A time-resolved polarimetric imaging apparatus and method for a wind tunnel flow field

Abstract

The application provides a time-resolved polarization imaging device and method for a wind tunnel flow field, which comprises the following steps: calibrating each polarization camera of a polarization camera array to obtain calibration parameters; determining the arrangement mode of the polarization camera array; performing a water tank calibration experiment on the arranged polarization camera array to correct the polarization signals obtained by each polarization camera at different imaging angles; installing the polarization camera array after the water tank calibration experiment to a wind tunnel measurement site to obtain calibration plate images and pre-shooting original images; performing image processing on the pre-shooting original images according to the calibration parameters and the calibration plate images to obtain pre-shooting results; evaluating whether the polarization camera array meets the measurement consistency requirement according to the pre-shooting results, and if yes, obtaining time-resolved polarization images of the wind tunnel flow field by using the polarization camera array. The time-resolved polarization images obtained by the application can be quantitatively analyzed, have strong anti-interference ability, and can provide accurate flow field information.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wind tunnel flow field measurement, in particular to a wind tunnel flow field time-resolved polarization imaging device and method. BACKGROUND

[0002] The wind tunnel is an important equipment for conducting ground test research of aircraft. In the wind tunnel, the aircraft aerodynamic performance research is usually needed to visualize and image the wind tunnel flow field simulating the aircraft flight environment, and to study the interaction between the aircraft and the surrounding flow field. The time-resolved flow field imaging technology refers to applying the flow field imaging method to shoot a series of flow field images with time correlation, using the time-resolved flow field images to analyze the change process of the flow field with time, and further studying the time-varying law of the aircraft aerodynamic characteristics. However, this technology is based on light intensity imaging. Since the light intensity signal is easily disturbed by the background and is difficult to quantify, the time-resolved flow field images not only have signal changes caused by the change of the flow field itself, but also have signal errors caused by problems such as uneven light intensity and imaging angle difference, which is not conducive to the quantitative analysis and processing of the time-resolved flow field images.

[0003] The polarization imaging technology is a new cross-discipline developed rapidly by combining traditional detection technology with polarization optical principles. It uses polarization information (polarization degree, polarization angle, etc.) instead of light intensity information, which can make up for the shortcomings of traditional light intensity visualization technology, and has broad application prospects and development potential. However, using a single polarization imaging system to continuously shoot sequential time-resolved images cannot meet the requirements of wind tunnel flow field imaging. The wind tunnel flow field (especially the high-speed wind tunnel flow field) changes very quickly with time, so the time-resolved imaging measurement must be completed in a very short time interval. On the one hand, the existing mature polarization imaging products cannot meet the requirements of time-resolved testing in terms of imaging rate. On the other hand, from the quantitative analysis point of view, the time-resolved imaging results should only be affected by the change of the flow field itself, but the existing time-resolved testing technology usually uses a multi-camera array shooting method, which has not solved the problems of imaging angle difference and inconsistency of polarization signals at different observation angles. Therefore, there is an urgent need for a method and system that can perform sequential polarization imaging for time-resolved wind tunnel flow field. SUMMARY

[0004] In view of the above shortcomings of the prior art, the present application provides a wind tunnel flow field time-resolved polarization imaging device and method, which effectively solves the problem that the existing polarization imaging technology cannot meet the requirements of time-resolved testing in terms of imaging rate in the wind tunnel time-resolved flow field imaging, and also solves the problem that the imaging angle difference and inconsistency of polarization signals at different observation angles cause the quantitative analysis to be impossible.

[0005] In a first aspect, the present application provides a time-resolved polarization imaging device for a wind tunnel flow field, comprising: a wind tunnel, a particle generator, a multi-cavity laser, a synchronization controller, a control system, and a polarization camera array, wherein the control system is connected to the synchronization controller and the polarization camera array, and the synchronization controller is connected to the multi-cavity laser and the polarization camera array.

[0006] The particle generator is configured to provide particles as tracers to the wind tunnel.

[0007] The multi-cavity laser is configured to sequentially illuminate the wind tunnel flow field according to a first working time sequence.

[0008] The polarization camera array is configured to sequentially image to obtain time sequence images according to a second working time sequence, and the polarization camera array comprises a plurality of split-focus plane polarization cameras, each of which is provided with a translation adjustment mechanism in three directions of X-axis, Y-axis and Z-axis, and a fine adjustment mechanism in three angular directions of pitch, roll and yaw, and each of which is equipped with an imaging lens of the same specification, and a tilt-shift (TS) structure is arranged between the split-focus plane polarization camera and the imaging lens.

[0009] The control system and the synchronization controller are configured to control the cooperative work of the multi-cavity laser and the polarization camera array.

[0010] In a second aspect, the present application provides a time-resolved polarization imaging method for a wind tunnel flow field, which is applied to the time-resolved polarization imaging device for a wind tunnel flow field according to the first aspect of the present application, and the method comprises the following steps:

[0011] Calibrating each polarization camera of the polarization camera array to obtain calibration parameters of each polarization camera.

[0012] Determining the arrangement mode of the polarization camera array according to the size of the experimental section of the wind tunnel and the flow field region to be measured.

[0013] Performing a water tank calibration experiment on the arranged polarization camera array to correct the polarization signals obtained by each polarization camera at different imaging angles.

[0014] Installing the polarization camera array after the water tank calibration experiment to the wind tunnel measurement site to obtain calibration plate images and pre-shooting original images of each polarization camera.

[0015] Performing camera error correction, polarization information calculation and image distortion correction processing on the pre-shooting original images according to the calibration parameters and the calibration plate images to obtain pre-shooting results.

[0016] According to the pre-shooting result, whether the polarization camera array shooting the same target information meets the measurement consistency requirement is evaluated, if not, the above steps are repeated, if yes, the time-resolved polarization image of the wind tunnel flow field is acquired by using the polarization camera array.

[0017] Further, each polarization camera of the polarization camera array is calibrated to obtain the calibration parameters of each polarization camera, specifically including:

[0018] A standard light source is used as input, and by measuring the output of each polarization camera under the standard light source signal condition, the calibration parameters of each polarization camera are obtained;

[0019] According to the standard light source and the calibration parameters, the calibration effect of each polarization camera is verified to determine the qualified polarization camera.

[0020] Further, the calibration parameters include micro-polarization sheet angle correction value, response correction coefficient and response offset.

[0021] Further, the arrangement mode of the polarization camera array includes: multiple polarization cameras arranged side by side, multiple polarization cameras arranged in two rows, and multiple polarization cameras arranged symmetrically on both sides of the wind tunnel test section.

[0022] Further, the polarization camera array after arrangement is subjected to a water tank calibration experiment to correct the polarization signals obtained by each polarization camera at different imaging angles, specifically including:

[0023] According to the state parameters of the tracer particles in the wind tunnel flow field, the state parameters of the tracer particles in the water tank are set;

[0024] The position of the laser sheet light illuminating the water tank, the position of the polarization camera array from the laser sheet light, and the shooting position of the polarization camera array are adjusted;

[0025] The polarization information of the scattering light of the tracer particles with known parameters under laser illumination at the same time is measured by the polarization camera array, and each polarization camera obtains the polarization signals of the same target at different angles;

[0026] According to the polarization signal difference and camera angle of each polarization camera, the polarization signal is corrected.

[0027] Further, after the water tank calibration experiment, the polarization camera array is installed to the wind tunnel measurement site to obtain the calibration plate image and the pre-shooting original image of each polarization camera, specifically including:

[0028] Adjusting the position of the laser sheet light of the multi-cavity laser illuminating the flow field of the wind tunnel, the position of the polarized camera array from the laser sheet light, and the position of the polarized camera array shooting position consistent with the corresponding position in the water tank calibration experiment.

[0029] Placing a calibration board for different polarized camera angle correction in the shooting area to obtain a calibration board image.

[0030] Running the wind tunnel and the time-resolved polarized imaging device to shoot the flow field image at the same laser sheet light illumination moment, and obtaining the pre-shooting original image of each polarized camera.

[0031] Further, the pre-shooting original image is corrected for camera error, polarized information is calculated, and image distortion correction processing is performed according to the calibration parameters and the calibration board image to obtain a pre-shooting result, specifically including:

[0032] The pre-shooting original image is corrected using the calibration parameters to eliminate the system error of each polarized camera.

[0033] The polarized information of the image after the system error of the polarized camera is eliminated is calculated according to the focal plane polarized imaging method, and the image after the polarized information is calculated is subjected to noise reduction processing.

[0034] The image after the noise reduction processing is corrected for spatial position distortion using the calibration board image to obtain the pre-shooting result of each polarized camera.

[0035] Further, the system error of the polarized camera includes a micro-polarization plate angle error, a sensor response error, and an offset error.

[0036] Further, whether the polarized camera array shooting the same target information meets the measurement consistency requirement is evaluated according to the pre-shooting result using an image consistency processing method, and the image consistency processing method includes one or more of a mean square error algorithm, a peak signal-to-noise ratio algorithm, and a cosine similarity algorithm.

[0037] The time-resolved polarized imaging device for the flow field of the wind tunnel provided by the application uses a focal plane polarized camera array to replace a common light intensity imaging camera array, and a high-precision synchronous controller is used to realize the cooperative work of each system. The polarized camera array can be freely and accurately adjusted. When the object plane and the image plane are not parallel, a tilt-shift TS structure is used to realize clear focusing of the entire imaging plane. The time-resolved polarized image obtained by the device can be quantitatively analyzed, and has strong anti-interference ability. When the time-resolved polarized image is subjected to image quantitative analysis, calculation, and machine learning, it can provide more accurate flow field information than traditional light intensity images. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some of the embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0039] Figure 1 is a schematic diagram of a time-resolved polarization imaging device for a wind tunnel flow field provided by an embodiment of the present application;

[0040] Figure 2 is a schematic diagram of a parallel layout of multiple polarization cameras provided by an embodiment of the present application;

[0041] Figure 3 is a comparison schematic diagram of the focusing effect of the time-resolved polarization imaging device provided by an embodiment of the present application on the entire imaging plane;

[0042] Figure 4 is a working timing diagram of the time-resolved polarization imaging device provided by an embodiment of the present application;

[0043] Figure 5 is a time-resolved shock wave flow field intensity image obtained by a conventional method provided by an embodiment of the present application;

[0044] Figure 6 is a time-resolved shock wave flow field polarization image obtained by a polarization imaging method provided by an embodiment of the present application;

[0045] Figure 7 is a same-time shock wave flow field intensity image obtained by a conventional camera array provided by an embodiment of the present application;

[0046] Figure 8 is a same-time shock wave flow field intensity image obtained by a polarization camera array provided by an embodiment of the present application. DETAILED DESCRIPTION

[0047] The embodiments of the present application will be described in detail below, and examples of the embodiments are shown in the drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present application, and should not be understood as a limitation on the present application.

[0048] It is to be understood that where an element such as a layer, region or substrate is described as being "on" another element, it can be directly on the other element or intervening elements can also be present. Where an element such as a layer, region or substrate is referred to as being "connected" to or "coupled" to another element, it can be directly connected to the other element or intervening elements can also be present. In contrast, where an element is referred to as being "directly on" or "directly connected to" to another element, there are no intervening elements present. It will be appreciated that for the purposes of this specification, the terms "on" and "directly on" are used in the same manner as "connected" and "directly connected" unless explicitly stated otherwise. The terms "vertical", "horizontal", "left", "right" and similar terms are used for explanation only and not to be understood as limiting.

[0049] In the present application, unless specifically defined otherwise, the terms "mounting", "connected", "connecting", "fixed", and the like are to be construed in a broad sense, for example, can be fixedly connected, or can be detachably connected, or can be integrated; can be mechanically connected, or can be electrically connected; can be directly connected, or can be indirectly connected through an intermediate medium; can be internal communication of two elements or interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0050] In addition, the terms "first", "second", etc. are used only for descriptive purposes and should not be construed as indicating or implying relative importance or an implied indication of the number of technical features indicated. Therefore, the features defined as "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless specifically defined otherwise.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the template herein is only for the purpose of describing specific embodiments and is not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0052] At present, a single polarization imaging system is used to continuously shoot a sequence of time-resolved images, which cannot meet the demand of imaging the flow field of a wind tunnel. The flow field of a wind tunnel, especially the flow field of a high-speed wind tunnel, changes very quickly over time, and the time-resolved imaging measurement must be completed within a very short time interval. On the one hand, the existing mature polarization imaging products cannot keep up with the requirements of time-resolved testing in terms of imaging rate. On the other hand, from the perspective of quantitative analysis, the time-resolved imaging results should not be disturbed by other factors except the change of the flow field itself. However, the existing time-resolved testing technology usually uses a multi-camera array shooting method, and the problems such as imaging angle difference, consistency of polarization signals at different observation angles, etc. have not been solved. The present application provides a time-resolved polarization imaging device and method for the flow field of a wind tunnel, which effectively solves the above problems of the existing polarization imaging technology in the time-resolved flow field imaging of a wind tunnel.

[0053] Embodiment one

[0054] The embodiment of the present application provides a time-resolved polarization imaging device of a wind tunnel flow field, Figure 1 is a schematic diagram of a time-resolved polarization imaging device of a wind tunnel flow field provided by the embodiment of the present application, as shown in the figure, the device comprises a wind tunnel, a particle generator, a multi-cavity laser, a synchronization controller, a control system and a polarization camera array. Figure 1

[0055] In the embodiment of the present application, the polarization camera array is composed of eight split focal plane polarization cameras arranged side by side, each split focal plane polarization camera has a small volume, and the space occupied by the polarization camera array is reduced. Each split focal plane polarization camera is equipped with a translation adjustment mechanism in X-axis, Y-axis and Z-axis directions and a fine adjustment mechanism in pitch, roll and yaw three angle directions, so that the 6-degree-of-freedom accurate adjustment of each split focal plane polarization camera can be realized. Meanwhile, each split focal plane polarization camera is equipped with imaging lenses of the same specification, so that the imaging error caused by the lenses is reduced.

[0056] Since the shooting area of the wind tunnel flow field is generally a plane, Figure 2 is a schematic diagram of parallel layout of multiple polarization cameras provided by the embodiment of the present application, as shown in the figure, Figure 2 in order to shoot the same area, each camera in the polarization camera array has a certain angle with the shooting plane, and the polarization camera cannot realize complete clear focusing on the entire plane, so a TS structure with a tilt-shift (TS) function is installed between each polarization camera and the imaging lens, so that the tilt angle between the objective lens or the lens and the imaging plane is adjusted, so that the focal depth of the objective lens or the lens is expanded to the entire imaging plane. Figure 3 is a comparison schematic diagram of focusing effects of the time-resolved polarization imaging device on the entire imaging plane, wherein Figure 3 (a) in the figure is the focusing effect of the imaging plane of the polarization camera array without using the TS structure, Figure 3 (b) in the figure is the focusing effect of the imaging plane of the polarization camera array using the TS structure, and it is found through comparison that the TS structure can obtain clearer and more accurate images, and the difference is more obvious when imaging irregular-shaped objects or objects not on the same plane.

[0057] ​In the embodiment of the present application, the particle generator adopts a nano tracer particle generator, the multi-cavity laser adopts an eight-cavity pulsed laser, and the control system adopts a computer system, the computer system is connected with a synchronous controller and a polarization camera array, the synchronous controller is connected with the multi-cavity laser and the polarization camera array, so that the whole device works cooperatively through the high-precision synchronous controller. Figure 4 is the working timing diagram of the time-resolved polarization imaging device provided by the embodiment of the present application, as shown in Figure 4 the eight-cavity pulsed laser illuminates the flow field according to the specified first working timing in turn, and the polarization camera array acquires time sequence images according to the specified second working timing in turn, wherein Δt represents the interval time between the lasers, and the specific parameters of Δt can be determined according to the main flow velocity of the flow field to be measured.

[0058] Further, the embodiment of the present application can also adopt the way of a light splitting prism to split the imaging signal to each camera of the polarization camera array, and the way of the light splitting prism combined with the multi-camera can eliminate parallax and facilitate the calculation of polarization information, but a specially designed light splitting prism and imaging light path are required, and each polarization camera needs to have a precise positioning and adjusting structure.

[0059] The time-resolved polarization imaging device for the wind tunnel flow field provided by the embodiment of the present application adopts a focal plane polarization camera array to replace an ordinary light intensity imaging camera array, a high-precision synchronous controller is used to realize the cooperative work of the whole device, the cameras of the polarization camera array are placed in parallel, the structure is simple, and the cameras can be freely and accurately adjusted, when the object plane and the image plane are not parallel, a tilt-shift TS structure is used to realize clear focusing of the whole imaging plane.

[0060] Embodiment two

[0061] The embodiment of the present application provides a time-resolved polarization imaging method for a wind tunnel flow field, which is applied to the time-resolved polarization imaging device for the wind tunnel flow field in the embodiment one, and the method comprises the following steps:

[0062] Step S1, calibrate each polarization camera of the polarization camera array to obtain the calibration parameters of each polarization camera; specifically including:

[0063] Each polarization camera in the polarization camera array is calibrated to reduce the error of the camera itself to the polarization imaging. The related calibration method can use the current commonly used polarization camera calibration method. In the embodiment of the present application, a standard light source with known parameters is used as input, wherein the parameters are Stokes parameters, and the standard light source includes an integrating sphere light source, an integrating sphere light source plus a linear polarizer, etc. The calibration parameters of each polarization camera are obtained by measuring the output of the polarization camera under the standard light source signal condition, which include the micro-polarization plate angle correction value, the response correction coefficient and the response offset, etc.

[0064] The calibration effect of each polarization camera by the standard light source and the calibration parameters is further verified. Specifically, the calibration of the polarization camera is determined to be qualified or not by comparing the measurement result of the incident light source with known Stokes parameters with the theoretical value. Only when the calibration meets the requirements, the polarization camera calibration is considered to be qualified.

[0065] Step S2: Determine the arrangement mode of the polarization camera array according to the size of the experimental section of the wind tunnel and the flow field region to be measured.

[0066] According to the size of the experimental section of the experimental wind tunnel and the flow field region to be measured, the arrangement mode of the polarization camera array is determined, wherein the polarization cameras are all polarization cameras that have passed the camera calibration. If the shooting area is large, multiple polarization cameras can be arranged side by side. If the shooting area is small, multiple polarization cameras arranged side by side may result in a large angle between the polarization cameras on both sides and the imaging plane, thereby causing large imaging distortion, which is not conducive to the later image analysis and processing. At this time, the polarization cameras can be considered to be arranged side by side in an upper and lower arrangement mode. At the same time, in order to meet the imaging of the polarization camera array through the actual wind tunnel optical window to the wind tunnel flow field region, the polarization camera array can be arranged symmetrically on both sides of the experimental section of the wind tunnel, and the relative angle between each polarization camera is determined.

[0067] Step S3: Perform a water tank calibration experiment on the polarization camera array arranged, and correct the polarization signals obtained by each polarization camera at different imaging angles.

[0068] Experimental research has found that the polarization signals in the scattered light of group tracer particles are different at different angles in space, which leads to the fact that the polarization signals obtained by the same polarization camera at different angles for shooting the same target may not be consistent. In the traditional light intensity signal measurement, since the light intensity signal is difficult to quantify, only the spatial structure of the flow field reflected by the signal is concerned, and little attention is paid to whether the light intensity signals obtained by different cameras for shooting the same spatial position are consistent. However, the polarization imaging can quantify the imaging signal, so the influence of the shooting angle of different polarization cameras on the polarization imaging will affect the measurement accuracy of the polarization camera array for polarization imaging.

[0069] In order to solve the above problems, after the position of each polarization camera is determined according to the distribution mode of the plurality of polarization cameras in step S2, a water tank calibration experiment is performed on the current polarization camera array. The water tank calibration experiment is to add hydrophilic nano-scale tracer particles in the water tank to form a nano-particle aqueous solution. The type, size and concentration of the nano-particles are determined according to the type, size and concentration of the tracer particles in the wind tunnel flow field to be measured. The nano-particles in the water tank are used to simulate the nano-particles in the wind tunnel flow field, and the water medium replaces the air medium in the wind tunnel flow field. Except that the refractive index of water is different from that of air, which will affect the intensity of scattered light, it has no effect on the polarization signal. At the same time, a laser is used to illuminate the water tank, and the position of the laser sheet light to illuminate the water tank, the distance between the polarization camera array and the position of the laser sheet light, and the shooting position of the polarization camera array are adjusted and recorded to ensure the reliability of the calibration test results. The polarization information of the nano-tracer particle scattered light under laser illumination at the same time is measured by the polarization camera array. Each polarization camera obtains the polarization signal of the same target at different viewing angles. The polarization information of the nano-particle scattered light at the same time is consistent at the same spatial position. The difference between the polarization signals received by each polarization camera is the difference between the polarization signals at different viewing angles of the polarization camera. According to the difference between the polarization signals and the camera time, the polarization signal obtained by each polarization camera is corrected. Through the water tank calibration experiment, it can be ensured that the polarization information obtained by each polarization camera for the same scattered light polarization signal at the same time is consistent, thereby solving the measurement error of the polarization signal caused by the angle of the polarization camera.

[0070] Step S4, after the water tank calibration experiment, the polarization camera array is installed to the wind tunnel measurement site, and the calibration plate image and the pre-shooting original image of each polarization camera are obtained.

[0071] After the polarization camera array after the water tank calibration experiment is installed to the wind tunnel measurement site, the laser sheet light of the multi-cavity laser is adjusted at the wind tunnel flow field shooting position, and the distance between the polarization camera array and the position of the laser sheet light, the shooting position of the polarization camera array are consistent with the corresponding positions in the water tank calibration experiment. After the installation of each device of the device is completed, the calibration plate for different camera angle correction is placed in the corresponding shooting area, and the chessboard calibration plate is used in the embodiment of the application to obtain the calibration plate image and store it. Then the wind tunnel and each device of the device are run, and the pre-shooting measurement is carried out. Each polarization camera shoots the flow field image at the same laser sheet light illumination time, which is used to verify the reliability of the shooting method and the measurement system, so as to obtain the pre-shooting original image of each polarization camera in the polarization camera array.

[0072] Step S5, according to the calibration parameters in step S1 and the calibration plate image in step S4, the pre-shooting original image is corrected for camera error, polarization information is calculated and image distortion is corrected, and the pre-shooting result is obtained, which specifically includes:

[0073] First, the micro-polarizer angle correction value, the responsivity correction coefficient and the response offset obtained by calibrating the camera in step S1 are used to correct the pre-shooting original image, so as to eliminate the system errors of the micro-polarizer angle error, the sensor responsivity error and the offset error of each polarized camera.

[0074] Then, the polarized information of the image after eliminating the system error of the polarized camera is calculated according to the defocus plane polarized imaging method, and the image after calculating the polarized information is subjected to noise reduction processing. The noise reduction processing method includes one or more of the commonly used spatial filter noise reduction method, the noise reduction method based on the attention mechanism and the noise reduction method based on the deep learning network.

[0075] Finally, the image after noise reduction processing is subjected to spatial position distortion correction by using the calibration plate image, so that the images obtained by each camera are completely consistent in the physical space, and the pre-shooting result of each polarized camera is obtained.

[0076] Step S6: whether the pre-shooting result meets the measurement consistency requirement is evaluated according to the pre-shooting result, if not, the steps S1-S5 are repeated, if yes, the time-resolved polarized image of the wind tunnel flow field is obtained by using the polarized camera array.

[0077] According to the theoretical condition, the pre-shooting result is the flow field result at the same time, and the result should be consistent. The pre-shooting result is evaluated by using the image consistency processing method, and the image consistency processing algorithm includes one or more of the image analysis methods such as the mean square error algorithm MSE, the peak signal-to-noise ratio algorithm PSNR and the cosine similarity algorithm. The measurement consistency requirement of the mean square error algorithm MSE is that the measured MSE value is close to 0, and the specific numerical requirement is set according to the actual situation. The measurement consistency requirement of the peak signal-to-noise ratio algorithm PSNR is that the larger the measured PSNR value is, the better, and the specific numerical requirement is set according to the actual situation. The measurement consistency requirement of the cosine similarity algorithm is that the measured cosine similarity value is close to 1, and the specific numerical requirement is set according to the actual situation. If the pre-shooting result does not meet the measurement consistency requirement, the steps S1-S5 are repeated until the measurement consistency requirement is met. If the results of each polarized camera meet the measurement consistency requirement, the wind tunnel test measurement can be carried out, the time-resolved polarized image of the wind tunnel flow field is obtained and analyzed.

[0078] In order to verify the feasibility of the wind tunnel flow field time-resolved polarized imaging device and method provided by the present application, the time-resolved polarized image is obtained and analyzed by using the above device and method in the embodiment of the present application, and the results are as follows:

[0079] Figure 5is the time-resolved shock wave flow field light intensity image obtained by the traditional method provided by the embodiment of the application, wherein the time interval t of each image is 15 microseconds, and the time-resolved shock wave flow field light intensity image can be used for qualitative (naked eye subjective) analysis of the change process of the flow field over time. Figure 6 is the time-resolved shock wave flow field polarization image obtained by the polarization imaging method provided by the embodiment of the application, and the time interval t of each image is also 15 microseconds. The image can be quantitatively analyzed. Figure 5 and Figure 6 Analysis, the time-resolved polarization imaging technology of the embodiment of the application meets the requirements of the traditional time-resolved imaging technology, can obtain the time-resolved wind tunnel flow field image, the polarization image is a quantitative result, and does not rely on any artificial processing method, while the light intensity image cannot be quantified and can only directly display the corresponding light intensity gray value.

[0080] Meanwhile, 4 traditional cameras and 4 polarization cameras are used to image and measure the flow field at the same time, Figure 7 is the light intensity image of the shock wave flow field at the same time obtained by the traditional camera array provided by the embodiment of the application, Figure 8 is the light intensity image of the shock wave flow field at the same time obtained by the polarization camera array provided by the embodiment of the application. In theory, the flow field at the same time is consistent, so the images obtained by the polarization camera array should be consistent in theory. In the embodiment of the application, the mean square error algorithm MSE, the peak signal-to-noise ratio algorithm PSNR and the cosine similarity algorithm are used to process the image consistency of the images obtained by the traditional camera array and the polarization camera array. The image consistency analysis is to compare two images at a time, and the analysis results are shown in the following table:

[0081]

[0082] In the above table, the images obtained by the 4 cameras of the traditional camera array and the 4 cameras of the polarization camera array are respectively denoted as P1, P2, P3 and P4.

[0083] In the mean square error algorithm MSE, the smaller the value, the higher the similarity of the two images. In theory, the MSE value of two identical images is 0. In the peak signal-to-noise ratio algorithm PSNR, the larger the value, the higher the similarity of the two images. In theory, the PSNR value of two identical images is ∞. In the cosine similarity result, the larger the value, the higher the similarity of the two images. In theory, the cosine similarity value of two identical images is 1. From the analysis results, the consistency between the images of the polarization imaging of the same flow field at the same time is better than that of the traditional light intensity imaging results, which shows that the polarization imaging has more advantages in quantitative analysis and anti-interference.

[0084] In summary, the time-resolved polarization imaging device of the flow field of the wind tunnel provided by the application adopts a polarization camera array to replace a common light intensity imaging camera array, and high-precision synchronous controllers are used to realize the cooperative work of each system, the polarization camera array can be freely and accurately adjusted, when the object plane and the image plane are not parallel, a tilt-shift (TS) structure is used to realize clear focusing of the entire imaging plane. The time-resolved polarization image obtained by the device can be quantitatively analyzed, and the anti-interference ability is strong, and the time-resolved polarization image can provide more accurate flow field information than traditional light intensity images when performing image quantitative analysis, calculation and subsequent machine learning.

[0085] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. It is expressly understood that the embodiments described herein are merely possible examples of the application and are thus not limiting to the scope of the application.

[0086] The above-described embodiments only express several implementation manners of the application, and the description is more specific and detailed, but it should not be understood as the limitation to the patent scope of the application. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the application, and these all belong to the protection scope of the application. Therefore, the patent protection scope of the application should be subject to the appended claims.

[0087] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, and not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the application.

Claims

1. A method of time-resolved polarimetric imaging of a flow field in a wind tunnel, characterized in that, The method is applied to a time-resolved polarization imaging device of a wind tunnel flow field, and the time-resolved polarization imaging device of the wind tunnel flow field comprises a wind tunnel, a particle generator, a multi-cavity laser, a synchronous controller, a control system, and a polarization camera array, the control system is connected to the synchronous controller and the polarization camera array, and the synchronous controller is connected to the multi-cavity laser and the polarization camera array, wherein: The particle generator is used to provide particles as tracers to the wind tunnel; The multi-cavity laser is used to sequentially illuminate the wind tunnel flow field according to a first working time sequence; The polarization camera array is used to sequentially image to obtain time sequence images according to a second working time sequence, the polarization camera array comprises a plurality of split-focus plane polarization cameras, each of the split-focus plane polarization cameras is provided with a translation adjustment mechanism in three directions of X-axis, Y-axis and Z-axis, and is simultaneously provided with fine adjustment mechanisms in three angular directions of pitch, roll and yaw, and each of the split-focus plane polarization cameras is equipped with imaging lenses of the same specification, and a tilt-shift (TS) structure is arranged between the split-focus plane polarization cameras and the imaging lenses; The control system and the synchronous controller are used to control the cooperative work of the multi-cavity laser and the polarization camera array; The method comprises the following steps: S1, calibrating each polarization camera of the polarization camera array to obtain calibration parameters of each polarization camera; S2, determining an arrangement mode of the polarization camera array according to the size of an experimental section of the wind tunnel and a flow field region to be measured; S3, performing a water tank calibration experiment on the polarization camera array after arrangement to correct polarization signals obtained by each polarization camera at different imaging angles; S4, installing the polarization camera array after the water tank calibration experiment to a wind tunnel measurement site to obtain calibration plate images and pre-shooting original images of each polarization camera; S5, performing camera error correction, polarization information calculation and image distortion correction processing on the pre-shooting original images according to the calibration parameters and the calibration plate images to obtain pre-shooting results; S6, evaluating whether the polarization camera array satisfies the measurement consistency requirement for shooting the same target information according to the pre-shooting results, if not, repeating the steps S1-S5, and if yes, acquiring time-resolved polarization images of the wind tunnel flow field by using the polarization camera array; The water tank calibration experiment on the polarization camera array after arrangement to correct polarization signals obtained by each polarization camera at different imaging angles comprises: setting state parameters of tracer particles in the water tank according to state parameters of the tracer particles in the wind tunnel flow field; adjusting the position of laser sheet light of the multi-cavity laser for illuminating the water tank, the position of the polarization camera array relative to the position of the laser sheet light, and the shooting position of the polarization camera array; measuring polarization information of scattered light of the tracer particles with known parameters at the same time under laser illumination by the polarization camera array, and each polarization camera obtains polarization signals of the same target at different angles; correcting the polarization signals according to the differences in the polarization signals of each polarization camera and the camera angles.

2. The method of time-resolved polarimetric imaging of a flow field in a wind tunnel of claim 1, wherein, The calibration of each polarized camera in the polarized camera array is performed to obtain calibration parameters of each polarized camera, specifically including: The calibration parameters of each polarized camera are obtained by measuring the output of each polarized camera under the condition of a standard light source signal, using the standard light source as input; The calibration effect of each polarized camera is verified according to the standard light source and the calibration parameters to determine the qualified polarized cameras.

3. The time-resolved polarimetric imaging method of a wind tunnel flow field of claim 2, wherein, The calibration parameters include micro-polarization sheet angle correction value, response correction coefficient and response offset.

4. The method of time-resolved polarimetric imaging of a flow field in a wind tunnel of claim 1, wherein, The arrangement mode of the polarized camera array includes side-by-side arrangement of multiple polarized cameras, parallel arrangement of multiple polarized cameras divided into upper and lower rows, and symmetrical arrangement of multiple polarized cameras on both sides of the wind tunnel test section.

5. The method of time-resolved polarimetric imaging of a flow field in a wind tunnel of claim 1, wherein, The polarized camera array after the water tank calibration experiment is installed in the wind tunnel measurement site to obtain calibration plate images and pre-shooting original images of each polarized camera, specifically including: Adjusting the position of the laser sheet light illuminating the wind tunnel flow field, the position of the polarized camera array relative to the laser sheet light, and the shooting position of the polarized camera array to be consistent with the corresponding position in the water tank calibration experiment; Placing calibration plates for different polarization camera angle corrections in the shooting area to obtain calibration plate images; Running the wind tunnel and the time-resolved polarization imaging device to shoot flow field images at the same laser sheet light illumination moment to obtain pre-shooting original images of each polarized camera.

6. The method of time-resolved polarimetric imaging of a flow field of a wind tunnel of claim 1, wherein, According to the calibration parameters and the calibration plate images, the pre-shooting original images are processed for camera error correction, polarization information calculation and image distortion correction to obtain pre-shooting results, specifically including: Using the calibration parameters to correct the pre-shooting original images to eliminate the system error of each polarized camera; According to the defocus plane polarization imaging method, the polarization information of the image after eliminating the system error of the polarization camera is calculated, and the image after calculating the polarization information is processed for noise reduction; Using the calibration plate images to correct the spatial position distortion of the image after noise reduction to obtain the pre-shooting result of each polarized camera.

7. The time-resolved polarimetric imaging method of a flow field of a wind tunnel of claim 6, wherein, The system error of the polarized camera includes micro-polarization sheet angle error, sensor response error and offset error.

8. The method of time-resolved polarimetric imaging of a flow field of a wind tunnel of claim 1, wherein, According to the pre-shooting results, whether the polarized camera array shooting the same target information meets the measurement consistency requirement is evaluated using an image consistency processing method, and the image consistency processing method includes one or more of mean square error algorithm, peak signal-to-noise ratio algorithm and cosine similarity algorithm.

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