Method for measuring displacement thickness and momentum loss thickness of highly compressible turbulent boundary layer

Abstract

A method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer belongs to the technical field of spaceflight tests, and comprises the following steps: acquiring boundary layer velocity field distribution for a boundary layer flow field at a specific position of a flat plate model in a hypersonic wind tunnel capable of continuously operating; wall surface temperature distribution after model heat balance is obtained according to the specific position of the flat plate model; boundary layer velocity field distribution and wall surface temperature distribution of a specific flow direction station are utilized, temperature distribution and density distribution of flow in a boundary layer in the normal direction are obtained by introducing related hypotheses, and then the displacement thickness and the momentum loss thickness are calculated according to the temperature distribution and the density distribution. The problem of obtaining the displacement thickness and the momentum loss thickness of the boundary layer in the wind tunnel environment from the test angle is solved.

Description

Technical Field

[0001] The invention relates to a method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer, and belongs to the technical field of aerospace testing. Background Art

[0002] For high-speed aircraft, the measurement of boundary layer parameters is very important, which is related to flow transition, wall friction and thermal protection. However, under high Mach number conditions, it is difficult to quantitatively measure boundary layer parameters non-invasively. Especially under turbulent conditions, it is crucial to obtain high-resolution boundary layer flow field parameter information. After decades of development, particle image velocimetry (PIV) has achieved rapid development with its advantages of both flow display and measurement. With the introduction of double-exposure lasers, digital cross-frame cameras and high-precision time-delay signal generators, and the continuous development and improvement of image processing technologies such as cross-correlation calculation and fast Fourier transform, particle image velocimetry has been greatly improved in image quality, velocity measurement range and velocity measurement accuracy, and has been applied in more and more fields. All aerodynamic research institutes in major foreign aerodynamic powers have adopted particle image velocimetry technology to carry out aerodynamic research. For PIV technology, the presence of tracer particles in the fluid being studied is essential. By taking particle images with a cross-frame camera and calculating the velocity field, the flow field characteristic parameters such as isovelocity lines, streamlines and vorticity of the corresponding flow field can be calculated with the help of corresponding mathematical calculation software.

[0003] At present, a large number of boundary layer research fields in domestic high-speed wind tunnels lack accurate quantitative measurement and analysis of boundary layer velocity types. The displacement thickness and momentum loss thickness in the boundary layer are determined by theoretical methods and numerical simulation methods. Affected by the model boundary and temperature effects, many test results do not match expectations and cannot provide data reference for transition research and heat reduction and drag reduction research. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to provide a method for measuring the displacement thickness and momentum loss thickness of a strongly compressible turbulent boundary layer, and to solve the problem of measuring and obtaining the displacement thickness and momentum loss thickness of the boundary layer in a wind tunnel environment from an experimental perspective.

[0005] The technical solution of the present invention is: a method for measuring displacement thickness and momentum loss thickness of a strongly compressible turbulent boundary layer, comprising:

[0006] In a continuously operating hypersonic wind tunnel, the boundary layer velocity field distribution is obtained for the boundary layer flow field at a specific position of a flat plate model;

[0007] For a specific position of the flat plate model, obtain the wall temperature distribution after the model reaches thermal equilibrium;

[0008] By using the boundary layer velocity field distribution and wall temperature distribution at a specific flow direction station, the temperature distribution and density distribution along the normal direction of the flow in the boundary layer are obtained by introducing relevant assumptions, and then the displacement thickness and momentum loss thickness are calculated based on the temperature distribution and density distribution.

[0009] Further, obtaining the boundary layer velocity field distribution includes:

[0010] (1) Using a particle generator to inject tracer particles of a specific size into the wind tunnel flow field;

[0011] (2) Selecting a dual-pulse laser light source and a high-resolution cross-frame camera for flow field illumination and image acquisition;

[0012] (3) The particle images obtained by the acquisition are used to analyze the instantaneous velocity field (u) of the boundary layer. x,y ,v x,y ) calculation to obtain the boundary layer velocity field distribution.

[0013] Furthermore, the TiO2 particles in the boundary layer flow field meet a delay time better than 1 microsecond.

[0014] Furthermore, the number of velocity vectors in the normal direction of the inner wall surface of the boundary layer thickness is not less than 20.

[0015] Furthermore, the selected cross-frame CCD camera satisfies the requirements that the resolution in the normal direction of the wall is not less than 2000 pixels and the focal length of the lens is f=200 mm, and in the normal direction, the optical axis of the camera and lens optical system is parallel to the surface of the flat plate model and just passes through the surface of the flat plate model, so that the particle image occupies only 1 / 2 of the height of the CCD camera frame.

[0016] Furthermore, the temperature distribution is Calculate; where, is the time-averaged velocity in the boundary layer The average temperature at the location, is the time-averaged wall temperature, is the time-averaged temperature at the boundary layer thickness δ, To restore the temperature, is the time average of the streamwise velocity at a specific streamwise position in the boundary layer, is the time-averaged velocity of the stream at the boundary layer thickness δ.

[0017] Furthermore, the density distribution is Calculate; where, is the time-averaged velocity of the boundary layer The time-averaged density at the location, ρ ∞ is the static density of the incoming flow, is the time-averaged velocity in the boundary layer The time-averaged temperature at the location, T∞ For the incoming flow to be quiet and warm.

[0018] Furthermore, the displacement thickness and momentum loss thickness are respectively

[0019]

[0020] Among them, δ * is the displacement thickness of the compressible boundary layer, θ is the momentum loss thickness of the compressible boundary layer, δ is the boundary layer thickness, is the time-averaged velocity of the boundary layer The time-averaged density at the location, ρ ∞ is the static density of the incoming flow, is the time average of the streamwise velocity at a specific streamwise position in the boundary layer, is the time-averaged velocity of the stream at the boundary layer thickness δ.

[0021] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of a method for measuring displacement thickness and momentum loss thickness of a strongly compressible turbulent boundary layer are implemented.

[0022] A device for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps of a method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer are implemented.

[0023] The advantages of the present invention compared with the prior art are:

[0024] (1) The present invention obtains a boundary layer velocity field with a high vector resolution through an optical system of a high-resolution cross-frame CCD camera and a telephoto macro lens;

[0025] (2) The present invention obtains the model wall temperature through infrared thermal imaging technology, introduces the Walz equation combined with the velocity distribution, obtains the boundary layer temperature distribution, further introduces the complete gas equation and the assumption that the pressure in the boundary layer is approximately constant, obtains the density distribution in the boundary layer, and finally obtains the displacement thickness and momentum loss thickness of the compressible flow turbulent boundary layer through integration. DETAILED DESCRIPTION

[0026] In order to better understand the above technical scheme, the technical scheme of the present invention is described in detail through specific embodiments below. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical scheme of the present invention, rather than limitations on the technical scheme of the present invention. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0027] The following is a further detailed description of a method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer provided by an embodiment of the present invention. The specific implementation method may include:

[0028] In a continuously operating hypersonic wind tunnel, the boundary layer velocity field distribution is obtained for the boundary layer flow field at a specific position of a flat plate model;

[0029] For a specific position of the flat plate model, obtain the wall temperature distribution after the model reaches thermal equilibrium;

[0030] By using the boundary layer velocity field distribution and wall temperature distribution at a specific flow direction station, the temperature distribution and density distribution along the normal direction of the flow in the boundary layer are obtained by introducing relevant assumptions, and then the displacement thickness and momentum loss thickness are calculated based on the temperature distribution and density distribution.

[0031] The solution provided in the embodiment of the present invention specifically includes:

[0032] 1. Step 1: In a continuously operating hypersonic wind tunnel, use PIV technology to obtain discrete, high-resolution boundary layer instantaneous velocity fields (u x,y ,v x,y ), where the subscripts x and y represent different flow direction positions and wall normal positions. The specific requirements for velocity field acquisition are as follows:

[0033] (1) Use a particle generator to inject tracer particles of a specific size into the wind tunnel flow field

[0034] Tracer particles with larger diameters are difficult to enter the hypersonic boundary layer or cannot represent the actual boundary layer airflow velocity. Therefore, the diameter of the tracer particles needs to be limited. Generally speaking, for TiO2 particles, the delay time should be better than 1 microsecond. The calculation method of the delay time τ is as follows:

[0035]

[0036] The density of the particle is ρ p , with a diameter of d p , the density of the fluid is ρ f , the dynamic viscosity coefficient is υ f .

[0037] (2) Select dual-pulse laser light source and high-resolution cross-frame camera for flow field illumination and image acquisition

[0038] The illumination laser light source is the Nd:YAG high-energy double-pulse laser commonly used in PIV technology, with a single pulse energy of no less than 300mJ.

[0039] In order to obtain high-resolution boundary layer spatial velocity profiles, the more velocity vectors along the wall normal in the boundary layer, the better. The vector resolution R of the velocity vector along the wall normal in the boundary layer is defined as v-δ as follows:

[0040] R v-δ =δ / r v

[0041] Among them, r v is the number of velocity vectors in the normal direction of the inner wall of the boundary layer thickness, δ is the boundary layer thickness obtained by judging the velocity field, and for the PIV velocity field, the boundary layer thickness δ is defined by the normal coordinate of the position close to 99% of the mainstream velocity value.

[0042] Optimize v ≥20, so that the vector resolution R v-δ The selected cross-frame CCD camera should have a relatively small value and fully resolve the boundary layer velocity type. Accordingly, the cross-frame CCD camera should have a relatively high resolution, preferably a cross-frame CCD camera with a resolution of not less than 2000 pixels in the normal direction of the wall, and the optical system should have a relatively high magnification. The lens focal length f = 200mm is preferably used for shooting with a telephoto macro lens. In the normal direction, the optical axis of the camera and lens optical system is parallel to the surface of the flat plate model and just passes through the surface of the flat plate model, so that the particle image only occupies about 1 / 2 of the CCD camera frame height to avoid the influence of wall reflection. The PIV system composed of the camera is used to collect particle images during wind tunnel operation and record the cross-frame time Δt of the PIV system.

[0043] (3) The particle images obtained by the acquisition are used to analyze the instantaneous velocity field (u) of the boundary layer. x,y ,v x,y )calculate

[0044] According to the principle of PIV technology, the displacement field (du) of a pair of particle images recorded by the cross-frame camera is calculated by the cross-correlation algorithm. x,y ,dv x,y ), combined with the cross-frame time Δt, the boundary layer instantaneous velocity field (u x,y ,v x,y )=(du x,y / Δt,dv x,y / Δt). Since the cross-correlation algorithm requires a certain judgment interval, the pixel resolution of the vector is much larger than 1 pixel. Although the pixel resolution of the vector can be improved by increasing the overlap rate of the judgment area, the improvement effect is limited.

[0045] 2. Step 2: In a continuously operating hypersonic wind tunnel, use infrared thermal imaging technology to obtain the wall temperature distribution T w (x,z), z represents the spanwise coordinate of the plate model. The specific requirements for obtaining the surface temperature of the plate are as follows:

[0046] The model is recommended to use an approximate insulating material such as PEEK. In a hypersonic wind tunnel that can operate continuously, select infrared thermal image data after the wind tunnel has been running for a period of time and the model has reached temperature equilibrium, and consider the model surface reflectivity for temperature correction. For environments where the model predicts that the temperature is not high (within 100°C), the insulating tape method can be used, that is, in a calibration environment, a piece of insulating tape with a known emissivity is attached to the model surface for a certain period of time, and then the emissivity of the infrared thermal imager is adjusted to make the surface temperature of the model being tested the same or similar to the surface temperature of the insulating tape. At this time, the thermal imager emissivity is considered to be the correct emissivity of the model being tested. Under this parameter, the surface temperature distribution T of the test model is measured. w (x,z) are considered to be measured values ​​within the allowable error range.

[0047] 3. Step 3: Calculate the displacement thickness and momentum loss thickness using the velocity data and wall temperature data at a specific flow direction station. The specific implementation is as follows:

[0048] (1) The temperature distribution in the turbulent boundary layer is calculated using the velocity model and wall temperature.

[0049] The high-resolution boundary layer space velocity field has been obtained above. However, since the velocity field measured by PIV cannot reach the time resolution, the stream velocity u at a specific stream position x0 is selected here. y x=x0 to average a large number of time series samples As an approximation of the time average The time-averaged velocity of the stream at the boundary layer thickness δ is definition is the time-averaged velocity in the boundary layer The average temperature at the location, is the time-averaged temperature at the boundary layer thickness δ, is the time-averaged wall temperature, The velocity field can be used to measure T at the cross section (z = z0) w (x,z) is obtained, is the recovery temperature, r is the rewarming factor, which is 0.89 for the compressible turbulent boundary layer, and the boundary layer temperature distribution is estimated using the Walz equation:

[0050]

[0051] in,

[0052]

[0053] T0 is the total temperature of the incoming flow, which is obtained by measuring the total temperature in the wind tunnel test. Ma is the Mach number of the incoming flow. ∞ is the static temperature of the incoming flow, which can be approximately considered γ is the specific heat ratio of air, and is generally taken as γ=1.4.

[0054] (2) Obtaining the density distribution using the assumption of perfect gas and the assumption of stable pressure in the boundary layer

[0055] After obtaining the boundary layer temperature distribution, through the perfect gas equation and the assumption of pressure approximation invariance, it can be considered that the pressure p is independent of y, so the boundary layer time-averaged velocity Time-averaged density at location Given by:

[0056]

[0057] Among them, ρ ∞ is the static density of the incoming flow, given by

[0058]

[0059] Among them, ρ0 is the total density of the incoming flow, P0 is the total pressure of the incoming flow, which is obtained by measuring the total pressure in the wind tunnel test, and R is the complete gas constant, which is taken as R=287.06J / (kg.K).

[0060] (3) The displacement thickness and momentum loss thickness are calculated using the calculation formulas for compressible flow displacement thickness and momentum loss thickness.

[0061] The required parameters are available, and the displacement thickness and momentum loss thickness in the boundary layer are calculated by the following integral formula:

[0062]

[0063] where δ * is the displacement thickness of the compressible boundary layer, θ is the momentum loss thickness of the compressible boundary layer, and the discreteness of the data needs to be considered during integration.

[0064] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

[0065] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A method for measuring displacement thickness and momentum loss thickness of a strongly compressible turbulent boundary layer, characterized in that: include: In a continuously operating hypersonic wind tunnel, the boundary layer velocity field distribution is obtained for the boundary layer flow field at a specific position of a flat plate model; For a specific position of the flat plate model, obtain the wall temperature distribution after the model reaches thermal equilibrium; By using the boundary layer velocity field distribution and wall temperature distribution at a specific flow direction station, the temperature distribution and density distribution along the normal direction of the flow in the boundary layer are obtained by introducing relevant assumptions, and then the displacement thickness and momentum loss thickness are calculated based on the temperature distribution and density distribution.

2. A method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 1, characterized in that: The step of obtaining the boundary layer velocity field distribution comprises: (1) Using a particle generator to inject tracer particles of a specific size into the wind tunnel flow field; (2) Selecting a dual-pulse laser light source and a high-resolution cross-frame camera for flow field illumination and image acquisition; (3) The particle images obtained by the acquisition are used to analyze the instantaneous velocity field (u) of the boundary layer. x,y ,v x,y ) calculation to obtain the boundary layer velocity field distribution.

3. The method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 2, characterized in that: The TiO2 particles in the boundary layer flow field meet a delay time of less than 1 microsecond.

4. The method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 2, characterized in that: The number of velocity vectors in the normal direction of the inner wall surface of the boundary layer thickness is not less than 20.

5. A method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 4, characterized in that: The corresponding cross-frame CCD camera selected meets the requirements that the resolution in the normal direction of the wall is not less than 2000 pixels and the focal length of the lens is f=200mm. In the normal direction, the optical axis of the camera and lens optical system is parallel to the surface of the flat plate model and just passes through the surface of the flat plate model, so that the particle image only occupies 1 / 2 of the height of the CCD camera frame.

6. The method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 1, characterized in that: The temperature distribution is Calculate; where, is the time-averaged velocity in the boundary layer The average temperature at the location, is the time-averaged wall temperature, is the time-averaged temperature at the boundary layer thickness δ, To restore the temperature, is the time average of the streamwise velocity at a specific streamwise position in the boundary layer, is the time-averaged velocity of the stream at the boundary layer thickness δ.

7. The method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 1, characterized in that: The density distribution is given by Calculate; where, is the time-averaged velocity of the boundary layer The time-averaged density at the location, ρ ∞ is the static density of the incoming flow, T is the time-averaged velocity in the boundary layer The time-averaged temperature at the location, T ∞ For the incoming flow to be quiet and warm.

8. The method for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer according to claim 1, characterized in that: The displacement thickness and momentum loss thickness are respectively Among them, δ * is the displacement thickness of the compressible boundary layer, θ is the momentum loss thickness of the compressible boundary layer, δ is the boundary layer thickness, is the time-averaged velocity of the boundary layer The time-averaged density at the location, ρ ∞ is the static density of the incoming flow, is the time average of the streamwise velocity at a specific streamwise position in the boundary layer, is the time-averaged velocity of the stream at the boundary layer thickness δ.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

10. A device for measuring displacement thickness and momentum loss thickness of a highly compressible turbulent boundary layer, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 8 are implemented.

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