Multi-camera background schlieren arbitrary coplanar view field splicing method

Abstract

The invention discloses a multi-camera background schlieren arbitrary coplanar field-of-view splicing method, which comprises the following steps of: synchronously acquiring background images and experimental images of different areas of a complex geometric structure flow field by adopting a plurality of cameras under the conditions of still air and fluid flow; taking a view field delimited according to an actual complex flow field structure as a spliced view field to obtain a full-view contour, and drawing an unstructured grid for the full-view contour; on the basis of the sub-displacement fields shot by the cameras respectively, displacement on each grid point is obtained through interpolation according to the regional position corresponding relation, and full-field displacement distribution after splicing is obtained; and calculating the density information of the flow field by solving a Poisson equation according to the spliced full-field displacement distribution. The problem of limitation of a single camera system in large-view-field complex flow field measurement is solved, high-precision measurement of a complex geometric structure and a large-scale flow field is achieved, the application range of the BOS technology is expanded, and the measurement precision and flexibility are improved.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a method for splicing multi-camera background schlieren arbitrary coplanar fields of view, which is used to achieve high-precision measurement of flow fields with complex geometric structures. Background Art

[0002] Background Oriented Schlieren (BOS) technology is a contactless density gradient visualization method based on the principle of optical measurement. This technology is widely used in aerodynamic experiments, aerospace engine combustion testing and other fields. However, in large-scale complex flow fields, the field of view of the traditional single-camera BOS system is limited and cannot cover the entire measurement area, resulting in incomplete acquisition of flow field data. At the same time, limited by the resolution and field of view of the camera, the measurement accuracy will be reduced, especially for flow fields with complex geometric boundaries (such as the scramjet engine combustion chamber described in patent CN105180211B). It is difficult for a single-camera system to cover multiple key areas at the same time, resulting in incomplete measurement results or reduced accuracy.

[0003] The above problems can usually be solved by field of view stitching. For example, Chinese patent publication number CN114964713B discloses a schlieren experimental method for large-scale flow field visual imaging, and Chinese patent publication number CN117782517B discloses a two-dimensional flow field test method and system based on multi-camera background schlieren. However, both of them are mainly aimed at stitching one-dimensional regularly arranged fields of view, which can only solve the measurement problem of long flow fields in large-scale flow fields, but cannot solve the measurement problem of other large-scale flow fields and flow fields with complex geometric structures. Summary of the invention

[0004] The purpose of the present invention is to provide a method for stitching multi-camera background schlieren arbitrary coplanar fields of view, which can effectively expand the field of view range of the background schlieren method, solve the problem of limited field of view of traditional single-camera systems in large-scale measurements, and improve measurement accuracy.

[0005] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0006] A method for stitching a multi-camera background schlieren arbitrary coplanar field of view, the method comprising the following steps:

[0007] Step A, under static air and fluid flow conditions, using multiple cameras to synchronously capture background images and experimental images of different regions of the complex geometric structure flow field;

[0008] Step B, using the field of view defined according to the actual complex flow field structure as the spliced ​​field of view, obtaining the full field of view outline, and drawing an unstructured grid for it; based on the data of the sub-displacement field captured by each camera, according to the corresponding relationship between the regional positions, the data on the sub-displacement field is interpolated to the grid points of each unstructured grid to obtain the spliced ​​full-field displacement distribution;

[0009] Step C, according to the displacement distribution of the entire field after splicing, the density information of the flow field is calculated by solving the Poisson equation.

[0010] Step A further comprises:

[0011] A multi-camera background schlieren system is constructed, wherein the multi-camera background schlieren system comprises a background plate, a complex geometric structure to be measured and N cameras, where N is an integer greater than or equal to 2; the background plate is placed behind the flow field of the complex geometric structure to be measured, the surface of the background plate is covered with random scattered point patterns, and completely covers the entire measured flow field;

[0012] Adjust the position and posture of each camera so that there is an overlapping area between the fields of view of adjacent cameras and all cameras can see the background pattern; the fields of view to be stitched captured by multiple cameras are any coplanar feature fields of view, but the positions of the cameras may or may not be coplanar, and there is no limitation on this.

[0013] Furthermore, the complex geometric structure includes a scramjet engine model combustion chamber.

[0014] Furthermore, the frame rate of the camera ranges from 30fps to 3000fps, depending on the flow speed.

[0015] Further, a robotic arm or a rigid bracket is used to fix the camera.

[0016] Furthermore, in step B, the process of obtaining the sub-displacement field includes the following steps:

[0017] The particle displacement of each camera is calculated, and according to the field of view of the camera on the background image, the functions u(x,y) and v(x,y) of the x-direction displacement u and the y-direction displacement v with respect to x and y are obtained, and the displacement matrix of the local flow field is obtained.

[0018] Furthermore, interpolation methods for interpolating data on the sub-displacement field to grid points of each unstructured grid include: bilinear interpolation, bicubic interpolation, and inverse distance weighted interpolation.

[0019] Furthermore, in step C, based on the spliced ​​flow field displacement information, the density distribution in the flow field is calculated by solving the Poisson equation and the Gladstone-Dale equation; based on the boundary conditions, the velocity and temperature distribution of the flow field are calculated to obtain complete flow field physical information.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] First, the multi-camera background schlieren arbitrary coplanar field of view stitching method of the present invention can stitch the coplanar local flow field images of arbitrary features (arbitrary size, arbitrary shape, arbitrary overlapping mode) taken by multiple cameras through full-field unstructured grid drawing and displacement field interpolation, and can cover more complex flow field areas in the same measurement process, significantly expanding the field of view of BOS.

[0022] Second, the present invention combines precise camera calibration and stitching algorithms to ensure the accuracy of the measurement data, solves the limitations of single-camera systems in measuring complex flow fields with a large field of view, and achieves high-precision measurement of complex geometric structures and large-scale flow fields.

[0023] Third, the multi-camera background schlieren arbitrary coplanar field of view stitching method of the present invention has a flexible system structure, and the number and position of cameras can be adjusted according to experimental requirements to adapt to different experimental scenarios; it can be widely used in aerodynamics, combustion and other measurement fields involving fluid density changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the installation diagram of the multi-camera arbitrary coplanar field of view background splicing schlieren system.

[0025] Figure 2 This is a flow chart of the multi-camera background schlieren arbitrary coplanar field of view stitching technology.

[0026] Figure 3 Flow chart for measuring flow field parameters of complex structures using arbitrary coplanar field stitching technology.

[0027] Figure 4 This is a schematic diagram of interpolation using the bilinear interpolation method.

[0028] The reference numerals in the figure are: 1 is a background plate for generating clear irregular background points; 2 is a complex structure flow field to be measured, and the flow field has different density gradients and velocity distributions; 31 to 34 are cameras. DETAILED DESCRIPTION

[0029] The embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0030] The present invention discloses a method for stitching a multi-camera background schlieren arbitrary coplanar field of view, the method comprising the following steps:

[0031] S1, under still air and fluid flow conditions, multiple cameras are used to synchronously capture background images and experimental images of different regions of the complex geometric structure flow field;

[0032] S2, the field of view defined according to the actual complex flow field structure is used as the stitched field of view to obtain the full field of view outline, and an unstructured grid is drawn for it; based on the sub-displacement field captured by each camera, the displacement on each grid point is interpolated according to the corresponding relationship of the regional position to obtain the full-field displacement distribution after stitching;

[0033] S3, according to the displacement distribution of the entire field after splicing, the density information of the flow field is calculated by solving the Poisson equation.

[0034] The following describes the device system and specific process of measuring the flow field in the combustion chamber of a scramjet engine model by using a specific embodiment. Figure 1 , including the following key components:

[0035] Background plate 1, characterized in that it uses a reflective material with high reflectivity and is covered with a random scattered pattern on the surface. Its size must be selected according to the experimental requirements, but it must ensure that the background can cover the entire measured flow field. Generally speaking, its length and width are not less than 110% of the maximum length and width of the measured flow field. The background plate should be placed behind the measured flow field to ensure that all cameras can clearly see the background pattern.

[0036] The measured complex structure flow field 2, in this embodiment, the measured complex structure flow field 2 refers to the combustion chamber of the scramjet engine model, which has a relatively complex structure, including an injection section, a flame stabilization cavity, an air bleed channel, and an expansion section connected in sequence. By designing the cavity structure, the supersonic airflow forms a reflow zone when flowing through the cavity, thereby achieving the effect of stabilizing the flame. Due to its complex internal structure, when the background schlieren method is applied to measure the internal flow field of the engine combustion chamber, the field of view of a camera often cannot cover the entire flow field and cannot capture the corresponding flow structure. Multiple cameras 31, 32, 33, 34: According to the geometric structure of the flow field to be measured, multiple cameras are arranged at different angles and positions, and the field of view of each camera covers different areas of the flow field. In this embodiment, the number of cameras is 4, and the multiple cameras are arranged at different angles and positions. The field of view of each camera covers different areas of the flow field. The number of cameras actually used is selected according to the complexity of the geometric structure of the flow field. In this embodiment, camera 31 is arranged in the left area of ​​the flow field, and its field of view covers part of the left flow field; camera 32 is arranged in the left area in the middle of the flow field, ensuring that there is a partial overlapping field of view with camera 31; camera 33 is arranged in the right area in the middle of the flow field, sharing a certain overlapping field of view with camera 32; camera 34 is arranged on the right side of the flow field, and its field of view covers part of the right flow field, and has an overlapping area with camera 33. The frame rate of the camera is 30fps to 3000fps, and the specific frame rate selection depends on the flow speed. The exposure time can be dynamically adjusted between 1 microsecond and 1 second. Use a high-precision robotic arm or a rigid bracket to fix the camera to ensure that the camera position is stable during the entire experiment.

[0037] like Figure 3 As shown, the specific measuring method of the present invention comprises the following steps:

[0038] Step S1, construction of a multi-camera background schlieren system: construct a BOS measurement system including multiple high-resolution cameras, and multiple cameras are arranged at different positions according to experimental requirements to cover different areas of the flow field. The multi-camera system can be flexibly adjusted according to measurement requirements, including increasing or decreasing the number of cameras and changing the camera arrangement to meet the needs of different experimental scenarios.

[0039] Step S2, camera position adjustment: adjust the focal length and height of the shooting camera to focus the background image on the photosensitive element, and ensure that there is an overlapping area between the fields of view of adjacent cameras to facilitate subsequent image stitching and flow field data fusion.

[0040] Step S3, image acquisition: Under still air and experimental conditions, all cameras synchronously acquire background images.

[0041] Step S4, calculate the displacement matrix of the local flow field: use the single-pixel high-resolution correlation method to calculate the particle displacement of each camera, and according to the field of view of the camera on the background image, obtain the functions u(x,y) and v(x,y) of the x-direction displacement u and the y-direction displacement v with respect to x and y.

[0042] Step S5, field of view stitching: In one embodiment of the present invention, the flow field of the scramjet combustion chamber is used as the stitched field of view, and an unstructured grid is drawn for it. The displacement on each grid point is obtained by interpolation of the sub-displacement fields captured by each camera to form a stitched displacement field.

[0043] The sub-displacement fields captured by the camera are regular pixel blocks and are structured. However, since the field of view is arbitrary, the stitching and overlapping methods are also arbitrary. It is difficult to structure the overlapping parts of the sub-displacement fields. To address this problem, the present invention innovatively proposes to draw the field of view into an unstructured grid and then obtain the stitched displacement field through interpolation.

[0044] In the present invention, there are multiple interpolation methods for interpolating the data on the sub-displacement field to the grid points of each unstructured grid, and bilinear interpolation, bicubic interpolation, inverse distance weighted interpolation, etc. can be used. Figure 4 This is a schematic diagram of the principle of using bilinear interpolation method to interpolate sub-displacement field data. In practical applications, other interpolation methods can also be used.

[0045] Since the sub-displacement matrix is ​​directly calculated by the camera, it is actually a structured displacement matrix. After drawing the unstructured grid, the grid nodes will fall into the pixel grid of the sub-displacement matrix. There are two cases at this time. One is that the grid nodes fall into a single matrix, such as Figure 4 (a); Another case is that the grid nodes fall where the matrices overlap, such as Figure 4 (b).

[0046] like Figure 4 As shown in (a), the goal is to obtain the displacement vector of the flow field at the grid node P(x,y), given that Q 11 (x1,y1),Q 12 (x1,y2),Q 21 (x2,y1),Q 22 The coordinates of (x2, y2) and the corresponding displacement vector. First, interpolate in the x direction to get R1 and R2, and then interpolate in the y direction to get P. The calculation formulas for R1 and R2 are as follows:

[0047]

[0048] By interpolating in the y direction, we get:

[0049]

[0050] After substitution, the final displacement vector result is obtained:

[0051]

[0052] like Figure 4 As shown in (b), if a grid node falls where the matrices overlap, there are eight coordinate points of two sub-matrices around the grid node, namely, the Q 11 (x1,y1),Q 12 (x1,y2),Q 21 (x2,y1),Q 22 (x2,y2) and Q′ in matrix two 11 (x11,y1′),Q′ 12 (x′1,y2′),Q′ 21 (x′2,y1′),Q′ 22 (x′2, y2′), then perform two bilinear interpolations on the grid node, that is, repeat the above operation on the four nodes of matrix one to obtain the interpolated displacement P(x, y), and then repeat the same operation on the four nodes of matrix two to obtain the interpolated displacement P′(x, y). The displacement obtained by taking the average of the two is the actual displacement of the unstructured grid nodes in the overlapping area.

[0053] Step S6, solving the flow field physical information: According to the spliced ​​flow field displacement information, the density distribution in the flow field is calculated by solving the Poisson equation and the Gladstone-Dale equation, and the velocity and temperature distribution of the flow field are further calculated according to other boundary conditions to obtain complete flow field physical information. Figure 2 The complete scramjet engine model combustion chamber field of view stitching process is more clearly displayed, including the process from image acquisition to solving the physical field of the full field of view. The process is as follows:

[0054] First, when the combustion chamber is not working, all cameras synchronously shoot the background pattern without deflection through the trigger signal; further, when the combustion chamber is working, all cameras synchronously shoot the background pattern with deflection. Using the single-pixel high-resolution correlation method, the images with deflection and without deflection are processed to obtain the displacement matrix measured independently by each camera, that is, Figure 2 The unspliced ​​displacement field in . According to the information of the previous camera calibration, this set of displacement matrices is arranged according to the actual spatial position. At this time, there is overlap between different displacement matrices, and there are duplications and misalignments in the matrices. At the same time, the flow field of the scramjet combustion chamber is used as the spliced ​​field of view to obtain the full field of view contour and draw an unstructured grid. The displacement on each grid point in the grid is obtained by interpolation of the sub-displacement field (all points on) obtained by each camera respectively, forming a spliced ​​displacement field. In this way, the splicing of overlapping areas is achieved. For the spliced ​​flow field displacement information, based on the unstructured grid, the Poisson equation of arbitrary shape is solved by the variational method to calculate the refractive index distribution in the flow field. The flow field density distribution is further calculated in combination with the Gladstone-Dale equation, and the complete flow field physical information is further calculated based on other boundary conditions. Through this method, the splicing of coplanar fields of view of arbitrary shapes, arbitrary sizes, and arbitrary overlapping methods is realized, and more complex flow field areas can be covered in the same measurement process, which significantly expands the application scope of BOS.

[0055] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0056] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems) and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0057] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0058] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0059] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0060] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for stitching arbitrary coplanar fields of view of multi-camera background schlieren, characterized in that: The method for stitching arbitrary coplanar fields of view of multi-camera background schlieren comprises the following steps: Step A, under still air and fluid flow conditions, using multiple cameras to synchronously capture background images and experimental images of different regions of the complex geometric structure flow field; Step B, using the field of view defined according to the actual complex flow field structure as the spliced ​​field of view, obtaining the full field of view outline, and drawing an unstructured grid for it; based on the data of the sub-displacement field captured by each camera, according to the corresponding relationship between the regional positions, the data on the sub-displacement field is interpolated to the grid points of each unstructured grid to obtain the spliced ​​full-field displacement distribution; Step C, according to the displacement distribution of the entire field after splicing, the density information of the flow field is calculated by solving the Poisson equation.

2. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: Step A further comprises: A multi-camera background schlieren system is constructed, wherein the multi-camera background schlieren system comprises a background plate, a complex geometric structure to be measured and N cameras, where N is an integer greater than or equal to 2; the background plate is placed behind the flow field of the complex geometric structure to be measured, the surface of the background plate is covered with random scattered point patterns, and completely covers the entire measured flow field; The position and posture of each camera are adjusted so that there is an overlapping area between the fields of view of adjacent cameras and all cameras can see the background pattern; the fields of view to be stitched captured by multiple cameras are coplanar arbitrary feature fields of view.

3. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: The complex geometry includes a model scramjet combustion chamber.

4. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: The frame rate of the camera ranges from 30fps to 3000fps, and the frame rate of the camera depends on the flow speed.

5. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: Use a robotic arm or a rigid stand to secure the camera.

6. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: In step B, the process of obtaining the sub-displacement field includes the following steps: The particle displacement of each camera is calculated, and according to the field of view of the camera on the background image, the functions u(x,y) and v(x,y) of the x-direction displacement u and the y-direction displacement v with respect to x and y are obtained, and the displacement matrix of the local flow field is obtained.

7. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: In step B, the interpolation methods for interpolating the data on the sub-displacement field to the grid points of each unstructured grid include: bilinear interpolation, bicubic interpolation and inverse distance weighted interpolation.

8. The method for stitching multi-camera background schlieren arbitrary coplanar fields of view according to claim 1, characterized in that: In step C, the density distribution in the flow field is calculated by solving the Poisson equation and the Gladstone-Dale equation according to the spliced ​​flow field displacement information; the velocity and temperature distribution of the flow field are calculated according to the boundary conditions to obtain complete flow field physical information.

Images