Nanosecond particle velocity measurement method based on orthogonal line polarization beam splitting

Abstract

The invention discloses a nanosecond particle velocity measurement method based on orthogonal line polarization beam splitting, and belongs to the technical field of particle image velocity measurement. The method comprises the following steps: providing two linear polarization state orthogonal double-pulse lasers, and respectively outputting S polarization pulse light beams and P polarization pulse light beams; the four pulse light beams are combined through the double-path laser beam combining mirror set and output to the sheet light mirror set, and a plane illumination jet flow field with the thickness smaller than or equal to 1 mm is formed; scattered light is separated according to a polarization state by using a polarization beam-splitting cubic prism, and particle images under S polarization and P polarization are respectively captured by two double-exposure cameras which are orthogonally arranged; five kinds of time sequence control are triggered through a multi-channel synchronous controller; processing the view field deviation of the two cameras by adopting a block polynomial alignment method; and calculating a particle velocity field based on a particle tracking velocity measurement algorithm. According to the invention, the polarization debugging error is reduced, the homogeneity error after beam combination is obviously reduced, the coincidence degree of the four beams of planar light is improved, and a high-precision universal alignment scheme is provided for the multi-camera PTV technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of particle image velocimetry, and in particular relates to a nanosecond particle velocity measurement method based on orthogonal linear polarization splitting. Background Art

[0002] In particle image velocimetry (PIV / PTV) technology, the time interval between two frames is a key factor affecting measurement accuracy. Current mainstream technologies have three limitations:

[0003] 1. Limitations of time resolution and velocity field calculation

[0004] PIV / PTV technology is based on dual-pulse lasers. Dual-pulse lasers offer high power and stability, producing two stable beams of 200mJ high-power laser light. The resulting dual-frame images are high-quality and are the primary type of PIV / PTV measurement. However, due to the minimum time interval between the two pulse beams of a dual-pulse laser, the time interval between the two frames can only reach 300ns to 400ns, making it difficult to shorten the interval further. Furthermore, due to the exposure frequency and data transmission rate limitations of the dual-exposure cameras used in the system, only 2 to 10 pairs of particle images can be obtained per second. The long time interval between the two paired images significantly limits the accuracy of the calculations.

[0005] PIV / PTV technology is based on high-frequency lasers. High-frequency lasers can achieve pulse frequencies of 1kHz and above, but the energy of a single pulse is negatively correlated with the frequency. A decrease in laser energy significantly impacts particle image quality. Furthermore, the resolution of high-speed cameras used with high-frequency lasers is negatively correlated with the imaging frequency, while the camera noise floor is positively correlated with the frequency. Therefore, as the imaging frequency increases, image quality degrades significantly, significantly impacting computational accuracy.

[0006] PIV technology based on laser beam combining. Laser beam combining technology combines different lasers into one beam through reasonable optical path design. This technology can combine multiple single-pulse / double-pulse laser beams to achieve high-frequency and high-intensity PIV. However, this technology currently often requires more stringent requirements on the layout of the receiving end camera to ensure that the corresponding particle images obtained can be cross-correlated in the later stage. In the calculation, PIV technology divides the tracer particle image into a grid and performs cross-correlation calculations on the paired tracer particle grid images to obtain the velocity information at each position in the flow field. The calculation result is strongly correlated with the grid division density and shape, and a velocity vector in the calculation result is the average velocity field within a grid, and the accuracy of the local velocity information is insufficient.

[0007] 2. Low camera field of view alignment accuracy

[0008] In basic framing imaging PIV / PTV technology, multi-camera field of view alignment has long been a technical bottleneck. Existing framing imaging-based PIV / PTV technologies primarily utilize pre-processing methods such as mechanical structure design alignment and post-processing methods such as calibration image alignment to achieve alignment of the fields of view captured by two different cameras. Methods using mechanical structure design alignment suffer from low precision and high levels of human error. Methods based on calibration image alignment require high quality calibration images and often employ full-image adjustments. This often results in high overlap in the center of the two camera images after alignment, but low overlap at the edges. Both methods struggle to achieve high-precision alignment of the fields of view of two cameras and suffer from significant errors.

[0009] 3. Insufficient dynamic range

[0010] When experimental parameters are fixed, the PIV / PTV system can only measure the velocity distribution of the flow field at a single time interval within the same train test. It cannot measure the velocity distribution of the flow field at different time intervals within the same train test, which is a problem of insufficient dynamic range. When capturing both small-scale and large-scale flow structures in the same flow field, existing PIV technology often requires adjusting parameters such as the pulse beam output time interval and the time interval between two camera exposures. This process requires the coordinated adjustment of multiple parameters, which is difficult to adjust and increases the experimental complexity.

[0011] Based on this, it is necessary to address the problems in the above technical background and design a high dynamic range particle velocity measurement technology based on orthogonal linear polarization splitting that can reduce the complexity of camera layout and achieve high dynamic range measurement, and design a method that can process the images captured by the two cameras to achieve high-precision alignment of the fields of view of the two cameras. Summary of the Invention

[0012] Technical issues to be solved:

[0013] To overcome the shortcomings of the prior art, the present invention provides a nanosecond particle velocity measurement method based on orthogonal linear polarization splitting. This method uses two dual-pulse lasers with orthogonal linear polarization states (rather than four single-pulse lasers) to ensure polarization consistency of the cognate light beams (the two beams output by the same laser have the same polarization state), reducing polarization adjustment errors. The two beams output by the same laser have the same optical path length, significantly reducing the co-beam error after combining and improving the overlap of the four planar beams. By independently controlling the trigger timing of the two lasers, the present invention achieves flexible time scale switching at the hardware level, breaking through the limitations of fixed parameters in traditional PIV systems and providing a high-precision universal alignment solution for multi-camera PTV technology.

[0014] The technical solution of the present invention is: a nanosecond particle velocity measurement method based on orthogonal linear polarization splitting, comprising:

[0015] Two dual-pulse lasers with orthogonal linear polarization states are provided to output S-polarized and P-polarized pulse beams respectively;

[0016] The four pulse beams are combined by a dual-path laser beam combining mirror group and output to the light sheet mirror group to form a plane light irradiation flow field with a thickness of ≤1mm;

[0017] The scattered light is separated by polarization state using a polarization splitter cube prism, and two orthogonally arranged double-exposure cameras capture particle images under S-polarization and P-polarization, respectively.

[0018] Five timing control schemes are triggered by a multi-channel synchronous controller, achieving dynamic adjustment from 1 nanosecond to 50 microseconds;

[0019] The block polynomial alignment method is used to deal with the field of view deviation between the two cameras;

[0020] The particle velocity field is calculated based on the particle tracking velocimetry algorithm.

[0021] A further technical solution of the present invention is that the two double-pulse lasers are a combined four-pulse light source, wherein:

[0022] The two pulsed lights output by the same laser have the same polarization state;

[0023] The two pulsed lights output by the same laser have the same optical path length.

[0024] A further technical solution of the present invention is: the dual-path laser beam combining lens assembly comprises:

[0025] The reflective beam combining mirror group is used to adjust the optical path overlap and realize the beam combining of four pulse beams;

[0026] Two half-wave plates, the first half-wave plate is used to adjust and control the transmission of S-polarized light, and the second half-wave plate is used to adjust and control the transmission of P-polarized light;

[0027] The polarization beam splitter reflects and combines the two pulse beams with S polarization and the two pulse beams with P polarization to the laser output mirror group;

[0028] The laser output mirror group outputs the combined pulse beam to the light sheet mirror group, and includes at least two total reflection mirrors.

[0029] A further technical solution of the present invention is that the light-sheet lens assembly includes a concave lens, a third half-wave plate, a convex lens, and a cylindrical concave lens sequentially arranged along the optical path; the polarization state of the combined pulse beam is adjusted by the third half-wave plate; and the combined pulse beam is converted into a plane light with a thickness of ≤1 mm by the concave lens, the convex lens, and the cylindrical concave lens;

[0030] A total reflection mirror is set on the optical path of the light-sheet mirror group to change the propagation path of the pulse light beam so that the plane light illuminates the target shooting plane.

[0031] A further technical solution of the present invention is: the signal composition in the timing signal control solution is:

[0032] A first pulse beam trigger signal S1 is used to control the first double-pulse laser to emit a first pulse beam;

[0033] A second pulse beam trigger signal S2 is used to control the first double-pulse laser to emit a second pulse beam;

[0034] A third pulse beam trigger signal P1 is used to control the second double-pulse laser to emit a third pulse beam;

[0035] A fourth pulse beam trigger signal P2 is used to control the second double-pulse laser to emit a fourth pulse beam;

[0036] The five timing control schemes include:

[0037] Solution 1: Trigger P1, P2, S1, and S2 in sequence, with the time interval P1-P2 being the same as the time interval S1-S2.

[0038] Solution 2: Trigger P1, S1, P2, and S2 in sequence, with the time intervals P1-S1 and P2-S2 independently adjustable;

[0039] Solution 3: Trigger P1, S1, P2, and S2 in sequence. The time interval P1-P2 is in the nanosecond level, and the time interval S1-S2 is in the microsecond level.

[0040] Solution 4: Trigger P1, S1, P2, and S2 in sequence, with the same time intervals P1-S1, S1-P2, and P2-S2;

[0041] Solution 5: Trigger P1, S1, P2, and S2 in sequence. The time intervals P1-S1 are in nanoseconds, P1-P2 are in hundreds of nanoseconds, and P1-S2 are in microseconds.

[0042] A further technical solution of the present invention is: the block polynomial alignment method comprises:

[0043] Adjusting the laser triggering timing and the camera exposure timing so that the first pulse beam emitted by the first double-pulse laser and the third pulse beam emitted by the second double-pulse laser are triggered simultaneously, and the second pulse beam emitted by the first double-pulse laser and the fourth pulse beam emitted by the second double-pulse laser are triggered simultaneously, while ensuring that the two double-exposure cameras are exposed synchronously;

[0044] Adjust the position and focus of the two dual-exposure cameras to achieve physical field of view pre-alignment, so that the field of view error is ≤30 pixels;

[0045] The flow field is captured under synchronous triggering conditions. The two cameras are marked as Camera A and Camera B, and the particle images Image_A and Image_B are obtained at the same moment respectively.

[0046] Perform particle identification on Image_A and Image_B respectively, extract the particle coordinate sequence, and match the corresponding particle points in the two images using the particle tracking velocimetry algorithm;

[0047] Divide Image_A and Image_B into multiple grid blocks. The grid shape is square or rectangular with an aspect ratio of ≤1.5. Set a 20% overlap area between adjacent grid blocks. Extract the coordinates of the particles in the grid.

[0048] The coordinates of the corresponding particles in the two images are fitted by a cubic polynomial;

[0049] In the formal experiment, the following operations are performed on each frame of image captured by Camera A and Camera B:

[0050] Divide Image_A and Image_B into grid blocks according to the above grid division steps;

[0051] Apply the mapping parameters of the corresponding grid to perform coordinate transformation;

[0052] The aligned image sequence is generated by stitching the grid blocks together through bilinear interpolation and smoothing the overlapping areas.

[0053] A further technical solution of the present invention is that the fitting of the coordinates of the corresponding particles in the two images by using a cubic polynomial comprises:

[0054] Based on the matching particle points in each grid block, the sequence (x a ,y a ) i With the sequence (x b ,y b ) i Using a cubic polynomial, the expression is as follows,

[0055] x b =β0+β1x a +β2y a +β3x a 2 +β4x a y a +β5y 2 +β6x 3 +β7x a 2 y a +β8x a y a2 +β9y a 3

[0056] y b =γ0+γ1x a +γ2y a +γ3x a 2 +γ4x a y a +γ5y 2 +γ6x 3 +γ7x a 2 y a +γ8x a y a 2 +γ9y a 3

[0057] Use the least squares method to obtain the parameter sequence (β,γ) in part_i i ;

[0058] The above operations are performed on all divided grid areas to obtain the parameter sequence (β,γ)1(β,γ)2(β,γ)3···(β,γ) corresponding to all areas n .

[0059] A further technical solution of the present invention is: the particle tracking velocimetry algorithm includes:

[0060] Particle sub-pixel positioning:

[0061] Background extraction for paired tracer particle images: Background mean filtering and background separation based on intrinsic orthogonal decomposition are used to extract image background and enhance the image.

[0062] Processing with a Difference of Gaussian filter or a Laplacian of Gaussian filter yields a filtered image whose response peak corresponds to the particle center;

[0063] Calculate the local maximum of the filtered image to obtain the center of mass position of the particle;

[0064] Taking the particle's center of mass coordinates as (i, j), a Gaussian function or quadratic function is used to fit the grayscale values ​​of the neighboring pixels to calculate the sub-pixel coordinate position. The expression is as follows:

[0065]

[0066] Where, F i,jis the grayscale value of the motion image formed by the tracer particles moving in the flow field at the coordinate (i, j) after removing the background image from the original image obtained by shooting. F is the motion grayscale image formed by the tracer particles moving in the flow field. F' i,j is the value of the filtered image F' obtained by processing F using a Gaussian difference filter or a Gaussian Laplacian filter at coordinate (i, j); i and j are the coordinates of the identified particle center of mass; (x, y) are the sub-pixel precision particle position coordinates obtained by the above fitting method;

[0067] Histogram-based particle matching:

[0068] According to the maximum flow rate and frame interval, the maximum displacement is calculated and a pre-matching candidate box is generated for each particle;

[0069] Calculate the distance between the particle to be matched and each particle in the candidate frame in the second frame image;

[0070] Calculate the displacement between other particles within the set range near the particle to be matched and each particle in its candidate box, and draw a displacement histogram; select the displacement with the highest frequency in the histogram as the reference displacement direction;

[0071] Displacement Gaussian fitting optimization:

[0072] Take the adjacent displacement values ​​of the reference displacement and perform Gaussian fitting to solve the best matching direction X of the particle fit and Y fit , the expression is as follows,

[0073]

[0074] Where dX max and dY max They are the candidate displacements with the largest number of occurrences in the histogram algorithm, dX max-1 、dX max+1 、dY max-1 、dY max+1 are the reference displacement directions dX of the matching particles respectively max and dY max The left and right adjacent values ​​in the histogram candidate displacement group; X fit and Y fit is the displacement of the best match obtained in the x and y directions respectively by the above method, i.e., the best match direction;

[0075] Speed ​​calculation:

[0076] According to the best matching direction X fit and Y fit In the second frame, the matching particles are located, and the particle velocity is calculated based on the positional relationship between the to-be-matched particles and the found matching particles in the image.

[0077] A particle velocity measurement system, comprising:

[0078] Polarization orthogonal light source module: includes two dual-pulse lasers, the first dual-pulse laser outputs two pulse laser beams with polarization state S; the second dual-pulse laser outputs two pulse laser beams with polarization state P;

[0079] Polarization beam combining module: It is a dual-path laser beam combining mirror group, including a reflective beam combining mirror group, a first half-wave plate, a second half-wave plate and a polarization beam splitter prism; the reflective beam combining mirror group adjusts the optical paths of the four pulse beams to coincide, and the polarization beam splitter prism reflects S-polarized light and transmits P-polarized light, achieving the simultaneous output of the four beams;

[0080] Light sheet generation module: a concave lens, a convex lens, a cylindrical concave lens and a total reflection mirror are arranged in sequence to convert the combined light beam into a plane light with a thickness of ≤1mm;

[0081] Orthogonal imaging module:

[0082] Polarization splitter cube prism, which receives the scattered light from the flow field and separates it according to the polarization state;

[0083] Two dual-exposure cameras are orthogonally arranged on both sides of the polarization beam splitter cube prism to capture particle images under S polarization and P polarization respectively;

[0084] XY high-precision translation stage, adjust the camera position so that the field of view pre-alignment error is ≤30 pixels;

[0085] Synchronous control module: includes a multi-channel synchronous controller and an acquisition computer. The multi-channel synchronous controller executes five timing control schemes; the acquisition computer is used to store images and run the block polynomial alignment algorithm and particle tracking velocimetry algorithm.

[0086] Beneficial effects

[0087] The beneficial effects of the present invention are as follows: It provides a high-dynamic-range nanosecond particle velocity measurement device based on orthogonal linear polarization spectrometry. Through the rational design of the optical path, this device significantly reduces the complexity of the optical path layout and receiving-end camera arrangement required for PIV technology based on laser beam combining. By rationally setting the timing, the present invention can achieve high-dynamic-range measurements on time scales from nanoseconds to microseconds within the same train experiment. Combined with a particle tracking velocimetry algorithm, it can determine the flow structure of the flow field from small to large time scales, significantly improving the dynamic range of PTV technology.

[0088] This method can achieve high-precision alignment of the fields of view of two cameras, significantly reducing the field of view error caused by shooting with cameras in different positions, and improving the measurement accuracy of PTV technology based on multi-camera shooting. The specific effect analysis is as follows:

[0089] 1. This invention covers three orders of magnitude in time scale: Through a timing control scheme, a single experiment can simultaneously capture nanosecond-level transient flow fields, hundreds of nanosecond-level transition flow fields, and microsecond-level steady-state flow fields, increasing the dynamic range by 100 times compared to traditional PIV (which only covers a single scale). One-touch switching between five timing modes eliminates the tedious process of adjusting pulse intervals across multiple experiments required by traditional techniques, improving experimental efficiency.

[0090] 2. The present invention is based on a calibration method of simultaneous particle images, eliminating the influence of mechanical installation errors, environmental vibrations and optical distortion, and is suitable for complex working conditions.

[0091] 3. The present invention uses a dual-pulse laser combination to ensure the polarization consistency of the homologous light beams and the optical path difference is ≤1mm, which reduces the homologous beam error compared to the four-laser beam combining system. BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Figure 1 Schematic diagram of the structure of a high dynamic range particle image tracking measurement device based on orthogonal linear polarization spectrometry technology provided in one embodiment of the present invention.

[0093] Figure 2 The figure is a schematic structural diagram of a dual-path laser beam combining solution provided in one embodiment of the present invention.

[0094] Figure 3 The figure is a schematic structural diagram of a light-sheet lens assembly provided in one embodiment of the present invention.

[0095] Figure 4 The figure is a schematic structural diagram of a vertical camera imaging unit provided in one embodiment of the present invention.

[0096] Figure 5 Schematic diagram of five timing signal control schemes provided in the present invention.

[0097] Figure 6 Schematic diagram of nanosecond velocity field measurement based on particle image tracking velocimetry in Example 1 of the present invention.

[0098] Figure 7 Schematic diagram of high dynamic range flow field velocity field measurement based on particle image tracking velocimetry in Example 2 of the present invention.

[0099] Explanation of the accompanying drawings: 11. First double-pulse laser, 12. Second double-pulse laser; 20. Dual-path laser beam combining mirror assembly, 211. First half-wave plate, 212. Second half-wave plate, 221, 222, 223. Total reflection mirrors, 23. Polarization beam splitter prism, 241, 242. Total reflection mirrors; 30. Light-sheet mirror assembly, 31. Third half-wave plate, 32. Concave lens, 33. Convex lens, 34. Cylindrical concave lens, 351. Total reflection mirror, 352. Total reflection mirror; 40. Vertical camera imaging unit, 411. First double-exposure camera, 412. Second double-exposure camera, 42. Polarization beam splitter prism, 431. XY precision translation stage, 432. XY precision translation stage; 50. Flow field with uniformly distributed tracer particles. DETAILED DESCRIPTION

[0100] The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0101] Based on the problems of the existing technology such as time resolution and velocity field calculation limitations, low camera field alignment accuracy, and insufficient dynamic range, the present invention provides a nanosecond particle velocity measurement method based on orthogonal linear polarization spectrometry, comprising:

[0102] Two dual-pulse lasers with orthogonal linear polarization states are provided to output S-polarized and P-polarized pulse beams respectively;

[0103] The four pulse beams are combined by a dual-path laser beam combining mirror group and output to the light sheet mirror group to form a plane light irradiation flow field with a thickness of ≤1mm;

[0104] The scattered light is separated by polarization state using a polarization splitter cube prism, and two orthogonally arranged double-exposure cameras capture particle images under S-polarization and P-polarization, respectively.

[0105] Five timing control schemes are triggered by a multi-channel synchronous controller, achieving dynamic adjustment of the time scale from 1 nanosecond to 50 microseconds;

[0106] The block polynomial alignment method is used to deal with the field of view deviation between the two cameras;

[0107] The particle velocity field is calculated based on the particle tracking velocimetry algorithm.

[0108] The present invention also provides a particle velocity measurement system, comprising:

[0109] Polarization orthogonal light source module: includes two dual-pulse lasers, the first dual-pulse laser outputs two pulse laser beams with polarization state S; the second dual-pulse laser outputs two pulse laser beams with polarization state P;

[0110] Polarization beam combining module: It is a dual-path laser beam combining mirror group, including a reflective beam combining mirror group, a first half-wave plate, a second half-wave plate and a polarization beam splitter prism; the reflective beam combining mirror group adjusts the optical paths of the four pulse beams to coincide, and the polarization beam splitter prism reflects S-polarized light and transmits P-polarized light, achieving the simultaneous output of the four beams;

[0111] Light sheet generation module: a concave lens, a convex lens, a cylindrical concave lens and a total reflection mirror are arranged in sequence to convert the combined light beam into a plane light with a thickness of ≤1mm;

[0112] Orthogonal imaging module:

[0113] Polarization splitter cube prism, which receives the scattered light from the flow field and separates it according to the polarization state;

[0114] Two dual-exposure cameras are orthogonally arranged on both sides of the polarization beam splitter cube prism to capture particle images under S polarization and P polarization respectively;

[0115] XY high-precision translation stage, adjust the camera position so that the field of view pre-alignment error is ≤30 pixels;

[0116] Synchronous control module: includes a multi-channel synchronous controller and an acquisition computer. The multi-channel synchronous controller executes five timing control schemes; the acquisition computer is used to store images and run the block polynomial alignment algorithm and particle tracking velocimetry algorithm.

[0117] The above technical solution is further described below with reference to the accompanying drawings:

[0118] In one embodiment, referring to Figure 2 As shown, in this embodiment, a high dynamic range particle image tracking and measurement device based on orthogonal linear polarization splitting technology includes a first dual-pulse laser 11 that outputs an S-polarized pulse beam, a second pulse laser 12 that outputs a P-polarized pulse beam, a dual-path laser beam combining mirror group 20, a light sheet mirror group 30, a vertical camera imaging unit 40, a flow field 50 in which tracer particles are evenly distributed, and a control and acquisition system.

[0119] In one embodiment, two dual-pulse lasers with mutually perpendicular linear polarization states are used. The first dual-pulse laser 11 emits a first pulse beam and a second pulse beam to the dual-path laser beam combining mirror group according to a given trigger signal sequence, and the second dual-pulse laser 12 emits a third pulse beam and a fourth pulse beam to the dual-path laser beam combining mirror group according to a given trigger signal sequence.

[0120] The use of a quad-pulse laser composed of a dual-pulse laser combination can ensure that the polarization state of the pulsed beam output by the same dual-pulse laser is the same. Compared with the publicly available quad-pulse laser, the use of a dual-pulse laser combination can ensure that the polarization state of the pulsed beam output by the same dual-pulse laser is the same, reducing the beam polarization angle error caused by polarization filter adjustment and improving the accuracy of beam polarization angle adjustment. At the same time, since a quad-pulse laser often requires a combination of four single-pulse lasers, due to the actual volume limitations of the laser, it is inevitable that there will be large differences in the optical path lengths of the four-beam pulsed lasers. There is a strong correlation between the co-beaming of the four-pulse laser and the optical path length, and the co-beaming error will continue to accumulate with the optical path length. The use of a dual-pulse laser can ensure that the optical path lengths of the pulsed beams output by the same dual-pulse laser are the same, effectively reducing the co-beaming error, improving the overlap of the four plane lights generated by the quad-pulse laser, and improving the system accuracy.

[0121] In one embodiment, referring to Figure 3 As shown, the dual-path laser beam combining mirror assembly 20 includes reflective beam combining mirror assemblies 221, 222, and 223, a first half-wave plate 211, a second half-wave plate 212, a polarization beam splitter prism 23, and summed-beam laser output mirror assemblies 241 and 242. The reflective beam combining mirror assembly includes at least three total reflection mirrors to combine the first and second pulse beams emitted by the first dual-pulse laser with the third and fourth pulse beams emitted by the second dual-pulse laser. The first half-wave plate is used to adjust and control the polarization states of the first and second pulse beams emitted by the first dual-pulse laser. The second half-wave plate is used to adjust and control the third and fourth pulse beams emitted by the second dual-pulse laser. The polarization beam splitter prism is used to reflect the first and second pulse beams with S polarization states to the beam combining laser output mirror assembly, and transmit the third and fourth pulse beams with P polarization states to the summed-beam laser output mirror assembly. The beam combining laser output mirror assembly includes at least two total reflection mirrors to output the combined pulse beam to the light-sheet mirror assembly.

[0122] In one embodiment, referring to Figure 4 As shown, the light-sheet lens assembly includes at least one half-wave plate 31, a concave lens 32, a convex lens 33, a cylindrical concave lens 34, and multiple full-reflection mirrors 351 and 352. The half-wave plate in the light-sheet lens assembly is the third half-wave plate, which is used to finally adjust the polarization state of the combined pulse beam to achieve the best shooting effect. The concave lens, convex lens and cylindrical concave lens are used in conjunction to convert the combined pulse beam into a plane light with a thickness of no more than 1 mm. Multiple full-reflection mirrors are used to change the propagation path of the combined pulse beam so that the plane light illuminates the target shooting plane.

[0123] In one embodiment, referring to Figure 5As shown, the vertical camera imaging unit 40 includes at least two cameras with dual-exposure mode, a polarization beam splitter cube prism 42, and two XY high-precision translation stages 431 and 432. The dual-exposure cameras include a first dual-exposure camera 411 and a second dual-exposure camera 412. The first and second dual-exposure cameras are arranged such that the camera sensors are parallel to the surface of the polarization beam splitter cube prism, the sensor centers are coplanar with the center of the polarization beam splitter cube prism surface, and the line connecting the center of the interface between the first dual-exposure camera and the polarization beam splitter cube prism is orthogonal to the line connecting the center of the interface between the second dual-exposure camera and the polarization beam splitter cube prism. The XY high-precision translation stage is used in conjunction with the dual-exposure cameras to precisely adjust the position of the dual-exposure cameras. The stage is fine-tuned to achieve a single-digit pixel difference in the field of view of the two cameras. Post-production cropping and algorithmic correction achieve an identical field of view.

[0124] The double exposure camera may be a camera with a charge coupled device image sensor (CCD), or a camera with a complementary metal oxide semiconductor image sensor (CMOS), or other shooting devices capable of realizing a double exposure function.

[0125] The control and acquisition system includes at least one multi-channel synchronous controller and an acquisition computer. The multi-channel synchronous controller is used to control and output signals to the first dual-pulse laser, the second dual-pulse laser, the first dual-exposure camera, the second dual-exposure camera, and other devices requiring signal control, thereby triggering the pulse beams of the dual-pulse lasers and triggering the dual-exposure cameras to capture images of tracer particles under the pulse beams. The acquisition computer is used to collect and store particle images captured by the system.

[0126] Specifically, the first half-wave plate acts on the first pulse beam and the second pulse beam, and adjusts the polarization states of the first pulse beam and the second pulse beam by controlling the angle of the half-wave plate to ensure that the two pulse beams are S-polarized. The second half-wave plate acts on the third pulse beam and the fourth pulse beam, and adjusts the polarization states of the third pulse beam and the fourth pulse beam by controlling the angle of the half-wave plate to ensure that the two pulse beams are P-polarized.

[0127] In the high-dynamic-range particle image tracking measurement device based on orthogonal linear polarization spectrometry in the above-described embodiment, two dual-pulse lasers with orthogonal linear polarization states emit four high-energy pulse beams. Since the trigger signals of the two dual-pulse lasers are independent, the first and third pulse beams can be triggered simultaneously or with nanosecond intervals, and the second and fourth pulse beams can also be triggered simultaneously or with nanosecond intervals. The two dual-pulse lasers with orthogonal linear polarization states emit four high-energy pulse beams. A dual-path laser beam combiner is used to adjust the polarization states and optical paths of the four high-energy pulse beams, achieving S-polarization for the first and second pulse beams and P-polarization for the third and fourth pulse beams. The four high-energy pulse beams are combined by the reflective beam combiner and then, through the combined laser output lens, travel along the same output path to the light-sheet lens. A tracer particle generator is used to seed the measured airflow with tracer particles. The four high-energy pulses illuminate the flow plane to be measured via the light-sheet lens, illuminating the tracer particle flow. The polarization-splitting cube prism in the vertical camera imaging unit functions to separate light scattered into the cube prism according to its polarization state, allowing P-polarized light to be transmitted while S-polarized light is reflected by a total reflection mirror with a 90° angle. The incident surface of the polarization-splitting cube prism in the vertical camera imaging unit is aligned with the center of the flow field to be measured, and the polarization-splitting cube prism should be placed horizontally on the ground. The two orthogonally arranged double-exposure cameras in the vertical camera imaging unit are arranged according to the design of the vertical camera imaging unit. Light scattered by the tracer particle stream after being illuminated by four high-energy pulses is split into two beams according to their polarization state by the polarization-splitting cube prism. Images of the tracer particle stream illuminated by the first and second pulse beams in the S polarization state are captured by the first double-exposure camera, while images of the tracer particle stream illuminated by the third and fourth pulse beams in the P polarization state are captured by the second double-exposure camera. Due to the effect of the polarization beam splitter cubic prism, the second double-exposure camera cannot capture the tracer particle flow images illuminated by the first and second pulse beams, and the first double-exposure camera cannot capture the tracer particle flow images illuminated by the third and fourth pulse beams.

[0128] In the above embodiment, the double exposure is used to sequentially expose two images. The first frame of the first double exposure camera is used to expose the tracer particle flow image illuminated by the first pulsed light beam, the second frame of the first double exposure camera is used to expose the tracer particle flow image illuminated by the second pulsed light beam, the first frame of the second double exposure camera is used to expose the tracer particle flow image illuminated by the third pulsed light beam, and the second frame of the second double exposure camera is used to expose the tracer particle flow image illuminated by the fourth pulsed light beam.

[0129] In one embodiment, the pulse width of the laser emitted by the four-pulse laser is 6ns-10ns.

[0130] In one embodiment, the pulse width of the laser emitted by the four-pulse laser is shorter than the exposure time of any image of the double-exposure camera, and the double-exposure camera performs exposure and shooting in a dark room.

[0131] In one embodiment, the P polarization state refers to a linear polarization state in which the vibration direction of light is parallel to the incident plane, and the S polarization state refers to a linear polarization state in which the vibration direction of light is perpendicular to the incident plane.

[0132] In one embodiment, the first pulse beam, the second pulse beam, the third pulse beam, and the fourth pulse beam are all high-energy pulse lasers, and the maximum energy of a single pulse laser is 200 mJ.

[0133] In one embodiment, five timing signal control schemes for different scenarios are provided based on the high dynamic range particle image tracking measurement device based on orthogonal linear polarization spectrometry technology. The signal components in the timing signal control scheme are:

[0134] The first pulse beam trigger signal S1 is used to control the first double-pulse laser to emit a first pulse beam.

[0135] The second pulse beam trigger signal S2 is used to control the first double-pulse laser to emit a second pulse beam.

[0136] The third pulse beam trigger signal P1 is used to control the second double-pulse laser to emit a third pulse beam.

[0137] The fourth pulse beam trigger signal P2 is used to control the second double-pulse laser to emit a fourth pulse beam.

[0138] The first double-exposure camera trigger signal Camera s is used to control the first double-exposure camera's first frame start exposure time in the double-exposure mode.

[0139] The first double-exposure camera trigger signal Camera p is used to control the first frame start exposure time of the second double-exposure camera in the double-exposure mode.

[0140] Reference Figure 1 As shown, the five timing signal control schemes provided in the present invention are:

[0141] Figure 1In the figure, Camera p represents a dual-exposure camera that receives light with polarization state P, and Camera s represents a dual-exposure camera that receives light with polarization state S. The straight lines to the right of Camera p and Camera s represent the exposure states of the dual-exposure cameras. A black line indicates that the dual-exposure camera is in the non-exposure state during this time range, while a dashed blue line indicates that the camera is in the exposure state during this time range. The green arrows indicate the emission time of the first or second pulse beam emitted by the first dual-pulse laser, and the yellow arrows indicate the emission time of the third or fourth pulse beam emitted by the second dual-pulse laser. Because the pulse width of the pulse beam is in the nanosecond range, which is a very small value compared to the microsecond camera exposure time, the arrows can be used to approximate the triggering time and duration of the pulsed laser. The specific laser pulse represented by each arrow is marked on the side of the arrow. The length of the red line segment indicates the time interval between the two corresponding laser pulses when calculated using the particle tracking velocimetry algorithm. Since each laser pulse generates a tracer particle flow image, the length of the red line segment also represents the time interval calculated using the particle tracking velocimetry algorithm. The length of each time interval is indicated by the length of the red line segment and marked with text above the line segment.

[0142] Figure 1 In (a), the timing signal control scheme sequentially triggers P1, P2, S1, and S2, where the time interval between S1 and S2 is dt2, and the time interval between P1 and P2 is dt1. In the first timing signal control scheme, the particle velocity v1 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation with the first and second pulse beams, as captured by the first double-exposure camera. The particle velocity v2 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation with the third and fourth pulse beams, as captured by the second double-exposure camera. In this scheme, dt1 and dt2 are generally required to be the same to achieve the same sampling time interval. Under this scheme, the system sampling rate can be increased to twice the sampling rate of a single dual-pulse laser PIV system.

[0143] Figure 1In the timing signal control scheme (b), P1, S1, P2, and S2 are triggered sequentially, with the time interval between P1 and S1 being dt1, and the time interval between P2 and S2 being dt2. In the second timing signal control scheme, the particle velocity v1 can be determined by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by the first pulse beam and the tracer particle flow image obtained by the second pulse beam, respectively. The particle velocity v2 can be determined by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by the second pulse beam and the tracer particle flow image obtained by the second double-exposure camera. Since the first and third pulse beams are emitted by the first and second double-pulse lasers, respectively, and the second and fourth pulse beams are emitted by the first and second double-pulse lasers, respectively, the theoretical minimum time interval dt1 between the two pulse beams can be zero. There are no specific requirements for dt1 and dt2 in this scheme; they are typically in the range of 10 nanoseconds to 100 microseconds. The v1 and v2 obtained under this scheme can correspond to extremely short time interval conditions.

[0144] Figure 1 The timing signal control scheme (c) sequentially triggers P1, S1, P2, and S2, with the time interval between P1 and P2 being dt1, and the time interval between S1 and S2 being dt2. In a third timing signal control scheme, particle tracking velocimetry can be used to determine the particle velocity v1 using the tracer particle flow images captured by the first double-exposure camera, obtained by irradiation with the first and second pulse beams. Particle tracking velocimetry can be used to determine the particle velocity v2 using the tracer particle flow images captured by the second double-exposure camera, obtained by irradiation with the first and second pulse beams. In this third timing signal control scheme, dt1 and dt2 are different time intervals. This scheme effectively mitigates the problem of measurement errors caused by large time differences in flow characteristics in strong shear flows. For example, PTV and PIV images can be captured separately for the near-wall region of the boundary layer and the outer region of the flow. In this scheme, dt1 and dt2 are typically of different orders of magnitude, such as nanosecond time intervals for dt1 and microsecond time intervals for dt2, to enable measurement of flow characteristics at different time scales.

[0145] Figure 1In the timing signal control scheme (d), P1, S1, P2, and S2 are triggered in sequence, where the time interval between P1 and S1 is dt1, the time interval between S1 and P2 is dt2, and the time interval between P2 and S2 is dt3. In the fourth timing signal control scheme, the particle velocity v1 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the third pulse beam and captured by the second double-exposure camera, and the tracer particle flow image obtained by irradiation of the first pulse beam and captured by the first double-exposure camera. The particle velocity v2 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the first pulse beam and captured by the first double-exposure camera, and the particle velocity v3 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the fourth pulse beam and captured by the second double-exposure camera, respectively. The fourth timing signal control scheme can achieve instantaneous time resolution within a four-frame interval, less than the minimum trigger interval of the two laser beams of a single dual-pulse laser. In this scheme, dt1, dt2, and dt3 are the same size.

[0146] Figure 1 In the timing signal control scheme (e), P1, S1, P2, and S2 are triggered in sequence, where the time interval between P1 and S1 is dt1, the time interval between P1 and P2 is dt2, and the time interval between P1 and S2 is dt3. In the fourth timing signal control scheme, the particle velocity v1 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the third pulse beam and captured by the second double-exposure camera and the tracer particle flow image obtained by irradiation of the first pulse beam and captured by the first double-exposure camera. The particle velocity v2 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the third pulse beam and captured by the second double-exposure camera and the tracer particle flow image obtained by irradiation of the fourth pulse beam and captured by the second double-exposure camera. The particle velocity v3 can be obtained by performing a particle tracking velocimetry algorithm on the tracer particle flow image obtained by irradiation of the third pulse beam and captured by the second double-exposure camera and the tracer particle flow image obtained by irradiation of the second pulse beam and captured by the first double-exposure camera. In this scheme, dt1, dt2, and dt3 represent time intervals of three different orders of magnitude. Typically, dt1 is in the nanosecond range, dt2 is in the hundreds of nanoseconds, and dt3 is in the microsecond range. The v1 obtained through the above operation can be used to determine the flow velocity field at very small time scales; dt2 and dt3 can be used to determine the flow velocity field at larger time scales. This scheme enables high-dynamic-range particle tracking measurement technology.

[0147] In the above-described timing signal control, Camera s is used to adjust the exposure triggering time of the first double-exposure camera so that, in double-exposure mode, the first exposure image frame of the first double-exposure camera contains only the tracer particle flow image illuminated by the first pulse beam, and the second exposure image frame contains only the tracer particle flow image illuminated by the second pulse beam. Camera p is used to adjust the exposure triggering time of the second double-exposure camera so that, in double-exposure mode, the first exposure image frame of the second double-exposure camera contains only the tracer particle flow image illuminated by the third pulse beam, and the second exposure image frame contains only the tracer particle flow image illuminated by the fourth pulse beam.

[0148] The above technical solution is further explained below with reference to examples:

[0149] The specific operations of Example 1 are as follows:

[0150] (1) Select a timing signal control scheme that is suitable for the experimental goal. In Example 1, the timing signal control scheme is selected as follows: Figure 1 As shown in (b), S1 is the trigger signal for controlling the first pulse beam emitted by the first double-pulse laser 11, S2 is the trigger signal for controlling the second pulse beam emitted by the first double-pulse laser 11, P1 is the trigger signal for controlling the third pulse beam emitted by the second double-pulse laser 12, and P2 is the trigger signal for the fourth pulse beam emitted by the second double-pulse laser 12. Figure 1 As shown in the schematic diagram of the timing signal control scheme in (b), the P1, S1, P2, and S2 signals are triggered in sequence, where the time interval between P1 and S1 is dt1, and the time interval between P2 and S2 is dt2. Here, dt1 can reach the nanosecond time scale.

[0151] (2) Adjust the polarization state of the pulse beam. Figure 3As shown, according to the timing signal control scheme, after receiving the trigger signal P1, the first dual-pulse laser 11 emits a first pulse beam, which passes through the first half-wave plate 211 and reaches the total reflection mirror 221. It is then reflected by the total reflection mirrors 221, 222, and 223 in the beam combining mirror assembly to the polarization beam splitter prism 23. The first half-wave plate 211 is adjusted to minimize the intensity of the pulse beam transmitted through the polarization beam splitter prism 23. At this time, the intensity of the first pulse beam reflected by the polarization beam splitter prism 23 to the total reflection mirror 241 in the beam combining laser output mirror assembly is maximized. The first pulse beam is reflected by the polarization beam splitter prism 23 to the total reflection mirror 241, and then reflected by the total reflection mirrors 241 and 242 in the beam combining laser output mirror assembly to the light-sheet mirror assembly. After receiving the trigger signal S1, the second dual-pulse laser 12 emits a third pulse beam, which passes through the second half-wave plate 212 and reaches the polarization beam splitter prism 23. Adjust the second half-wave plate 212 to minimize the intensity of the pulsed beam reflected by the polarization beam splitter prism 23. At this point, the intensity of the third pulsed beam transmitted through the polarization beam splitter prism 23 to the total reflection mirror 241 in the combined laser output mirror assembly is maximized. The third pulsed beam passes through the polarization beam splitter prism 23 and is then transmitted to the total reflection mirror 241 in the combined laser output mirror assembly. After passing through the polarization beam splitter prism 23 and being reflected by the total reflection mirrors 241 and 242 in the combined laser output mirror assembly to the light-sheet mirror assembly.

[0152] (3) Adjust the reflective beam combining mirror group to achieve laser beam combining. After completing step (2), adjust the total reflection mirrors 221, 222, and 223 to adjust the optical path of the first pulse beam so that the light spot position of the first pulse beam on the polarization beam splitter prism 23 is the same as the light spot position of the third pulse beam on the polarization beam splitter prism 23, and the light spot position of the first pulse beam on the total reflection mirror 241 is the same as the light spot position of the third pulse beam on the total reflection mirror 241, and the light spot position of the first pulse beam on the total reflection mirror 242 is the same as the light spot position of the third pulse beam on the total reflection mirror 242. At this time, it can be ensured that the optical paths of the first pulse beam and the third pulse beam after the polarization beam splitter prism 23 are the same, that is, the laser beam is combined.

[0153] (4) Adjust the polarization state of the light beam again. Repeat step (1) to adjust the first half-wave plate so that the intensity of the transmitted light of the first pulse light beam is minimized when passing through the polarization beam splitter prism 23, and adjust the second half-wave plate so that the intensity of the reflected light of the third pulse light beam is minimized when passing through the polarization beam splitter prism 23.

[0154] After receiving the trigger signal P2, the first dual-pulse laser 11 emits a second pulse beam. Since the second pulse beam and the first pulse beam are in the same state when emitted by the first dual-pulse laser 11, the optical path and polarization state adjustment of the second pulse beam through the dual-path laser beam combining mirror assembly are the same as those of the first pulse beam. After receiving the trigger signal S2, the second dual-pulse laser 12 emits a fourth pulse beam. Since the third pulse beam and the fourth pulse beam are in the same state when emitted by the second dual-pulse laser 12, the optical path and polarization state adjustment of the fourth pulse beam through the dual-path laser beam combining mirror assembly are the same as those of the third pulse beam.

[0155] (5) Adjust the lamella lens group. The structure of the lamella lens group is as follows: Figure 4 As shown. The first pulsed light beam is output to the light sheet mirror group through the beam combining laser output mirror group, and is reflected by the total reflection mirror 351 in the light sheet mirror group to the concave lens 32. The first pulsed laser is expanded by the concave lens 32, and after passing through the third half-wave plate 31, it reaches the convex lens 33 for convergence. The converged light is reflected by the total reflection mirror 352 to the cylindrical concave lens 34, which converts the light into plane light and illuminates the flow field 50 to be measured containing the tracer particles. The light spot of the light converged by the cylindrical concave lens changes from a light point to a line at the horizontal plane of the flow field to be measured. At this time, the position of the convex lens 33 is adjusted so that the light width does not exceed 1mm.

[0156] Since step (5) is independent of the polarization state of the light, after the lens group based on the first pulse beam is adjusted, the second pulse beam, the third pulse beam, and the fourth pulse beam after completing steps (3) and (4) can achieve the same effect as after the lens group of the first pulse beam is adjusted.

[0157] (6) Arrange the vertical camera imaging unit. The vertical camera imaging unit structure is as follows Figure 5 As shown, it includes a first double exposure camera 411, a second double exposure camera 412, a polarization beam splitter prism 42, an XY precision translation stage 431, and an XY precision translation stage 432. The first double exposure camera 411 and the second double exposure camera 412 in the vertical camera imaging unit are arranged in such a way that the two camera sensors are parallel to the surface of the polarization beam splitter cube prism 42, the sensor center and the polarization beam splitter cube prism surface center are on the same horizontal plane, and the line connecting the center of the interface between the first double exposure camera 411 and the polarization beam splitter cube prism 42 is perpendicular to the line connecting the center of the interface between the second double exposure camera 412 and the polarization beam splitter cube prism 42. The XY high-precision translation stage is used in conjunction with the double exposure camera to precisely adjust the position of the double exposure camera, and the fine-tuning translation stage is used to achieve a single-digit pixel difference in the field of view of the two cameras. After post-cropping and algorithm correction, the field of view is the same. The vertical camera imaging unit should face the plane where the plane light in step (5) is located, that is, the second double exposure camera should face the plane where the plane light in step (5) is located.

[0158] (7) Cooperate with the vertical camera imaging unit to adjust the polarization state of the four pulse beams. Adjust the third half-wave plate 31 so that the first double-exposure camera 411 can only capture the tracer particle flow image illuminated by the first pulse beam and the second pulse beam, and the second double-exposure camera 412 can only capture the tracer particle flow image illuminated by the third pulse beam and the fourth pulse beam.

[0159] (8) Adjust the exposure signal of the dual-exposure camera to capture the tracer particle image of the flow field to be measured. Adjust the trigger signal Camera s that triggers the first frame exposure of the first dual-exposure camera so that the tracer particles illuminated by the first pulse laser are only exposed in the first frame, and the tracer particles illuminated by the second pulse laser are only exposed in the second frame. Adjust the trigger signal Camera p that triggers the first frame exposure of the second dual-exposure camera so that the tracer particles illuminated by the third pulse laser are only exposed in the first frame, and the tracer particles illuminated by the fourth pulse laser are only exposed in the second frame. After completing the exposure signal adjustment, start capturing the tracer particle flow image.

[0160] (9) Using a block polynomial alignment method based on particle images taken at the same moment, the field of view and image obtained by the second double-exposure camera are mapped to the field of view obtained by the first double-exposure camera in accordance with the steps to reduce the error caused by the inconsistency of the field of view between the images taken by the two cameras.

[0161] (10) Figure 6 This is a schematic diagram of the particle tracking velocimetry in this example, where img1 is the first frame of the tracer particle flow image taken by the second double-exposure camera after alignment, img2 is the first frame of the tracer particle flow image taken by the first double-exposure camera, img3 is the second frame of the tracer particle flow image taken by the second double-exposure camera after alignment, and img4 is the second frame of the tracer particle flow image taken by the first double-exposure camera.

[0162] In Example 1, img1 and img2 are paired images, and img3 and img4 are paired images. In img1, img2, img3, and img4, the position of the tracer particle is represented by a local high-grayscale area that is approximately circular in the image. For the paired tracer particle flow images obtained by shooting, the image background is first extracted and enhanced using the background mean filtering method and the background extraction method based on intrinsic orthogonal decomposition, and then the Gaussian difference filter and Gaussian Laplace filter are used to detect the particle position. Both methods will produce a filtered image F with a strong positive response to objects of scale δ. t ′, the region is considered to be particles with a size near δ. By calculating F t′ to obtain the particle's center of mass position. To improve the accuracy of the particle position and obtain the sub-pixel precision position of the particle, a quadratic function or Gaussian function is used to fit the pixel grayscale values ​​around the center of mass, and the maximum value of the fitted function is used as the particle's position. When the particle's center of mass coordinates are (i, j) pixels, the particle's sub-pixel precision position is:

[0163]

[0164] Where, F i,j is the grayscale value of the moving image of the tracer particles in the flow field at the coordinate (i, j) after removing the background image from the original image. F is the moving grayscale image of the tracer particles in the flow field. i,j is the value of the filtered image F' at coordinate (i, j) obtained by processing F using a Difference of Gaussians (DoG) or Laplace of Gaussians (LoG) filter. Using these two methods to process F will produce a sudden change in the local particle image at a scale of δ, resulting in a filtered image F'. Its essence is the gradient distribution or second-order derivative distribution of the image grayscale, reflecting the edge and center positions of the particle. i and j are the coordinates of the identified particle's center of mass. (x, y) are the sub-pixel precision particle position coordinates obtained using the above fitting method.

[0165] After obtaining the particle positions in the images, the velocity of each particle in the paired images is calculated using a histogram-based dual-frame particle matching algorithm.

[0166] In the first step, the maximum displacement f of the particle in the image is determined based on the maximum flow velocity and the time interval between two frames. A pre-matching candidate box is generated for each particle based on the maximum displacement f.

[0167] The second step is to find the distance between the particle to be matched P and each particle in the candidate box in the second frame image.

[0168] The third step is to find the displacements between other particles within a certain range near the particle to be matched P and each particle in its candidate box, group these candidate displacements, draw a histogram, and solve the displacements dX max and dY that exist the most times. max As the reference displacement direction of the pre-matching particle P. The candidate displacement that appears most times in the histogram is the most likely displacement of the particle P to be matched in the two frames of images.

[0169] The fourth step is to take the most likely displacement of the particle P to be matched between the two frames and its left and right adjacent displacements in the histogram using Gaussian fitting to solve the best matching direction X of the particle P. fit and Y fit, in order to improve the accuracy of matching. Equation (1-2) is the Gaussian fitting formula:

[0170]

[0171] Where dX max and dY max They are the candidate displacements with the largest number of occurrences in the histogram algorithm, dX max-1 、dX max+1 、dY max-1 、dY max+1 are the reference displacement directions dX of particle P respectively max and dY max The left and right neighboring values ​​in the histogram candidate displacement group.

[0172] The fifth step is to find the best matching direction X of the particle P to be matched. fit and Y fit , search for matching particles Q in the second frame of particle image. The particle velocity is calculated based on the positional relationship between the particle to be matched P and the found matching particle Q in the image.

[0173] The particle tracking velocimetry algorithm can be used to measure the particle velocity v1 on a nanosecond time scale from the paired images img1 and img2, and the particle tracking velocimetry algorithm can be used to measure the particle velocity v2 on a nanosecond time scale from the paired images img3 and img4.

[0174] Example 2 The specific scheme is as follows:

[0175] Reference Figure 2 As shown, in one embodiment of the present application, a high dynamic range particle image tracking measurement device based on orthogonal linear polarization spectrometry technology includes:

[0176] A first double-pulse laser 11 outputting an S-polarized pulse beam, a second pulse laser 12 outputting a P-polarized pulse beam, a dual-path laser beam combining lens assembly 20, a light sheet lens assembly 30, a vertical camera imaging unit 40, and a flow field 50 with uniformly distributed tracer particles.

[0177] The specific operations of Example 2 are as follows:

[0178] (1) Select a timing signal control scheme that is suitable for the experimental goal. In Example 2, the timing signal control scheme is selected as follows: Figure 1 As shown in (d), S1 is the trigger signal for controlling the first pulse beam emitted by the first double-pulse laser 11, S2 is the trigger signal for controlling the second pulse beam emitted by the first double-pulse laser 11, P1 is the trigger signal for controlling the third pulse beam emitted by the second double-pulse laser 12, and P2 is the trigger signal for the fourth pulse beam emitted by the second double-pulse laser 12. Figure 1 As shown in the schematic diagram of the timing signal control scheme in (d), the P1, S1, P2, and S2 signals are triggered in sequence, where the time interval between P1 and S1 is dt1, the time interval between S1 and P2 is dt2, and the time interval between P2 and S2 is dt,3.

[0179] (2) Adjust the polarization state of the pulse beam. Figure 3 As shown, according to the timing signal control scheme, after receiving the trigger signal P1, the first dual-pulse laser 11 emits a first pulse beam, which passes through the first half-wave plate 211 and reaches the total reflection mirror 221. It is then reflected by the total reflection mirrors 221, 222, and 223 in the beam combining mirror assembly to the polarization beam splitter prism 23. The first half-wave plate 211 is adjusted to minimize the intensity of the pulse beam transmitted through the polarization beam splitter prism 23. At this time, the intensity of the first pulse beam reflected by the polarization beam splitter prism 23 to the total reflection mirror 241 in the beam combining laser output mirror assembly is maximized. The first pulse beam is reflected by the polarization beam splitter prism 23 to the total reflection mirror 241, and then reflected by the total reflection mirrors 241 and 242 in the beam combining laser output mirror assembly to the light-sheet mirror assembly. After receiving the trigger signal S1, the second dual-pulse laser 12 emits a third pulse beam, which passes through the second half-wave plate 212 and reaches the polarization beam splitter prism 23. Adjust the second half-wave plate 212 to minimize the intensity of the pulsed beam reflected by the polarization beam splitter prism 23. At this point, the intensity of the third pulsed beam transmitted through the polarization beam splitter prism 23 to the total reflection mirror 241 in the combined laser output mirror assembly is maximized. The third pulsed beam passes through the polarization beam splitter prism 23 and is then transmitted to the total reflection mirror 241 in the combined laser output mirror assembly. After passing through the polarization beam splitter prism 23 and being reflected by the total reflection mirrors 241 and 242 in the combined laser output mirror assembly to the light-sheet mirror assembly.

[0180] (3) Adjust the reflective beam combining mirror group to achieve laser beam combining. After completing step (2), adjust the total reflection mirrors 221, 222, and 223 to adjust the optical path of the first pulse beam so that the light spot position of the first pulse beam on the polarization beam splitter prism 23 is the same as the light spot position of the third pulse beam on the polarization beam splitter prism 23, and the light spot position of the first pulse beam on the total reflection mirror 241 is the same as the light spot position of the third pulse beam on the total reflection mirror 241, and the light spot position of the first pulse beam on the total reflection mirror 242 is the same as the light spot position of the third pulse beam on the total reflection mirror 242. At this time, it can be ensured that the optical paths of the first pulse beam and the third pulse beam after the polarization beam splitter prism 23 are the same, that is, the laser beam is combined.

[0181] (4) Adjust the polarization state of the light beam again. Repeat step (1) to adjust the first half-wave plate so that the intensity of the transmitted light of the first pulse light beam is minimized when passing through the polarization beam splitter prism 23, and adjust the second half-wave plate so that the intensity of the reflected light of the third pulse light beam is minimized when passing through the polarization beam splitter prism 23.

[0182] After receiving the trigger signal P2, the first dual-pulse laser 11 emits a second pulse beam. Since the second pulse beam and the first pulse beam are in the same state when emitted by the first dual-pulse laser 11, the optical path and polarization state adjustment of the second pulse beam through the dual-path laser beam combining mirror assembly are the same as those of the first pulse beam. After receiving the trigger signal S2, the second dual-pulse laser 12 emits a fourth pulse beam. Since the third pulse beam and the fourth pulse beam are in the same state when emitted by the second dual-pulse laser 12, the optical path and polarization state adjustment of the fourth pulse beam through the dual-path laser beam combining mirror assembly are the same as those of the third pulse beam.

[0183] (5) Adjust the lamella lens group. The structure of the lamella lens group is as follows: Figure 4 As shown. The first pulsed light beam is output to the light sheet mirror group through the beam combining laser output mirror group, and is reflected by the total reflection mirror 351 in the light sheet mirror group to the concave lens 32. The first pulsed laser is expanded by the concave lens 32, and after passing through the third half-wave plate 31, it reaches the convex lens 33 for convergence. The converged light is reflected by the total reflection mirror 352 to the cylindrical concave lens 34, which converts the light into plane light and illuminates the flow field 50 to be measured containing the tracer particles. The light spot of the light converged by the cylindrical concave lens changes from a light point to a line at the horizontal plane of the flow field to be measured. At this time, the position of the convex lens 33 is adjusted so that the light width does not exceed 1mm.

[0184] Since step (5) is independent of the polarization state of the light, after the light-sheet mirror group is adjusted based on the first pulse light beam, the second pulse light beam, the third pulse light beam, and the fourth pulse light beam after completing steps (3) and (4) can achieve the same effect as the first pulse light beam after adjustment by the light-sheet mirror group.

[0185] (6) Arrange the vertical camera imaging unit. The vertical camera imaging unit structure is as follows Figure 5 As shown, it includes a first double exposure camera 411, a second double exposure camera 412, a polarization beam splitter prism 42, an XY precision translation stage 431, and an XY precision translation stage 432. The first double exposure camera 411 and the second double exposure camera 412 in the vertical camera imaging unit are arranged in such a way that the two camera sensors are parallel to the surface of the polarization beam splitter cube prism 42, the sensor center and the polarization beam splitter cube prism surface center are on the same horizontal plane, and the line connecting the center of the interface between the first double exposure camera 411 and the polarization beam splitter cube prism 42 is perpendicular to the line connecting the center of the interface between the second double exposure camera 412 and the polarization beam splitter cube prism 42. The XY high-precision translation stage is used in conjunction with the double exposure camera to precisely adjust the position of the double exposure camera, and the fine-tuning translation stage is used to achieve a single-digit pixel difference in the field of view of the two cameras. After post-cropping and algorithm correction, the field of view is the same. The vertical camera imaging unit should face the plane where the plane light in step (5) is located, that is, the second double exposure camera should face the plane where the plane light in step (5) is located.

[0186] (7) Cooperate with the vertical camera imaging unit to adjust the polarization state of the four pulse beams. Adjust the third half-wave plate 31 so that the first double-exposure camera 411 can only capture the tracer particle flow image illuminated by the first pulse beam and the second pulse beam, and the second double-exposure camera 412 can only capture the tracer particle flow image illuminated by the third pulse beam and the fourth pulse beam.

[0187] (8) Adjust the exposure signal of the double-exposure camera to capture the tracer particle image of the flow field to be measured. Adjust the trigger signal Camera s that triggers the first frame exposure of the first double-exposure camera so that the tracer particles illuminated by the first pulse laser are only exposed in the first frame, and the tracer particles illuminated by the second pulse laser are only exposed in the second frame. Adjust the trigger signal Camera p that triggers the first frame exposure of the second double-exposure camera so that the tracer particles illuminated by the third pulse laser are only exposed in the first frame, and the tracer particles illuminated by the fourth pulse laser are only exposed in the second frame. After completing the exposure signal adjustment, start capturing the tracer particle flow image.

[0188] (9) Using a block polynomial alignment method based on particle images taken at the same moment, the field of view and image obtained by the second double-exposure camera are mapped to the field of view obtained by the first double-exposure camera in accordance with the steps to reduce the error caused by the inconsistency of the field of view between the images taken by the two cameras.

[0189] (10) Figure 7 This is a schematic diagram of the particle tracking velocimetry in this example, where img5 is the first frame of the tracer particle flow image taken by the second double-exposure camera after alignment, img6 is the first frame of the tracer particle flow image taken by the first double-exposure camera, img7 is the second frame of the tracer particle flow image taken by the second double-exposure camera after alignment, and img8 is the second frame of the tracer particle flow image taken by the first double-exposure camera.

[0190] In Example 2, img5 and img6 are paired images, img5 and img7 are paired images, and img5 and img8 are paired images. In img5, img6, img7, and img8, the position of the tracer particles is represented by a local high-grayscale area that is approximately circular in the image. For the paired tracer particle flow images obtained by shooting, the image background is first extracted and enhanced using the background mean filtering method and the background extraction method based on intrinsic orthogonal decomposition, and then the Gaussian difference filter and Gaussian Laplace filter are used to detect the particle position. Both methods will produce a filtered image F with a strong positive response to objects of scale δ. t ′, the region is considered to be particles with a size near δ. By calculating F t′ to obtain the particle's center of mass position. To improve the accuracy of the particle position and obtain the sub-pixel precision position of the particle, a quadratic function or Gaussian function is used to fit the pixel grayscale values ​​around the center of mass, and the maximum value of the fitted function is used as the particle's position. When the particle's center of mass coordinates are (i, j) pixels, the particle's sub-pixel precision position is:

[0191]

[0192] After obtaining the particle positions in the images, the velocity of each particle in the paired images is calculated using a histogram-based dual-frame particle matching algorithm.

[0193] In the first step, the maximum displacement f of the particle in the image is determined based on the maximum flow velocity and the time interval between two frames. A pre-matching candidate box is generated for each particle based on the maximum displacement f.

[0194] The second step is to find the distance between the particle to be matched P and each particle in the candidate box in the second frame image.

[0195] The third step is to find the displacements between other particles within a certain range near the particle to be matched P and each particle in its candidate box, group these candidate displacements, draw a histogram, and solve the displacements dX max and dY that exist the most times. max As the reference displacement direction of the pre-matching particle P. The candidate displacement with the largest number of occurrences in the histogram is the most likely displacement of the particle P to be matched in the two frames of images.

[0196] The fourth step is to take the most likely displacement of the particle P to be matched between the two frames and its left and right adjacent displacements in the histogram using Gaussian fitting to solve the best matching direction X of the particle P. fit and Y fit , in order to improve the accuracy of matching. Equation (1-2) is the Gaussian fitting formula:

[0197]

[0198] Where dX max and dY max They are the candidate displacements with the largest number of occurrences in the histogram algorithm, dX max-1 、dX max+1 、dY max-1 、dY max+1 are the reference displacement directions dX of particle P respectively max and dY max The left and right neighboring values ​​in the histogram candidate displacement group.

[0199] The fifth step is to find the best matching direction X of the particle P to be matched. fit and Y fit, search for matching particles Q in the second frame of particle image. The particle velocity is calculated based on the positional relationship between the particle to be matched P and the found matching particle Q in the image.

[0200] By applying the particle tracking velocimetry algorithm to the paired images img5 and img6, img6 and img7, and img7 and img8 in sequence, the particle velocities v4, v5, and v6 at time intervals of different orders of magnitude can be obtained, thereby realizing high-dynamic flow field velocity field measurement.

[0201] Example 3:

[0202] A block polynomial alignment method based on particle images taken simultaneously. The specific method is designed as follows:

[0203] Step 1: Adjust the laser trigger timing and camera exposure timing so that the first pulse beam and the third pulse beam emitted by the two dual-pulse lasers are triggered simultaneously, and the second pulse beam and the fourth pulse beam are triggered simultaneously. The exposure timing of the two dual-exposure cameras is the same.

[0204] Step 2: Use the pre-processing method of mechanical structure design alignment to adjust the actual position and focus of the two cameras so that the physical field of view error between the two cameras does not exceed 30 pixels.

[0205] Step 2: Shoot using the same timing as in Step 1. The two cameras are labeled Camera A and Camera B. There are no specific requirements for labeling Camera A and Camera B. The image captured by Camera A is labeled Image_A, and the image captured by Camera B is labeled Image_B.

[0206] Step 4: Use the particle identification algorithm in the particle tracking velocimetry algorithm for Image_A and record the identified particle coordinate sequence as (x a ,y a ). Use the particle identification algorithm part of the particle tracking velocimetry algorithm for Image_B and record the identified particle coordinate sequence as (x b ,y b ). Through the particle tracking velocimetry algorithm, find the sequence (x b ,y b ) and the sequence (x a ,y a ) coordinates of the points that match the coordinates and follow the sequence (x a ,y a ) order according to the corresponding point rearrangement sequence (x b ,y b ).

[0207] Step 5: Grid the field of view. Grid division usually uses a square or rectangular grid with an aspect ratio of no more than 1.5 to divide the original image horizontally and vertically. There is a 20% overlap between adjacent grids during segmentation. Perform the same grid division on Image_A and Image_B. Each grid area is recorded as part_1, part_2, part_3...part_n, that is, the original image is divided into n blocks. There is a 20% overlap between adjacent blocks. The coordinates of the particles in each grid are extracted, that is, the sequence (x a ,y a ) coordinates and sequence (x b ,y b ) coordinates are extracted to obtain (x a ,y a ) i (x a ,y a )2(x a ,y a )3···(x a ,y a ) n and (x b ,y b )1(x b ,y b )2(x b ,y b )3···(x b ,y b ) n , where (x b ,y b ) i (x b ,y b ) i The coordinates of the particle corresponding to part_i in Image_A and Image_B.

[0208] Step 6: Use the following cubic polynomial to fit the coordinates of corresponding points in the two images:

[0209]

[0210] For the sequence (x a ,y a ) i With the sequence (x b ,y b ) i Use the (1-3) polynomial and the least squares method to obtain the parameter sequence (β,γ) in part_i iThe above operation is performed on all the divided grid areas to obtain the parameter sequence (β,γ)1(β,γ)2(β,γ)3···(β,γ) corresponding to all areas. n .

[0211] Step 7: In the formal experiment, perform step (e) on the image sequences {Image_A} and {Image_B} obtained by Camera A and Camera B to obtain n grid blocks. The parameters (β, γ) corresponding to each grid block are i This can be obtained by step (f). The pixel positions in each block of each image in the image sequence {Image_A} are mapped using a cubic polynomial (1-3), and the corresponding position of each pixel in the image sequence {Image_B} after alignment is obtained. The mapped blocks are stitched together, and bilinear interpolation is used on the overlapping parts of each mapped block and its adjacent blocks to make the transition areas between the blocks smooth, and the final aligned image sequence {Image_A} is obtained. done , this image sequence is the image sequence obtained by block polynomial alignment of {Image_A} to {Image_B}.

[0212] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and purpose of the present invention.

Claims

1. A nanosecond particle velocity measurement method based on orthogonal linear polarization spectrometry, characterized in that: include: Two dual-pulse lasers with orthogonal linear polarization states are provided to output S-polarized and P-polarized pulse beams respectively; The four pulse beams are combined by a dual-path laser beam combining mirror group and output to the light sheet mirror group to form a plane light irradiation flow field with a thickness of ≤1mm; The scattered light is separated by polarization state using a polarization splitter cube prism, and two orthogonally arranged double-exposure cameras capture particle images under S-polarization and P-polarization, respectively. Five timing control schemes are triggered by a multi-channel synchronous controller, achieving dynamic adjustment from 1 nanosecond to 50 microseconds; The block polynomial alignment method is used to deal with the field of view deviation between the two cameras; The particle velocity field is calculated based on the particle tracking velocimetry algorithm.

2. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 1, characterized in that: The two double-pulse lasers are a combined four-pulse light source, wherein: The two pulsed lights output by the same laser have the same polarization state; The two pulsed lights output by the same laser have the same optical path length.

3. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 1, characterized in that: The dual-path laser beam combining mirror assembly comprises: The reflective beam combining mirror group is used to adjust the optical path overlap and realize the beam combining of four pulse beams; Two half-wave plates, the first half-wave plate is used to adjust and control the transmission of S-polarized light, and the second half-wave plate is used to adjust and control the transmission of P-polarized light; The polarization beam splitter reflects and combines the two pulse beams with S polarization and the two pulse beams with P polarization to the laser output mirror group; The laser output mirror group outputs the combined pulse beam to the light sheet mirror group, and includes at least two total reflection mirrors.

4. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 1, wherein: The light-sheet lens assembly includes a concave lens, a third half-wave plate, a convex lens, and a cylindrical concave lens sequentially arranged along the optical path. The polarization state of the combined pulse beam is adjusted by the third half-wave plate, and the concave lens, the convex lens, and the cylindrical concave lens are used to convert the combined pulse beam into a plane light with a thickness of ≤1 mm. A total reflection mirror is set on the optical path of the light-sheet mirror group to change the propagation path of the pulse light beam so that the plane light illuminates the target shooting plane.

5. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 1, characterized in that: The signal components in the timing signal control scheme are as follows: A first pulse beam trigger signal S1 is used to control the first double-pulse laser to emit a first pulse beam; A second pulse beam trigger signal S2 is used to control the first double-pulse laser to emit a second pulse beam; A third pulse beam trigger signal P1 is used to control the second double-pulse laser to emit a third pulse beam; A fourth pulse beam trigger signal P2 is used to control the second double-pulse laser to emit a fourth pulse beam; The five timing control schemes include: Solution 1: Trigger P1, P2, S1, and S2 in sequence, with the time interval P1-P2 being the same as the time interval S1-S2. Solution 2: Trigger P1, S1, P2, and S2 in sequence, with the time intervals P1-S1 and P2-S2 independently adjustable; Solution 3: Trigger P1, S1, P2, and S2 in sequence. The time interval P1-P2 is in the nanosecond level, and the time interval S1-S2 is in the microsecond level. Solution 4: Trigger P1, S1, P2, and S2 in sequence, with the same time intervals P1-S1, S1-P2, and P2-S2; Solution 5: Trigger P1, S1, P2, and S2 in sequence. The time intervals P1-S1 are in nanoseconds, P1-P2 are in hundreds of nanoseconds, and P1-S2 are in microseconds.

6. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 1, characterized in that: The block polynomial alignment method comprises: Adjusting the laser triggering timing and the camera exposure timing so that the first pulse beam emitted by the first double-pulse laser and the third pulse beam emitted by the second double-pulse laser are triggered simultaneously, and the second pulse beam emitted by the first double-pulse laser and the fourth pulse beam emitted by the second double-pulse laser are triggered simultaneously, while ensuring that the two double-exposure cameras are exposed synchronously; Adjust the position and focus of the two dual-exposure cameras to achieve physical field of view pre-alignment, so that the field of view error is ≤30 pixels; The flow field is captured under synchronous triggering conditions. The two cameras are marked as Camera A and Camera B, and the particle images Image_A and Image_B are obtained at the same moment respectively. Perform particle identification on Image_A and Image_B respectively, extract the particle coordinate sequence, and match the corresponding particle points in the two images using the particle tracking velocimetry algorithm; Divide Image_A and Image_B into multiple grid blocks. The grid shape is square or rectangular with an aspect ratio of ≤1.

5. Set a 20% overlap area between adjacent grid blocks. Extract the coordinates of the particles in the grid. The coordinates of the corresponding particles in the two images are fitted by a cubic polynomial; In the formal experiment, the following operations are performed on each frame of image captured by Camera A and Camera B: Divide Image_A and Image_B into grid blocks according to the above grid division steps; Apply the mapping parameters of the corresponding grid to perform coordinate transformation; The aligned image sequence is generated by stitching the grid blocks together through bilinear interpolation and smoothing the overlapping areas.

7. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 6, characterized in that: The fitting of the coordinates of the corresponding particles in the two images by using a cubic polynomial comprises: Based on the matching particle points in each grid block, the sequence (x a ,y a ) i With the sequence (x b ,y b ) i Using a cubic polynomial, the expression is as follows, x b =β0+β1x a +β2y a +β3x a 2 +β4x a y a +β5y 2 +β6x 3 +β7x a 2 y a +β8x a y a 2 +β9y a 3 y b =γ0+γ1x a +γ2y a +γ3x a 2 +γ4x a y a +γ5y 2 +γ6x 3 +γ7x a 2 y a +γ8x a y a 2 +γ9y a 3 Use the least squares method to obtain the parameter sequence (β,γ) in part_i i ; The above operations are performed on all divided grid areas to obtain the parameter sequence (β,γ)1(β,γ)2(β,γ)3···(β,γ) corresponding to all areas n .

8. The method for measuring nanosecond particle velocity based on orthogonal linear polarization spectrometry according to claim 7, characterized in that: The particle tracking velocimetry algorithm includes: Particle sub-pixel positioning: Background extraction for paired tracer particle images: Background mean filtering and background separation based on intrinsic orthogonal decomposition are used to extract image background and enhance the image. Processing with a Difference of Gaussian filter or a Laplacian of Gaussian filter yields a filtered image whose response peak corresponds to the particle center; Calculate the local maximum of the filtered image to obtain the center of mass position of the particle; Taking the particle's center of mass coordinates as (i, j), a Gaussian function or quadratic function is used to fit the grayscale values ​​of the neighboring pixels to calculate the sub-pixel coordinate position. The expression is as follows: Where, F i,j is the grayscale value of the motion image formed by the tracer particles moving in the flow field at the coordinate (i, j) after removing the background image from the original image obtained by shooting. F is the motion grayscale image formed by the tracer particles moving in the flow field. F' i,j is the value of the filtered image F' obtained by processing F using a Gaussian difference filter or a Gaussian Laplacian filter at coordinate (i, j); i and j are the coordinates of the identified particle center of mass; (x, y) are the sub-pixel precision particle position coordinates obtained by the above fitting method; Histogram-based particle matching: According to the maximum flow rate and frame interval, the maximum displacement is calculated and a pre-matching candidate box is generated for each particle; Calculate the distance between the particle to be matched and each particle in the candidate frame in the second frame image; Calculate the displacement between other particles within a set range near the particle to be matched and each particle in its candidate box, and draw a displacement histogram; select the displacement with the highest frequency in the histogram as the reference displacement direction; Displacement Gaussian fitting optimization: Take the adjacent displacement values ​​of the reference displacement and perform Gaussian fitting to solve the best matching direction X of the particle fit and Y fit , the expression is as follows, Where dX max and dY max They are the candidate displacements with the largest number of occurrences in the histogram algorithm, dX max-1 、dX max+1 、dY max-1 、dY max+1 are the reference displacement directions dX of the matching particles respectively max and dY max The left and right adjacent values ​​in the histogram candidate displacement group; X fit and Y fit is the displacement of the best match obtained in the x and y directions respectively by the above method, i.e., the best match direction; Speed ​​calculation: According to the best matching direction X fit and Y fit In the second frame, the matching particles are located, and the particle velocity is calculated based on the positional relationship between the to-be-matched particles and the found matching particles in the image.

9. A particle velocity measurement system, characterized in that: include: Polarization orthogonal light source module: includes two dual-pulse lasers, the first dual-pulse laser outputs two pulse laser beams with polarization state S; The second dual-pulse laser outputs two pulse laser beams with polarization state P; Polarization beam combining module: It is a dual-path laser beam combining mirror group, including a reflective beam combining mirror group, a first half-wave plate, a second half-wave plate and a polarization beam splitter prism; the reflective beam combining mirror group adjusts the optical paths of the four pulse beams to coincide, and the polarization beam splitter prism reflects S-polarized light and transmits P-polarized light, achieving the simultaneous output of the four beams; Light sheet generation module: a concave lens, a convex lens, a cylindrical concave lens and a total reflection mirror are arranged in sequence to convert the combined light beam into a plane light with a thickness of ≤1mm; Orthogonal imaging module: Polarization splitter cube prism, which receives the scattered light from the flow field and separates it according to the polarization state; Two dual-exposure cameras are orthogonally arranged on both sides of the polarization beam splitter cube prism to capture particle images under S polarization and P polarization respectively; XY high-precision translation stage, adjust the camera position so that the field of view pre-alignment error is ≤30 pixels; Synchronous control module: includes a multi-channel synchronous controller and an acquisition computer. The multi-channel synchronous controller executes five timing control schemes; the acquisition computer is used to store images and run the block polynomial alignment algorithm and particle tracking velocimetry algorithm.

Images