Plasma wind tunnel flow field structure measuring device and method

Abstract

The invention discloses a plasma wind tunnel flow field structure measuring device and method, and the method comprises the steps: taking laser speckles generated by a high-energy pulse laser as a background, respectively collecting background images before and during the operation of a flow field, enabling the non-uniform density distribution of the flow field during the operation of the flow field to cause the deflection of light, enabling the laser speckles in the background images to generate displacement, and carrying out the measurement of the structure of the flow field. And the displacement is calculated by using an optical flow algorithm, so that the flow structure distribution of the flow field can be reflected. The flow field measurement sensitivity is effectively improved, and the method can be suitable for flow structure measurement of a plasma wind tunnel ultra-low-density flow field.

Description

Technical Field

[0001] The invention belongs to the field of aerospace experimental technology, and in particular relates to a plasma wind tunnel flow field structure measurement device and method. Background Art

[0002] During the atmospheric reentry of aerospace vehicles, the surrounding air is extremely heated. The high temperature causes the vibration energy of air molecules to be excited, the molecules to dissociate, and even atomic ionization occurs, and the air becomes a thermochemical reaction gas with nonlinear, non-equilibrium, and multi-scale characteristics. At present, there is still a lack of mathematical physics equations with sufficient accuracy to describe it. Therefore, ground tests of high-enthalpy equipment are still the main research method for high-enthalpy flow field simulation. Plasma wind tunnels are important high-enthalpy simulation equipment and are widely used in thermal protection systems and material performance assessment tests. However, in general, the focus of plasma wind tunnel simulation is the enthalpy value of the flow field, and the simulation ability of the flow field structure characteristics is relatively weak. Moreover, due to the extreme flow field conditions of plasma wind tunnels, such as extremely high total temperature, extremely low density, complex gas components, strong self-luminescence, and strong electromagnetic interference, the requirements for experimental testing technology are very high. In conventional wind tunnels, shadow or schlieren methods are usually used to measure flow field structures, but the shadow or schlieren methods have insufficient measurement sensitivity and high requirements for the measurement optical environment, and cannot be applied to plasma wind tunnels.

[0003] With the deepening of thermal protection research, the requirements for the quality of plasma wind tunnel flow fields are getting higher and higher, and more attention is paid to the details of the flow field structure. Therefore, the demand for flow display and measurement technology suitable for plasma wind tunnels is very urgent. There is an urgent need for a flow structure measurement method with high measurement sensitivity, simple and stable structure, which is suitable for the extremely low density flow field of plasma wind tunnels. Summary of the invention

[0004] The technical problem solved by the present invention is: in view of the problem that the existing technology cannot measure the flow structure of the extremely low-density flow field of a plasma wind tunnel, a plasma wind tunnel flow field structure measurement device and method are provided, which have high measurement sensitivity and anti-interference ability and can obtain the flow structure distribution of the extremely low-density flow field of a plasma wind tunnel.

[0005] The object of the present invention is achieved by the following technical solutions: A plasma wind tunnel flow field structure measurement device, comprising a high-energy pulse laser, a light guide arm, a convex lens, a background plate, an optical window, a filter, a high-speed camera and a synchronous controller;

[0006] The high-energy pulse laser beam emitted by the high-energy pulse laser is guided by the light guide arm and then emitted, and forms divergent surface light after passing through the convex lens. The surface light is reflected on the background plate to form laser speckle with uneven brightness distribution. The light speckle passes through the optical window and the test flow field in turn, and then is filtered by the filter to filter the flow field self-luminescence and stray light interference. Finally, the laser speckle image on the background plate is photographed by a high-speed camera to obtain the measurement result. The high-energy pulse laser and the high-speed camera are connected through a synchronous controller to control the synchronization of the laser light emission and the camera shutter time.

[0007] The high-energy pulse laser has a wavelength of 532 nm, a pulse width of 10 ns, and a maximum pulse energy greater than 500 mJ.

[0008] The light guide arm is connected to a high-energy pulse laser, so that the laser beam is emitted at the end after multiple total reflections inside the light guide arm. The emitted laser beam has a diameter D L <5mm; adjust the position and angle of the end of the light guide arm so that the angle between the laser beam and the background plate is not less than 45°.

[0009] The diameter of the convex lens is D c =1 / 2~1D L , focal length is f c = 5~10mm, the laser beam forms a divergent surface light after passing through, the diameter of the surface light is D m =L1D L / f c , L1 is the distance from the convex lens to the background plate.

[0010] The background plate is a metal plate with a surface roughness of Ra=6.4-12.8. The distance from the center of the test flow field is L2 = 0.5 ~ 1.5m; the laser speckle size formed on the background plate is expressed as

[0011]

[0012] Where λ is the laser wavelength, f L is the focal length of the lens, and D is the exit pupil of the imaging system.

[0013] The optical windows are two pieces of optical glass, which are respectively installed on both sides of the test section where the test flow field is located. The optical glass material is quartz, and the light transmittance is >90%; the filter is a 532±5nm bandpass filter.

[0014] The test flow field is a plasma flow field generated by a plasma wind tunnel, and the flow field density is 10 -4 kg / m 3 Magnitude.

[0015] The resolution of the high-speed camera is greater than 2000×2000, the image exposure time is less than 10μs, the distance between the high-speed camera and the test flow field center is L3=0.5-1.5m; the focal length of the lens meets When the best imaging resolution is achieved, D c The size of the high-speed camera imaging chip.

[0016] The synchronous controller ensures that the shutter of the high-speed camera is synchronized with the light emission time of the high-energy pulse laser, and the control signal delay is less than 1ns.

[0017] A method for measuring the flow field structure of a plasma wind tunnel using the device comprises:

[0018] Before the experiment, turn on the high-energy pulse laser and high-speed camera. The high-energy pulse laser emitted by the high-energy pulse laser is emitted through the light guide arm and then expanded into surface light after passing through the convex lens. The surface light is shone on the background plate to form laser speckle. The high-speed camera takes the image of the laser speckle as a reference image.

[0019] The wind tunnel is turned on to generate a plasma flow field. The laser speckle light is deflected after passing through the plasma flow field. A high-speed camera is used to capture the laser speckle image at this time as a test image.

[0020] After closing the wind tunnel, the optical flow algorithm is used to calculate the displacement of the laser speckle in the test image relative to the reference image, and the image displacement distribution cloud map is obtained, that is, the flow field structure of the plasma is obtained.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention uses the laser speckle generated by a high-energy pulsed laser as the background, collects background images before and during the flow field operation, and uses the optical flow algorithm to calculate the position offset of the laser speckle in the background image caused by the uneven density distribution of the flow field, thereby inferring the flow structure distribution of the flow field. The present invention overcomes the limitations of extreme conditions such as extremely low density and strong self-luminescence of plasma wind tunnels, effectively improves the sensitivity of flow field measurement, and makes up for the problem that plasma wind tunnels could not measure flow field structures in the past, and realizes flow structure measurement that can be applied to extremely low-density flow fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a plasma wind tunnel flow field structure measurement device provided by the present invention;

[0024] Figure 2 A schematic diagram of the principle of laser speckle formation provided by the present invention;

[0025] Figure 3 The laser speckle background image provided by the present invention;

[0026] Figure 4 A flow chart of calculating the displacement of a background image using the optical flow algorithm provided by the present invention;

[0027] Figure 5 This is a flow chart of the plasma wind tunnel flow field structure measurement method provided by the present invention. DETAILED DESCRIPTION

[0028] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings.

[0029] Figure 1 The present invention provides a schematic diagram of a plasma wind tunnel flow field structure measurement device. The device is composed of a high-energy pulse laser 1, a light guide arm 2, a convex lens 3, a background plate 4, an optical window 6, a filter 7, a high-speed camera 8 and a synchronous controller 9. The high-energy pulse laser beam emitted by the high-energy pulse laser 1 is guided by the light guide arm 2 and then emitted. After passing through the convex lens 3, it forms a divergent surface light. The surface light is reflected on the background plate 4 to form a laser speckle with uneven brightness distribution. After passing through the optical window 5 and the test flow field 6, it is filtered by the filter 7 to filter the flow field self-luminescence and stray light interference. Finally, the laser speckle image on the background plate 4 is photographed by the high-speed camera 8. The high-energy pulse laser 1 and the high-speed camera 8 are connected through the synchronous controller 9 to control the synchronization of the laser light emission and the camera shutter time.

[0030] The high-energy pulse laser 1 has a wavelength of 532nm, a pulse width of 10ns, and a maximum pulse energy of more than 500mJ. Due to the monochromaticity of the laser, the use of a filter of the corresponding wavelength can effectively shield the flow field self-luminescence; the extremely narrow pulse width can greatly reduce the integration time of imaging and reduce stray light interference; the high pulse energy can significantly improve the brightness and contrast of the background image, which is beneficial to the background displacement calculation.

[0031] The light guide arm 2 is connected to the high energy pulse laser 1, so that the laser beam is emitted at the end after multiple total reflections inside the light guide arm. The emitted laser beam has a diameter D L <5mm. Adjust the position and angle of the end of the light guide arm so that the angle between the laser beam and the background plate 4 is not less than 45°.

[0032] The diameter of the convex lens 3 is D c =1 / 2~1D L , focal length is f c = 5~10mm, the laser beam forms a divergent surface light after passing through, the diameter of the surface light is D m =L1D L / f c , L1 is the distance from the convex lens to the background plate.

[0033] The background plate 4 is a metal plate with a surface roughness of Ra = 6.4 to 12.8. The distance from the center of the test flow field 6 is L2 = 0.5 ~ 1.5m.

[0034] The optical windows 5 are two pieces of optical glass, which are respectively installed on both sides of the test section. The optical glass material is quartz, and the light transmittance is greater than 90%.

[0035] The test flow field 6 is a plasma flow field generated by a plasma wind tunnel, and the flow field density is about 10 -4 kg / m 3 Magnitude.

[0036] The bandpass of the above-mentioned filter 7 is 532±5nm, which can filter out almost all light waves of other bands except the laser, and can effectively shield the self-luminous interference generated by the test flow field.

[0037] The high-speed camera 8 has a resolution greater than 2000×2000, an image exposure time <10μs, a distance of L3=0.5-1.5m from the center of the test flow field 6, and a lens focal length that satisfies When the optimal imaging resolution is achieved, D c The aperture is the size of the imaging chip of a high-speed camera. The smaller the aperture, the better while ensuring the brightness of the image.

[0038] The above-mentioned synchronization controller 9 ensures that the shutter of the high-speed camera 8 is synchronized with the light emission time of the high-energy pulse laser 1, and the control signal delay is <1ns.

[0039] Figure 2 The schematic diagram of the principle of laser speckle formation is that the laser emitted by the high-energy pulse laser 1 is coherent light. When the coherent light hits the surface of a rough object, the uneven surface reflects the light in all directions like countless small mirrors, so that bright spots are generated where the light converges and dark spots are generated where the light disperses. After passing through the lens, a speckle image is formed on the imaging surface. Due to the randomness of the surface distribution of the object, the generated background speckle image is also completely randomly distributed.

[0040] The imaging size of laser speckle can be expressed as:

[0041]

[0042] Where λ is the laser wavelength, f L is the focal length of the lens, and D is the exit pupil of the imaging system.

[0043] Figure 3 This is a laser speckle background image, which is generated by the above-mentioned device. The image is covered with randomly distributed laser speckles, and the diameter of the laser speckles is about 6 to 8 pixels.

[0044] Figure 4Flowchart for calculating the displacement of the background image using the optical flow algorithm. Take an image before the test flow field is run as a reference image, and take an image during the test flow field as a test image. Calculate the partial derivatives of the image grayscale value in the three directions of x, y, and t, and then set the size and step size of the calculation window. Assume that the optical flow of all pixels in the window is the same. Solve the basic optical flow equations to get the optical flow value in the window. Then move the window position according to the step size, calculate the optical flow value of the next window, and traverse the entire image to get the optical flow distribution of the entire image. Determine whether the result meets the accuracy requirements. If not, reset the window size and step size until the result meets the requirements. In this way, the optical flow distribution of the laser speckle background image can be obtained, which reflects the flow structure distribution of the test flow field.

[0045] Figure 5 The flow chart of the plasma wind tunnel flow field structure measurement method provided by the present invention is as follows: before the test begins, the high-energy pulse laser 1 and the high-speed camera 8 are turned on first. The high-energy pulse laser emitted by the high-energy pulse laser 1 is emitted through the light guide arm 2, and then expanded into surface light after passing through the convex lens 3. The surface light is shone on the background plate 4 to form laser speckle, and the high-speed camera 8 takes the image of the laser speckle as a reference image. Then, the wind tunnel is turned on to generate a plasma flow field. The light of the laser speckle is deflected after passing through the plasma flow field. The laser speckle image at this time is taken by the high-speed camera 8 as a test image. After closing the wind tunnel, the optical flow algorithm is used to calculate the displacement of the laser speckle in the test image relative to the reference image, and the image displacement distribution cloud map is obtained, and the flow field structure of the plasma can be obtained.

[0046] The present invention uses the laser speckle generated by a high-energy pulsed laser as the background, collects background images before and during the flow field operation, and uses the optical flow algorithm to calculate the position offset of the laser speckle in the background image caused by the uneven density distribution of the flow field, thereby inferring the flow structure distribution of the flow field. The present invention overcomes the limitations of extreme conditions such as extremely low density and strong self-luminescence of plasma wind tunnels, effectively improves the sensitivity of flow field measurement, and makes up for the problem that plasma wind tunnels could not measure flow field structures in the past, and realizes flow structure measurement that can be applied to extremely low-density flow fields.

[0047] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A plasma wind tunnel flow field structure measurement device, characterized in that: It includes a high-energy pulse laser, a light-guiding arm, a convex lens, a background plate, an optical window, a filter, a high-speed camera and a synchronous controller; The high-energy pulse laser beam emitted by the high-energy pulse laser is guided by the light guide arm and then emitted, and forms divergent surface light after passing through the convex lens. The surface light is reflected on the background plate to form laser speckle with uneven brightness distribution. The light speckle passes through the optical window and the test flow field in turn, and then is filtered by the filter to filter the flow field self-luminescence and stray light interference. Finally, the laser speckle image on the background plate is photographed by a high-speed camera to obtain the measurement result. The high-energy pulse laser and the high-speed camera are connected through a synchronous controller to control the synchronization of the laser light emission and the camera shutter time.

2. A plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The high-energy pulse laser has a wavelength of 532 nm, a pulse width of 10 ns, and a maximum pulse energy greater than 500 mJ.

3. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The light guide arm is connected to a high-energy pulse laser, so that the laser beam is emitted at the end after multiple total reflections inside the light guide arm. The emitted laser beam has a diameter D L <5mm; adjust the position and angle of the end of the light guide arm so that the angle between the laser beam and the background plate is not less than 45°.

4. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The diameter of the convex lens is D c =1 / 2~1D L , focal length is f c = 5~10mm, the laser beam forms a divergent surface light after passing through, the diameter of the surface light is D m =L1D L / f c , L1 is the distance from the convex lens to the background plate.

5. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The background plate is a metal plate with a surface roughness of Ra=6.4-12.

8. The distance from the center of the test flow field is L2 = 0.5 ~ 1.5m; the laser speckle size formed on the background plate is expressed as Where λ is the laser wavelength, f L is the focal length of the lens, and D is the exit pupil of the imaging system.

6. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The optical windows are two pieces of optical glass, which are respectively installed on both sides of the test section where the test flow field is located. The optical glass material is quartz, and the light transmittance is >90%; the filter is a 532±5nm bandpass filter.

7. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The test flow field is a plasma flow field generated by a plasma wind tunnel, and the flow field density is 10 -4 kg / m 3 Magnitude.

8. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The resolution of the high-speed camera is greater than 2000×2000, the image exposure time is less than 10μs, the distance between the high-speed camera and the test flow field center is L3=0.5-1.5m; the focal length of the lens meets When the best imaging resolution is achieved, D c The size of the high-speed camera imaging chip.

9. The plasma wind tunnel flow field structure measurement device according to claim 1, characterized in that: The synchronous controller ensures that the shutter of the high-speed camera is synchronized with the light emission time of the high-energy pulse laser, and the control signal delay is less than 1ns.

10. A method for measuring the flow field structure of a plasma wind tunnel using the device of claim 1, characterized in that: include: Before the experiment, turn on the high-energy pulse laser and high-speed camera. The high-energy pulse laser emitted by the high-energy pulse laser is emitted through the light guide arm and then expanded into surface light after passing through the convex lens. The surface light is shone on the background plate to form laser speckle. The high-speed camera takes the image of the laser speckle as a reference image. The wind tunnel is turned on to generate a plasma flow field. The laser speckle light is deflected after passing through the plasma flow field. A high-speed camera is used to capture the laser speckle image at this time as a test image. After closing the wind tunnel, the optical flow algorithm is used to calculate the displacement of the laser speckle in the test image relative to the reference image, and the image displacement distribution cloud map is obtained, that is, the flow field structure of the plasma is obtained.

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