A visual quantitative measurement system and method for inferring the airflow angle at the cascade outlet

Abstract

A visual quantitative measurement system and method for the airflow angle at the outlet of a reverse thrust cascade belongs to the field of engine aerodynamics. The present invention comprises a binocular photogrammetry system, a digital pulse delay signal generator, a computer, a square guide rail, and a wrench slider. The binocular photogrammetry system is slidably mounted on the square guide rail via the wrench slider. The binocular photogrammetry system is connected to the digital pulse delay signal generator. Images collected by the binocular photogrammetry system are input and stored in the computer. The digital pulse delay signal generator is connected to the computer. In order to overcome the problem that existing wire technology can only provide intuitive qualitative display, the present invention realizes the quantitative measurement capability of the airflow angle at the outlet of the reverse thrust cascade of the entire machine on the ground platform of the reverse thrust full-ring performance test piece.

Description

Technical Field

[0001] The present invention relates to an airflow angle measurement method, in particular to a visual quantitative measurement system and method for reverse thrust cascade outlet airflow angle, belonging to the field of engine aerodynamic technology. Background Art

[0002] Modern aircraft engines widely use cascade thrust reversers for braking. These devices are compact and flexible, highly reliable, and provide smooth reverse thrust. They can effectively control the reverse thrust airflow and achieve high reverse thrust efficiency. The effective area ratio and reverse thrust efficiency when the reverse thrust is fully open are important indicators in the reverse thrust design requirements, determining the compatibility with the engine during reverse thrust operation and the reverse thrust provided to the aircraft. Due to the limited domestic experience in reverse thrust design, it is not enough to support the accuracy of CFD numerical simulation methods in reverse thrust performance simulation. Therefore, it is necessary to conduct reverse thrust reduction full-ring performance tests to verify and optimize the reverse thrust aerodynamic design and provide technical reserves for reverse thrust aerodynamic design. The cascade density and outlet airflow angle are directly related to the axial reverse thrust coefficient and flow coefficient, and have a significant impact on reverse thrust performance. Therefore, measuring the outlet airflow direction of cascades of different densities when the reverse thrust is fully open is an important part of the test.

[0003] Thread tracing is a method for observing flow patterns. By attaching a thread to a model surface, the direction of the airflow can be determined by its movement. However, the traditional thread tracing method provides only a visual, qualitative display and cannot meet the analysis requirements of the impact of actual airflow deflection angles on thrust reverser blade performance. Therefore, it is urgent to develop a visual, quantitative measurement method for the airflow angle at the thrust reverser blade outlet. This method can accurately and quantitatively determine the circumferential and axial angles of the airflow at the blade outlet, addressing the measurement and analysis requirements of full-ring thrust reverser performance testing. Summary of the Invention

[0004] To overcome the problem that existing wire technology can only provide intuitive qualitative displays, the present invention provides a visual quantitative measurement system and method for the airflow angle at the outlet of the thrust reverser blade cascade, enabling quantitative measurement of the airflow angle at the outlet of the full-ring thrust reverser performance test piece on the ground platform. A brief overview of the present invention is provided below to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to identify key or important parts of the present invention, nor is it intended to limit the scope of the present invention.

[0005] The technical solution of the present invention:

[0006] Solution 1: A visual quantitative measurement system for the airflow angle at the reverse thrust cascade outlet, comprising a binocular photogrammetry system, a digital pulse delay signal generator, a computer, a square guide rail, and a wrench slider. The binocular photogrammetry system is slidably installed on the square guide rail via the wrench slider, the binocular photogrammetry system is connected to the digital pulse delay signal generator, and the images collected by the binocular photogrammetry system are input into the computer for storage, and the digital pulse delay signal generator is connected to the computer.

[0007] Preferably, the binocular photogrammetry system includes a first imaging device and a second imaging device, and a baseline distance exists between the first imaging device and the second imaging device.

[0008] Preferably, the first imaging device and the second imaging device are both fixedly connected to the square guide rail via a wrench slider.

[0009] Preferably, the first imaging device includes a first camera, a first lens, a first filter, a first annular light source, a first fixing ring, a first fixing plate, and a first connecting bracket, wherein the first filter is connected to the first lens via a thread, the first lens is connected to the first camera via a thread or a bayonet, the first camera is fixedly mounted on the first fixing plate, and the first annular light source is placed on the first fixing ring and connected to the first fixing plate via the connecting bracket;

[0010] The second imaging device includes a second camera, a second lens, a second filter, a second annular light source, a second fixing ring, a second fixing plate and a second connecting bracket. The second filter is connected to the second lens via a thread, the second lens is connected to the second camera via a thread or a bayonet, the second camera is fixedly mounted on the second fixing plate, the second annular light source is placed on the second fixing ring and connected to the second fixing plate via a second connecting bracket.

[0011] Solution 2: A visual quantitative measurement method for the reverse propulsion cascade outlet airflow angle is implemented based on the visual quantitative measurement system for the reverse propulsion cascade outlet airflow angle described in Solution 1, and includes the following steps:

[0012] Step 1, thread pasting: Paste the thread along the flow direction of the cascade block to be measured at the circumferential center of the grid at the trailing edge of the flow-direction blade of the cascade block, and then install the cascade block on the support ring of the reverse thrust full-ring performance test piece;

[0013] Step 2: Arrange and calibrate the binocular photogrammetry system: Arrange the binocular photogrammetry system on the circumferential outside of the cascade block after the threads have been attached in Step 1. Align the first and second imaging devices of the binocular photogrammetry system with the cascade grille to be measured, so that the cascade grille of the cascade block is within the camera's field of view. Adjust the focal lengths of the first and second lenses to ensure a clear cascade image, and adjust the first and second ring light sources for fill light according to on-site conditions. Before the test, calibrate the camera parameters of the binocular photogrammetry system using a checkerboard calibration plate. During the test, ensure that there is no relative displacement between the first and second imaging devices in the binocular photogrammetry system.

[0014] Step 3, grid coordinate system calibration: Before the test, increase the exposure time of the first camera and the second camera, collect images, define the origin of the grid coordinate in the cascade block as (m,n), which is surrounded by the corner points (m,n), (m+1,n), (m,n+1) and (m+1,n+1), define the axis of the model coordinate system along the airflow direction as the positive direction of the x-axis, the vertical cascade block plane pointing to the axis direction as the opposite direction of the z-axis, and the y-axis satisfies the right-hand rule, that is, the positive direction of x points to (m,n+1), and the positive direction of y points to (m+1,n), reconstruct the three-dimensional coordinates of the corner points (m,n), (m+1,n), and (m,n+1) in the camera coordinate system, and calculate the axis vector of the current cascade block grid and According to the right-hand rule, we can obtain Then obtain the grid coordinate system vector matrix;

[0015] Step 4: Test image acquisition: Reduce the exposure time to ensure that there is no motion blur in the image when the silk thread moves; after the drop pressure ratio is judged to be stable, control the first camera and the second camera to perform synchronous acquisition to obtain the silk thread image during the test process

[0016] Step 5: 3D reconstruction of the thread endpoints: The image coordinates of the threads are obtained based on line detection technology. The thread targets are matched with the same name according to the corresponding constraints. After matching, the image coordinates of the two end points of the thread in the grid (m, n) are extracted. The 3D coordinates of the thread endpoints in the camera coordinate system are reconstructed using triangulation.

[0017] Step 6, calculate the outlet airflow angle: define the airflow circumferential angle θ as the angle between the projection of the wire segment vector on the grid yoz plane and the y-axis, and the airflow axial angle φ as the angle between the wire segment vector and the x-axis. Based on the line endpoint coordinates obtained in step 5 and the grid coordinate system vector matrix obtained in step 3, the outlet airflow angle represented by the wire at the current position is obtained.

[0018] Preferably, in step 1, the length of the thread pasted at the circumferential center of the grid at the trailing edge of the flow blade of the cascade block along the flow direction of the cascade block should not exceed the blade spacing, that is, the distance between the corresponding points of two adjacent flow blades along the frontal line.

[0019] Preferably: in step 3: before the test, increase the exposure time to take a clear cascade image I 1 , I 2 , to I 1 , I 2 Perform image processing and obtain the image coordinates of the three corner points at the grid positions (m,n), (m+1,n), and (m,n+1) by extracting the corner points and and The three-dimensional coordinates P of these three corner points in the camera coordinate system are obtained based on triangulation reconstruction. m,n 、P m+1,n and P m,n+1 , where P m,n and P m+1,n Circumferentially adjacent, P m,n and P m,n+1 Flow to adjacent

[0020] Calculate the axis vector of the coordinate system of the current grid:

[0021]

[0022]

[0023]

[0024] The coordinate system vector matrix of the cascade block grid is obtained from this

[0025] Preferably: in step 5: obtain the two camera images of the silk thread in the system based on the Steger line center detection algorithm The line center image coordinates l 1 and l 2 After matching the silk targets with the same name according to the corresponding constraints, the image coordinates of the two end points of the silk thread in the grid (m,n) are extracted. and The three-dimensional coordinates L of the two end points of the silk thread in the camera coordinate system are obtained based on triangulation reconstruction. mn,1 and L mn,2 , where L mn,1 is the end point of the thread, L mn,2 The free end of the thread.

[0026] Preferably, in the steps: the three-dimensional coordinates of the endpoints of the thread in the camera coordinate system are rotated by the relationship R mn Convert to the grid coordinate system to get the new thread endpoint coordinate E mn,1 =L mn,1 / R mn and E mn,2 =L mn,2 / R mn , then in the grid coordinate system, the thread vector under the action of airflow during the test is That is, the outlet airflow angle represented by the current position thread is obtained;

[0027] The angle between the projection of this vector on the grid yoz plane and the y-axis is:

[0028]

[0029] The angle between the projection of this vector on the grid xoy plane and the x-axis is:

[0030]

[0031] The present invention has the following beneficial effects:

[0032] 1. This method eliminates the need for special optical path construction and modification. By detecting the center of the threads in the image, extracting the endpoints of the line segments for 3D reconstruction and post-processing, the circumferential and axial angles of the airflow at the cascade outlet can be obtained. This reduces the complexity of the experimental equipment, minimizes errors and manual intervention, and improves measurement accuracy, operational flexibility, and efficiency.

[0033] 2. The present invention realizes the quantitative measurement of the airflow angle at the outlet of the entire reverse thrust cascade on the ground platform, solves the measurement and analysis needs of the reverse thrust full-ring performance test, can efficiently and accurately measure the circumferential angle and axial angle of the airflow at the cascade outlet, and optimizes the experimental process and result quality of the airflow angle measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a flow chart of a visual quantitative measurement method for inferring the airflow angle at the cascade outlet;

[0035] Figure 2 Schematic diagram of the structure of the cascade block in the present invention, wherein (a) is an indication diagram of the grid circumferential center position of the cascade block, and (b) is a schematic diagram of the grid coordinate system;

[0036] Figure 3 Schematic diagram of the structure of the binocular photogrammetry system of the present invention;

[0037] Figure 4 is a schematic structural diagram of a first imaging device and a second imaging device of the present invention;

[0038] In the figure, 1-binocular photogrammetry system, 2-digital pulse delay signal generator, 3-computer, 4-square guide rail, 5-wrench slider, 6-blade block, 7-streamward blade trailing edge, 8-grid circumferential center position, 11-first imaging device, 12-second imaging device, 111-first camera, 112-first lens, 113-first filter, 114-first annular light source, 115-first fixing ring, 116-first fixing plate, 117-first connecting bracket, 121-second camera, 122-second lens, 123-second filter, 124-second annular light source, 125-second fixing ring, 126-second fixing plate, 127-second connecting bracket. DETAILED DESCRIPTION

[0039] To make the objectives, technical solutions, and advantages of the present invention more clearly apparent, the present invention is described below using specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely illustrative and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0040] Specific implementation method 1: Combination Figure 1-Figure 4 The present embodiment is described. A visual quantitative measurement system for reverse-estimating the airflow angle of a blade cascade outlet is provided in the present embodiment, and includes a binocular photogrammetry system 1, a digital pulse delay signal generator 2, a computer 3, a square guide rail 4, and a wrench slider 5. The binocular photogrammetry system 1 is slidably installed on the square guide rail 4 through the wrench slider 5. The binocular photogrammetry system 1 is connected to the digital pulse delay signal generator 2. The image collected by the binocular photogrammetry system 1 is input into the computer 3 for storage. The digital pulse delay signal generator 2 is connected to the computer 3, and the computer 3 is used to input a working signal to the digital pulse delay signal generator 2.

[0041] The binocular photogrammetry system 1 includes a first imaging device 11 and a second imaging device 12 . The first imaging device 11 and the second imaging device 12 have the same structure and have a baseline distance.

[0042] The digital pulse delay signal generator 2 is used to provide an external trigger signal to the first imaging device 11 and the second imaging device 12 to perform image acquisition, so as to ensure synchronous image acquisition of the first imaging device 11 and the second imaging device 12;

[0043] The first imaging device 11 and the second imaging device 12 are both fixedly connected to the square guide rail 4 via the wrench slider 5 .

[0044] The first imaging device 11 includes a first camera 111, a first lens 112, a first filter 113, a first annular light source 114, a first fixing ring 115, a first fixing plate 116 and a first connecting bracket 117. The first filter 113 is connected to the first lens 112 via a thread, the first lens 112 is connected to the first camera 111 via a thread or a bayonet, the first camera 111 is fixedly mounted on the first fixing plate 116, and the first annular light source 114 is placed on the first fixing ring 115 and connected to the first fixing plate 116 via a connecting bracket 117.

[0045] The second imaging device 12 includes a second camera 121, a second lens 122, a second filter 123, a second annular light source 124, a second fixing ring 125, a second fixing plate 126 and a second connecting bracket 127. The second filter 123 is connected to the second lens 122 via a thread, and the second lens 122 is connected to the second camera 121 via a thread or a bayonet. The second camera 121 is fixedly mounted on the second fixing plate 126. The second annular light source 124 is placed on the second fixing ring 125 and is connected to the second fixing plate 126 via a second connecting bracket 127.

[0046] The first ring light source 114 provides additional lighting for the first ring camera 111, and similarly, the second ring light source 124 provides additional lighting for the second camera 121. Taking the first ring light source 114 and the first ring camera 111 as an example, since the first ring light source 114 directly surrounds the front of the first ring lens 112, the light can be evenly irradiated onto the silk thread in the same direction, which helps to reduce shadows and light spots, make the silk thread details clearer, and improve image quality.

[0047] Specific implementation method 2: Combination Figure 3-Figure 4 This embodiment describes a visual quantitative measurement method for reverse-estimating the airflow angle at the cascade outlet. This method is implemented based on the visual quantitative measurement system for reverse-estimating the airflow angle at the cascade outlet described in the first embodiment, and includes the following steps:

[0048] Step 1, thread pasting: Paste the thread along the flow direction of the cascade block 6 to be measured at the grid circumferential center position 8 of the trailing edge 7 of the flow-direction blade of the cascade block 6, and then install the cascade block 6 on the support ring of the reverse thrust full-ring performance test piece;

[0049] Step 2: Arrange and calibrate the binocular photogrammetry system 1: Arrange the binocular photogrammetry system 1 on the outer side of the cascade block 6 after the thread is attached in step 1. Align the first imaging device 11 and the second imaging device 12 of the binocular photogrammetry system 1 with the cascade grille to be measured so that the cascade grille of the cascade block 6 is within the camera's field of view; adjust the focal lengths of the first lens 112 and the second lens 122 to make the cascade image clear, and adjust the first ring light source 114 and the second ring light source 124 for fill light according to the on-site conditions; before the test, use a checkerboard calibration plate to calibrate the camera parameters of the binocular photogrammetry system 1; during the test, ensure that there is no relative displacement between the first imaging device 11 and the second imaging device 12 in the binocular photogrammetry system 1;

[0050] Step 3, grid coordinate system calibration: Before the test, increase the exposure time of the first camera 111 and the second camera 121, collect images, define the origin of the grid coordinate in the cascade block 6 as (m,n), which is surrounded by the corner points (m,n), (m+1,n), (m,n+1) and (m+1,n+1), define the axis of the model coordinate system along the airflow direction as the positive direction of the x-axis, the plane perpendicular to the cascade block 6 points to the axis direction as the opposite direction of the z-axis, and the y-axis satisfies the right-hand rule, that is, the positive direction of x points to (m,n+1), and the positive direction of y points to (m+1,n), reconstruct the three-dimensional coordinates of the corner points (m,n), (m+1,n), and (m,n+1) in the camera coordinate system, and calculate the axis vector of the current cascade block 6 grid and According to the right-hand rule, we can obtain Then obtain the grid coordinate system vector matrix;

[0051] Step 4, test image acquisition: reduce the exposure time to ensure that the image is free of motion blur when the silk thread moves; after the pressure drop ratio is judged to be stable, the first camera 111 and the second camera 121 in the control system are synchronously acquired to obtain the silk thread image during the test process

[0052] Step 5: 3D reconstruction of the thread endpoints: The image coordinates of the threads are obtained based on line detection technology. The thread targets are matched with the same name according to the corresponding constraints. After matching, the image coordinates of the two end points of the thread in the grid (m, n) are extracted. The 3D coordinates of the thread endpoints in the camera coordinate system are reconstructed using triangulation.

[0053] Step 6, calculate the outlet airflow angle: define the airflow circumferential angle θ as the angle between the projection of the wire segment vector on the grid yoz plane and the y-axis, and the airflow axial angle φ as the angle between the wire segment vector and the x-axis. Based on the line endpoint coordinates obtained in step 5 and the grid coordinate system vector matrix obtained in step 3, the outlet airflow angle represented by the wire at the current position is obtained.

[0054] In step 1, a thread is attached to the cascade block 6 at the circumferential center 8 of the grid, at the trailing edge 7 of the flow blade. The thread length should not exceed the blade pitch, i.e., the distance along the frontal line between corresponding points on two adjacent flow blades. This ensures that the thread only indicates the direction of the cascade's outlet airflow and prevents oscillation or even reversal due to excessive thread length. The cascade block 6 is then installed in the designated position on the support ring of the reverse thrust full-ring performance test piece.

[0055] In step 2, the binocular photogrammetry systems 1 are positioned away from the airflow. Each binocular photogrammetry system 1 includes a first camera 111 and a second camera 121. The common field of view of the binocular photogrammetry systems 1 is aligned with the grille of the cascade segment 6 to be measured. First and second lenses 112, 122, with appropriate focal lengths are selected, and the focal length and aperture are adjusted to ensure the field of view meets the measurement requirements. Appropriate fill light is applied based on site conditions. One binocular photogrammetry system 1 can measure the viewing angle of one quadrant of the test piece. If a full-circle test is required, four binocular photogrammetry systems 1 are required.

[0056] In step 3: before the test, increase the exposure time to capture a clear cascade image I 1 , I 2 , to I 1 , I 2 Perform image processing and obtain the image coordinates of the three corner points at the grid positions (m,n), (m+1,n), and (m,n+1) by extracting the corner points and and The three-dimensional coordinates P of these three corner points in the camera coordinate system are obtained based on triangulation reconstruction. m,n 、P m+1,n and P m,n+1 , where P m,n and P m+1,n Circumferentially adjacent, P m,n and P m,n+1 Flow to adjacent

[0057] Calculate the axis vector of the coordinate system of the current grid:

[0058]

[0059]

[0060]

[0061] The coordinate system vector matrix of the cascade block 6 grid is obtained

[0062] In step 5: Based on the Steger line center detection algorithm, the two camera images of the silk thread in the system are obtained respectively. The line center image coordinates l 1 and l 2 After matching the silk targets with the same name according to the corresponding constraints, the image coordinates of the two end points of the silk thread in the grid (m,n) are extracted. and The three-dimensional coordinates L of the two end points of the silk thread in the camera coordinate system are obtained based on triangulation reconstruction. mn,1 and L mn,2 , where L mn,1 is the end point of the thread, L mn,2 The free end of the thread.

[0063] In the steps: the three-dimensional coordinates of the endpoints of the silk thread in the camera coordinate system are rotated by the relationship R mn Convert to the grid coordinate system to get the new thread endpoint coordinate E mn,1 =L mn,1 / R mn and E mn,2 =L mn,2 / R mn , then in the grid coordinate system, the thread vector under the action of airflow during the test is That is, the outlet airflow angle represented by the current position thread is obtained;

[0064] The angle between the projection of this vector on the grid yoz plane and the y-axis is:

[0065]

[0066] The angle between the projection of this vector on the grid xoy plane and the x-axis is:

[0067]

[0068] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be permuted and combined. Those skilled in the art can exhaust all possibilities based on the mathematical knowledge of permutations and combinations. Therefore, the present invention will no longer describe the technical solutions after permutations and combinations one by one, but it should be understood that the technical solutions after permutations and combinations have been disclosed by the present invention.

[0069] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A visual quantitative measurement method for inferring the airflow angle at the cascade outlet, characterized in that: The following steps are involved: Step 1, thread pasting: stick the thread at the grid circumferential center position (8) of the trailing edge (7) of the flow-direction blade of the cascade block (6) along the flow direction of the cascade block (6) to be measured, and then install the cascade block (6) on the support ring of the reverse thrust full-ring performance test piece; Step 2, arrangement and calibration of the binocular photogrammetry system (1): the binocular photogrammetry system (1) is arranged on the circumferential outer side of the cascade block (6) after the thread is pasted in step 1, and the first imaging device (11) and the second imaging device (12) of the binocular photogrammetry system (1) are aligned with the cascade grid to be measured so that the cascade grid of the cascade block (6) is located within the camera field of view; the focal lengths of the first lens (112) of the first imaging device (11) and the second lens (122) of the second imaging device (12) are adjusted to make the cascade image clear, and the first annular light source (114) of the first imaging device (11) and the second annular light source (124) of the second imaging device (12) are adjusted according to the on-site conditions to perform fill light; before the test, the camera parameters of the binocular photogrammetry system (1) are calibrated using a checkerboard calibration plate; during the test, it is necessary to ensure that there is no relative displacement between the first imaging device (11) and the second imaging device (12) in the binocular photogrammetry system (1); Step 3, grid coordinate system calibration: Before the test, increase the exposure time of the first camera (111) of the first imaging device (11) and the second camera (121) of the second imaging device (12), collect images, and define the origin of the grid coordinates in the cascade block (6) as , which consists of the corner points 、 and The axis of the model coordinate system is defined as the positive direction of the x-axis along the airflow direction, the plane of the vertical blade block (6) points to the axis direction as the opposite direction of the z-axis, and the y-axis satisfies the right-hand rule, that is, the positive direction of x points to , the positive y direction points to , reconstruct corner points 、 Calculate the axis vector of the grid of the current cascade block (6) in the three-dimensional space coordinates of the camera coordinate system and , according to the right-hand rule, we get , and then obtain the grid coordinate system vector matrix; Step 4, test image acquisition: reduce the exposure time to ensure that the image is free of motion blur when the silk thread moves; after the drop pressure ratio is judged to be stable, control the first camera (111) and the second camera (121) to perform synchronous acquisition to obtain the silk thread image during the test process , ; Step 5, 3D reconstruction of the thread endpoints: Based on the line detection technology, the image coordinates of the threads are obtained respectively. The thread targets are matched with the same name according to the corresponding constraints. After matching, the grid is extracted. The image coordinates of the two end points of the inner silk thread are used to reconstruct the three-dimensional coordinates of the silk thread end points in the camera coordinate system through triangulation; Step 6, outlet airflow angle calculation: define the airflow circumferential angle For the silk line segment vector in the grid Projection of the plane and Axis angle, airflow axial angle The vector of the silk line segment is The angle between the axes is obtained according to the three-dimensional coordinates of the thread endpoint in the camera coordinate system obtained in step 5 and the grid coordinate system vector matrix obtained in step 3, that is, the outlet airflow angle represented by the thread at the current position is obtained.

2. A visual quantitative measurement method for reverse cascade outlet airflow angle according to claim 1, characterized in that: In the step 1, a thread is pasted at the circumferential center position (8) of the grid of the trailing edge (7) of the flow blade of the cascade block (6) along the flow direction of the cascade block (6). The length of the thread should not exceed the blade spacing, that is, the distance between the corresponding points of two adjacent flow blades along the frontal line direction.

3. The visual quantitative measurement method for reverse cascade outlet airflow angle according to claim 2, characterized in that: In step 3: Before the test, increase the exposure time to capture a clear cascade image. , ,right , Perform image processing and obtain a grid by extracting corner points 、 The image coordinates of the three corner points of the position 、 , 、 and and , based on triangulation reconstruction, the three-dimensional coordinates of these three corner points in the camera coordinate system are obtained 、 and ,in and Circumferentially adjacent, and Flow to adjacent Calculate the axis vector of the coordinate system of the current grid: ； ； ； The coordinate system vector matrix of the grid of the blade block (6) is obtained from this , .

4. The visual quantitative measurement method for reverse cascade outlet airflow angle according to claim 3, characterized in that: In step 5: Based on the Steger line center detection algorithm, the two camera images of the silk thread in the system are obtained respectively. , The line center image coordinates on and , after matching the wire targets with the same name according to the corresponding constraints, the grid is extracted Image coordinates of the two end points of the inner wire 、 and 、 , based on triangulation, the three-dimensional coordinates of the two end points of the silk thread in the camera coordinate system are reconstructed and ,in For the sticking end of the silk thread, The free end of the thread.

5. The visual quantitative measurement method for reverse cascade outlet airflow angle according to claim 4, characterized in that: In step 6: the three-dimensional coordinates of the endpoints of the silk thread in the camera coordinate system are rotated by Convert to the grid coordinate system to get the new thread endpoint coordinates = / and = / , then in the grid coordinate system, the thread vector under the action of airflow during the test is , that is, obtaining the outlet airflow angle represented by the current position thread; The vector is in the grid Projection of the plane and Angle of the axes: ； The vector is in the grid Projection of the plane and Angle of the axes: 。 6. A visual quantitative measurement method for reverse cascade outlet airflow angle according to claim 5, characterized in that: The first imaging device (11) and the second imaging device (12) are both fixedly connected to the square guide rail (4) via a wrench slider (5).

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