schematic representation of an embodiment of a measuring device according to the invention 1 , A flow 2 a fluid moves at least with a flow component and / or partially parallel to an outer surface 3 a flow guide 4 , The flow control surface 4 is with a body 5 formed on the on the flow 2 facing side a deformation layer 6 is applied. For the illustrated embodiment, the main body 5 just trained and the deformation layer 6 with constant wall thickness, whereby any other geometries and contours are possible. The deformation layer 6 owns on the flow 2 facing outer surface 3 or in these marking elements 7 who are a first group 8th form while on or in an inner surface 9 the deformation layer 6 in the transition region to the main body 5 markers 10 are provided, that of a second group 11 assigned. Assuming an ideally rigid body 5 it comes as a result of an elastic formation of the deformation layer 6 and in the area of the outer surface 3 with the deformation layer 6 interacting flow 2 to a deformation of the deformation layer 6 in in 1 horizontal direction. Here, the deformation increases according to the course 12 from 0 in the area of the inner surface 9 towards the outer surface 3 , for example, linear as shown. As a result, there is a reference position x _{REF of} a marking element 13 in the area of the outer surface 3 changed to a position x. For clarity, the marking element 13 in a deformed position with reference numerals 13 ' in). Also for the other marking elements 7 in the area of the outer surface 3 a deformation corresponding to that in the region of the marking elements 7 acting wall shear stress. On the other hand change the marking elements 10 in the area of the inner surface 9 their position not according to the flow 2 , A change in the position x _V takes place only when the entire flow guide 4 emotional.

About an image capture organ 14 becomes a picture 25 from the marking elements 7 . 10 . 13 before the influence of the flow 2 on the deformation layer 6 recorded, which exemplifies in 2 is shown, as well as another picture 26 upon exposure to the flow 2 on the marking elements 7 . 10 . 13 recorded in which 3 is shown. From the image capture organ 14 be the pictures 25 . 26 at least partially match or image data obtained therefrom 15 to an image recognition device 16 transmitted.

In the image recognition device 16 an identification of the positions x _V , x, x _{REF of} the marking elements takes place 7 . 10 . 13 before and during the action of the flow 2 , For example, for the pictures according to 2 and 3 the crossing of a threshold value of a gray level for the individual pixels are checked, wherein exceeding the threshold value can indicate a position of a marking element. Have the marking elements 7 and 10 different radiopacity or radiation reflection properties, can distinguish the marking elements via an existing gray or color level 7 . 10 of different groups 8th . 11 respectively. The reference positions x _{REF, 1} ; X _{REF, 2} , ... x _{REF, N} as well as the positions x ₁ , x ₂ , ... x _N are called data 17 to a deformation detection device 18 which determines deformations Δx from the difference between x and X _REF (Δx = x ₁ -x _{REF, 1} ; ΔX ₂ = x ₂ -x _{REF, 1} ; ... Δx _N = x _{N -x REF, N.}

The thus determined deformations Δx are called data 19 to a wall voltage detection device 20 which converts the determined deformations Δx into a wall shear stress τ via a dependence specified a priori. The wall shear stresses τ are then used as data 21 to an output device 22 to hand over.

at Such an output device may be, for example to a printer, a screen o. Ä. Act where the determined Wandschubspannungen issued in a suitable form for a user can be. This can be done, for example, in the form of maximally determined wall shear stresses, averaged wall shear stresses, wall shear stress distributions, Wall shear stress gradients, Fields of wall shear stresses, three-dimensional representations take place, the output also take place in the form of vector fields can be where for each measuring point the direction of the determined wall shear stress by a Arrow direction is indicated and the amount of the determined wall shear stress by the length of the Arrow is shown. Also possible is the matrix-like Representation of the amount of the determined wall shear stress over a Color coding.

The facilities 16 . 18 . 20 . 22 can be completely or partially in a CPU 23 be integrated or part of a network. The image recognition device 16 and the deformation detection means 18 together form a processing unit 24 , on the basis of at least one image of the image acquisition organ 14 Positions of the marking elements 7 . 10 . 13 , Changes in the positions of the marking elements 7 . 10 . 13 and / or changes in the relative position of the marker elements 7 . 10 . 13 different groups 8th . 11 can be determined.

A dependence between the deformations Δx and the wall shear stress τ can be a priori analytically determined and specified or generated by a suitable calibration measurement and in the wall voltage determination device 20 be implemented.

To determine a shear stress gradient of adjacent marking elements, the deformation layer must 6 can not only experience a deformation that occurs due to a shear stress between sub-layers of the deformation layer and can be approximated by a first conversion characteristic between the wall shear stress and the resulting deformation. Rather, a deformation between the adjacent marking elements must be possible. For a relative deformation of adjacent marking elements in the direction of the coordinate x in 1 Thus, a deformability due to a normal stress in a surface having a surface normal in the direction of the coordinate x must be given, which can be approximated by a second conversion characteristic between the wall shear stress variation and the resulting relative deformation of the adjacent marking elements. There may be a constant flow in the area of the outer surface 3 leads to a different deformation than a corresponding, but only local flow and thus the first and second conversion characteristics differ, in the wall voltage determination means an improved approximation of the wall shear stresses occur when both the first conversion characteristic and the second conversion characteristic or appropriate combinations find ever according to the extent of the deformation gradient of the deformation Δx. This results in that for a given deformation .DELTA.x of a marking element, the amount of a determined wall shear stress is different depending on the deformation gradient to an adjacent marking element.

During measurement methods known from the prior art using a gel deformable by the flow, an undesirable deformation distorting the measurement result as a result of a normal to the outer surface 3 acting flow can occur, for the inventive design deformation Δx is not generated by such flow components, although the deformation layer 6 can deform, but only in the vertical direction in 1 but not in a horizontal direction.

While in the 2 and 3 Only the positions and deformations in the horizontal direction x are indicated, of course, according to and alternatively or additionally, an evaluation of positions and deformations with respect to a y-coordinate, ie in the vertical direction in the 2 and 3 , respectively.

An exemplary embodiment for determining the wall shear stress on the basis of detecting a shift of at least one marking element is explained below.
If a laminar plate boundary layer is considered for clarification, it is known that the local wall shear stress (τ) according to Blasius is a function of the flow velocity (U _∞ ) of the density of the flowing medium (ρ) and the local Reynolds number (Re _χ ), formed with the lead length (x) and the kinematic viscosity (ν). τ 0 = 0.322 (ρU ∞ 2 ) / (√Re χ ) with Re χ = U ∞ x / ν

Under standard conditions, a wall shear stress of the order τ ≈ 0.5 N / m ² follows for an air flow with U _∞ = 20 m / s.

The resulting deformation of an elastic layer on the plate surface (Δx) is a linear function of the elastic properties of the material, given as shear moduli (G), the layer thickness (h) and τ. G = τ / (Δx / h)

Thus, for a known G in the range G = 50 - 5000 Pa, for layer thicknesses of h = 0.1 - 10 mm, a deformation of the order Δx = O (10 ^-6 m) can be estimated as a result of the wall shear stress mentioned above.

On the other hand, a measured deformation Δx or a field of deformations Δx at a plurality of measurement locations x _{i, j} and knowledge of G and h can be inferred to those at the location / locations x shear stress τ or τ _{i, j} . Of course, another conversion using linear or non-linear dependencies τ = f (Δx) or experimentally determined maps is possible.

1

measuring device

2

flow

3

outer surface

4

flow guide

5

body

6

deformation layer

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marker

8th

first group

9

palm

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marker

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second group

12

course

13

marker

14

Image capture organ

15

image data

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Image recognition device

17

dates

18

Deformation detection device

19

dates

20

Wall voltage detector

21

dates

22

output device

23

CPU

24

processing unit

25

image

26

image

Claims (13)

Measuring device ( 1 ) for a wall shear stress one of a flow ( 2 ) of a fluid impinged flow control surface ( 4 ) with a) through the flow ( 2 ) of the fluid elastically deformable deformation element, which is designed as an elastic solid and in the region of the flow guide ( 4 ), b) at least one marking element ( 7 ; 13 ), its absolute position (x) or relative position with respect to another marking element ( 10 ) is dependent on the elastic deformation of the deformation element, and c) a detection member, via which a change in the position of a marking element ( 7 ) or the relative position of two marking elements ( 7 . 10 ) is detectable. Measuring device ( 1 ) according to claim 1, characterized in that the deformation element as a flow guide ( 4 ) at least partially covering surface layer ( 6 ) is trained. Measuring device ( 1 ) according to claim 2, characterized in that two groups ( 8th . 11 ) of marking elements ( 7 . 10 ) at different depths in the deformation layer ( 6 ) are arranged. Measuring device ( 1 ) according to claim 3, characterized in that a first group ( 8th ) of marking elements ( 7 ) on or in the fluid facing outer surface ( 3 ) of the deformation layer ( 6 ), while a second group ( 11 ) of marking elements ( 10 ) on or in the side facing away from the fluid (inner surface 9 ) of the deformation layer ( 6 ) is arranged. Measuring device ( 1 ) according to one of the preceding claims, characterized in that Marking elements ( 7 . 10 . 13 ) are formed as discrete local marking elements. Measuring device ( 1 ) according to one of claims 1 to 4, characterized in that marking elements are formed as global continuous marking elements. Measuring device ( 1 ) according to any one of the preceding claims, characterized in that the sensing member is provided with an image sensing member ( 14 ) is trained. Measuring device ( 1 ) according to claim 7, characterized in that a processing unit ( 24 ) from which at least one image ( 25 . 26 ) of the image-capturing organ ( 14 ) - Positions of the marking elements ( 7 ) and / or - changes in the positions of the marking elements ( 7 ) and / or - changes in the relative position of the marking elements ( 7 . 10 ) of different groups ( 8th . 11 ) are determinable. Measuring device ( 1 ) according to one of the preceding claims, characterized in that the deformation layer ( 6 ) is formed with a polymer material. Measuring device according to one of the preceding claims, characterized in that the deformation layer ( 6 ) is formed with microfibers. Method for detecting a wall shear stress of a flow ( 2 ) of a fluid impinged flow control surface ( 6 ) with a measuring device ( 1 ) according to one of the preceding claims with the following method steps: a) specification of a reference position (x _REF ) for a marking element ( 7 ), b) admission of the flow control surface ( 4 ) and the deformation element ( 6 ) with the flow ( 2 c) detecting an altered position (x) of the marking element or elements ( 7 ) due to deformation of the deformation element caused by the flow ( 2 d) generating a measured value for the wall shear stress taking into account a comparison of the changed position (x) of the marking element ( 7 ) with the reference position (x _REF ) of the marking element ( 7 ). A method according to claim 11, characterized in that at least a portion of the deformation element (deformation layer 6 ) via an image capture device ( 14 ) and in a processing unit ( 24 ) via automated processing of the image of the image-capturing agency ( 14 ) the position change of the marking element ( 7 ) is determined. Method according to claim 12, characterized in that a) several marking elements ( 7 ) stochastically via the flow control surface ( 4 ) and b) during automated processing of the image to determine a change in position (Δx) of the position of marking elements ( 7 ) a cross-correlation function or autocorrelation function of signals or data obtained from the image is used.