CN113324727B - Schlieren image processing method for compressed corner supersonic flow field structure - Google Patents
Schlieren image processing method for compressed corner supersonic flow field structure Download PDFInfo
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- CN113324727B CN113324727B CN202110538315.1A CN202110538315A CN113324727B CN 113324727 B CN113324727 B CN 113324727B CN 202110538315 A CN202110538315 A CN 202110538315A CN 113324727 B CN113324727 B CN 113324727B
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- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M9/00—Aerodynamic testing; Arrangements in or on wind tunnels
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Abstract
A schlieren image processing method for a compressed corner supersonic flow field structure comprises the following steps: the video of the schlieren is gathered; video framing operation and image selection; calculating the refractive index gradient; judging rms convergence; calculating root mean square distribution of the schlieren intensity field; and (5) judging the size and the position of the separation area. According to the invention, by obtaining the pulsation change level of the supersonic flow field structure, the visual quantitative measurement of the size of the separation area and the judgment of the position are realized.
Description
Technical Field
The invention relates to the technical field of supersonic flow field diagnosis, in particular to a schlieren image processing method for a compressed corner supersonic flow field structure.
Background
Flow separation is an important problem in the field of supersonic flow, and has very important significance for judgment, prediction and diagnosis of the size and the position of a separation area. The precise, real-time and rapid measuring flow field structure is realized by a non-contact means, and the method has wide application in the fields of aerospace, military and the like. Research on a measuring method of the size of a separation area is always a hot spot problem in the field of supersonic speeds at home and abroad. Non-contact flow field diagnostic methods generally fall into two broad categories: the first type is that the parallel light beam passes through the compressible supersonic flow field, the light velocity is disturbed by the density gradient change of the flow field to deflect, the deflected light beam is imaged by the receiving device, and the illuminance of the deflected light beam on the imaging surface is changed, so that the flow field structure required by people is obtained, but the flow field structure with larger density gradient change can be extracted well, such as shock waves. The method and the device have simple structure and are convenient to operate. A disadvantage is that low pulsation level structures, such as the size and location of the separation zone, cannot be determined. Schlieren techniques are among such methods. The other type is to indirectly measure the transient speed distribution of the flow field by injecting nano-scale tracer particles into the flow field, illuminating the flow field in a short time by nanosecond pulse laser and utilizing the displacement of the tracer particles in a very short time interval. The technology has higher measurement precision, can accurately capture all structures of the supersonic flow field, including the size of a separation area, and has the defects of high requirements on the quality, the size, the distribution, the refractive index and the uniformity of trace particles, complex testing device and inconvenient operation. PIV, laser doppler velocimetry, belongs to this class of methods. In recent years, a certain progress has been made in obtaining a supersonic flow structure distribution by processing an original image by means of a computer image post-processing technique or the like.
The schlieren method is a common method for displaying and measuring the flow field, can realize rapid airflow visualization, and can be used as a non-contact method for measuring the supersonic flow field structure. The schlieren technique can be used as a quantitative analysis means in the measurement of supersonic flow separation areas with the help of modern rapidly developing digital image post-processing techniques. Although the test means such as supersonic PIV test the separation area more accurately, it is expensive to construct, maintain and maintain, and the operation is very complicated. In supersonic flow experiments, it is often desirable to obtain the size of the separation zone in a timely manner and to observe its phenomenon quickly and intuitively. Therefore, the design of the experimental device of the non-contact schlieren measurement separation area and the schlieren picture post-processing method which have high cost performance and simple operation is more significant.
Disclosure of Invention
In order to overcome the defect of the schlieren method in the quantitative analysis technology and avoid adopting more expensive PIV technology, the invention provides a schlieren image processing method aiming at a compressed corner supersonic flow field structure, which comprises the following steps:
1. schlieren video acquisition
The high-speed camera 409 internally comprises an embedded video acquisition system, so that a plurality of functions such as shearing transplanting, multitasking parallelism, real-time acquisition and the like are realized; based on PC high-performance processing, the front-end video data is transmitted to the PC end through a high-speed local area network for acquisition; the PC acquisition system realizes video data sharing through a high-speed local area network, and the PC end also comprises a data analysis microcomputer, so that test grain video can be watched in real time after authorization;
2. video framing operation and image selection
The PC end post-processing software comprises video framing operation, outputs video clips at different moments, can select the size of output image data according to the needs, and finally derives a picture file;
3. refractive index gradient calculation
The light beam passes through the compressed corner flow test area in a parallel light beam mode, the refractive index gradient of the light beam changes due to air flow disturbance of the smooth test area, and in the light ray tracing process of the refractive index discrete distribution medium, the refractive index and gradient of a required space point are solved by adopting a distance weighted interpolation and Barron gradient operator; the solving result is a digital gray matrix, and finally, a black-and-white image containing flow field structural features is displayed on the imaging of a high-speed camera;
rms Convergence determination
Further performing convergence analysis on the image intensity of the obtained continuous schlieren image sequence; i mean And I RMS Respectively an image intensity average value and an image intensity root mean square; the data volume contains S samples, residual volume
Which is defined as the image intensity field average value +.>
Mean value of the schlieren intensity field +.>
The absolute value of the difference;
root mean square value of schlieren intensity field
Root mean square value of intensity field of schlieren with N-1->
Absolute value of difference +.>
Wherein (i, j) represents the ith (i) in the horizontal direction and the vertical direction of the image, respectively th ) And j (j) th ) A pixel value; determination of
And->
The residual quantity is not more than 0.1, namely convergence;
5. computing root mean square distribution of schlieren intensity field
On the basis that the root mean square residual error of the schlieren intensity field in the step 4 has convergence, the root mean square I of each pixel position of the schlieren intensity field is further calculated RMS Is S;
the mathematical meaning of the above formula is image intensityThe pulsation level, the corresponding physical meaning then represents the pulsation level of the flow field structure corresponding to each region; it should be noted that only proof I (I, j) RMS Having convergence, the above formula has physical meaning, otherwise nonsensical;
6. separation zone size and location determination
Root mean square I of schlieren intensity field obtained according to step 5 RMS Is to draw I RMS The contour cloud picture is marked with contour lines, the flow field structure can be further locked according to the distribution of the pulsation intensity in the cloud picture, obvious boundary characteristics can appear in different pulsation intensity areas in the cloud picture, and obvious contour areas appear near compression corners, so that the flow field structure corresponding to the areas can be identified as separation areas; the location and size of the separation zone can be determined from the boundaries of the pulsating intensity at the compression corners in the cloud image.
According to the invention, a single-amplitude high-resolution steady-state structure and a plurality of continuous transient structures in the flow field are acquired through a high-frame-rate high-speed schlieren technology, wherein the plurality of continuous transient structures can better capture the change rule of the flow structure, and the visualized quantitative measurement of the size of the separation area and the judgment of the position are realized by obtaining the pulsation change level of the supersonic flow field structure.
The method can measure the evolution process of the ultrasonic flow structure with microsecond magnitude time scale, and overcomes the technical defect that the traditional high-speed schlieren technology can not effectively estimate the separation area. Meanwhile, on the premise of lacking PIV test conditions, the ultrasonic separation zone measuring device based on the schlieren technology is easy to build, high in reliability, large in measurement information quantity, high in speed and high in accuracy, and is very suitable for academic research and industrial application.
Drawings
FIG. 1 is a schematic view of the overall structure of a supersonic separation zone generation apparatus of the present invention;
FIG. 2 is a schematic diagram of the positional relationship of the experimental chamber 4 and the compressed corner flow test model 5;
FIG. 3 is a schematic view of a high-speed schlieren-based supersonic separation zone measurement apparatus;
fig. 4 is a transient schlieren view of a supersonic flow field structure acquired by a high speed camera, wherein fig. 4 (a) to (f) respectively show transient flow field structures with ramp angles of 20 °, 22 °, 24 °, 26 °, 28 °, 30 °;
FIG. 5 shows a corresponding 30 compressed corner flow I mean And I RMS Schematic diagram of residual variation with the number of schlieren sequences;
fig. 6 is a supersonic flow field pulsatile horizontal distribution, wherein fig. 6 (a) to (f) show I under supersonic flow conditions with ramp angles of 20 °, 22 °, 24 °, 26 °, 28 °, 30 °, respectively RMS Distribution.
Reference numerals: 1. an expansion section; 2. expansion section flange (wind tunnel starting device); 3. a supersonic nozzle; 4. an experiment cabin; 5. compressing the corner flow test model; 6. a vacuum chamber; 7, a vacuum pump group; 200. an outlet of the supersonic jet pipe; 201. a flat plate model; 202. a ramp; 203. a support base; 401. a xenon light lamp; 402. a grating; 403. a first concave spherical mirror; 404. a first optical glass; 405. a second optical glass; 406. a second concave spherical mirror; 407. a plane mirror; 408. a knife edge; 409. a high-speed camera; 4010. an image post-processing device.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the present patent; for the purpose of better illustrating the embodiments, certain elements of the drawings may be omitted, enlarged or reduced and do not represent the size of the actual product; it will be appreciated by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The drawings are for illustrative purposes only and are not to be construed as limiting the present patent; for the purpose of better illustrating the embodiments, certain elements of the drawings may be omitted, enlarged or reduced and do not represent the size of the actual product; it will be appreciated by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
As shown in fig. 1, the invention provides a supersonic separation area generating device, which comprises an expansion section flange 2, a supersonic jet pipe 3, an experiment cabin 4, a compression corner flow test model 5, a vacuum cabin 6 and a vacuum pump set 7.
The inlet of the wind tunnel is in an atmospheric environment, and the vacuum pump set 7 is adopted to reduce the air pressure of the vacuum chamber 6 to be lower than the starting air pressure of the wind tunnel. The device comprises an expansion section 1, an expansion section flange 2, a supersonic jet pipe 3, an experimental cabin 4, a compression corner flow test model 5 and a vacuum cabin 6 in sequence from front to back. It is well known to those skilled in the art that the expansion section flange 2 actually belongs to the expansion section 1 for connecting together two parts of the expansion section 1. The basic principle of the wind tunnel is that: by additionally installing a diaphragm at the expansion section flange 2 (i.e. the diaphragm is clamped between the expansion section flange 2 and can play a role of isolating the experimental cabin 4 from the atmosphere) as a wind tunnel start switch, when the diaphragm is opened (i.e. the diaphragm is broken by external force, at this time, the atmosphere can pass through the broken diaphragm to enter the experimental cabin 4), under the driving of pressure difference, the atmosphere firstly passes through the expansion section 1 and then enters the experimental cabin 4 under the acceleration of the supersonic jet pipe 3, the required inflow condition for experiments is generated near the compression corner flow test model 5 in the experimental cabin 4, and finally, the atmosphere enters the vacuum cabin 6. Before the wind tunnel experiment, the vacuum pump unit 7 is used to form a vacuum environment for the vacuum chamber 6. The composition, structure, location, and manner of connection of the above-described components, except for the compressed corner flow test model 5, are well known to those skilled in the art and will not be described in detail. In order to ensure the air tightness of the whole device, each interface and the threaded hole are required to be sealed. The wind tunnel connection structure below the components of fig. 1, which is not marked with reference numerals, is a fixed facility and is not within the scope of the invention.
The experiment cabin 4 is of a closed cylinder structure, the cylinder is transversely placed, round holes are formed in the left end face and the right end face, the supersonic jet pipe outlet 200 extends into the experiment cabin 4 from the round hole in the left end face, the diffuser located at the left end of the vacuum cabin 6 extends into the experiment cabin 4 from the round hole in the right end face, and joints of the left round hole and the right round hole are sealed, for example, in a welding mode. A compressed corner flow test model 5 is placed in the experimental cabin 4.
The compressed corner flow test model 5 structure is illustrated in fig. 2. The compression corner flow test model 5 includes a flat plate model 201, a slope 202 mounted on the upper side of the rear end of the flat plate model 201, and a support base 203 for supporting the flat plate model 201. The support base 203 is in the shape of a support frame capable of supporting the ramp 202, and is shown in one embodiment of the present inventionOne embodiment is
A shaped support base 203, the upper end of which is fixedly provided with a flat model 201, the lower end of which is +.>
The plate is fixedly connected with the inner wall of the experiment cabin 4, and the flat plate model 201 is horizontally arranged. The overall length of the flat model 201 is in the range of 220 to 600mm, preferably 440mm, and the width is in the range of 50 to 160mm, preferably 110mm. The height of the slope 202 ranges from 10 to 30mm, the preferred value is 20mm, the slope angle ranges from 10 degrees to 30 degrees, and in the example, six angles of 20 degrees, 22 degrees, 24 degrees, 26 degrees, 28 degrees and 30 degrees are selected for replacement. The height of the supporting base 203 is based on that the flat model 201 is positioned at the center of the outlet of the supersonic nozzle 3 in the experimental cabin 4 (the flat model 201 can extend into the outlet of the supersonic nozzle 3 to be zero-spaced or keep a small space); in one embodiment of the invention, the support base 203 has a height ranging from 1080mm to 1280mm, preferably 1180mm. To facilitate changing the angle of the ramp 202, the compression corner flow test pattern 5 is designed to be removable to ensure that the flat plate pattern 201, the ramp 202 and the support base 203 are removable, i.e., the ramp 202 is easily removable from the flat plate pattern 201 and the flat plate pattern 201 is easily removable from the support base 203, and the support base 203 can be made partially removable, e.g., in one embodiment of the invention>
Shaped support base 203 +.>
Part is easy to be from->
And partially disassembled. In one embodiment of the invention, bolts are used between the flat plate model 201 and the slope 202 and between the flat plate model 201 and the supporting base 203, and the flat plate modelThe compression corner structure is formed by bolting between the plate mold 201 and the ramp 202.
As shown in fig. 3, the present invention provides a high-speed schlieren-based supersonic separation area measurement device, which includes: xenon lamp 401, grating 402, first concave spherical reflector 403, first optical glass 404, second optical glass 405, second concave spherical reflector 406, plane mirror 407, knife edge 408, high-speed camera 409, and image post-processing device 4010.
As shown in fig. 4, the supersonic separation zone generating device shown in fig. 1 is placed vertically with the expansion section 1 down and the experiment compartment 4 up (for simplicity, the vacuum compartment 6 is omitted); the xenon light lamp 401, the grating 402, the first concave spherical reflecting mirror 403 and the first optical glass 404 are arranged on one side (for example, the right side) of the supersonic flow field; a second concave spherical mirror 406, a second optical glass 405, a flat mirror 407, a knife edge 408, and a high speed camera 409 are placed on the other side (e.g., left side) of the supersonic flow field. The first concave spherical reflecting mirror 403, the first optical glass 404, the second optical glass 405 and the second concave spherical reflecting mirror 406 are respectively positioned at the left side and the right side of the experimental cabin 4, and have the same horizontal optical axis, so that the experimental cabin 4 is clamped between the two optical glass 405 and the first optical glass 404, and the space between the two optical glass and the experimental cabin 4 can be provided, or the space can be provided. The xenon lamp 401 and the grating 402 are positioned at the upper right position outside the experimental cabin 4, the xenon lamp 401 emits converged xenon light opposite to the grating 402, the grating 402 is positioned at the focal plane of the converged xenon light, and the xenon light forms standard circular light spots after passing through the grating; then projected onto the first concave spherical reflecting mirror 403, the xenon light is reflected by the first concave spherical reflecting mirror 403 to form a parallel light beam to propagate leftwards; the parallel light beam sequentially transmits through the first optical glass 404, the experiment cabin 4 and the second optical glass 405, so that the output optical signal contains the flow field structure information of the compressed corner flow test model 5; the optical signal is then transmitted to the second concave spherical reflector 406, reflected by the second concave spherical reflector 406, and becomes converging light, and diverges and continues to propagate to the planar mirror 407 located at the lower right of the second concave spherical reflector 406; the plane mirror 407 reflects the light signal containing the light signal to the high speed camera 409; due to the need of the schlieren system, a knife edge 408 is arranged at a proper position between the plane mirror 407 and the high-speed camera 409 for cutting light, so that the high-speed camera 409 can image clearly (the principle and the position of the knife edge 408 are well known to those skilled in the art and are not described again); the high speed camera 409 collects optical signals containing flow field structure information of the compressed corner flow test model 5 and transmits them to the image post-processing device 4010.
In one embodiment of the present invention, the grating 402 is provided with an adjusting device (not shown) to adjust the size of the grating and control the amount of light entering, and a position fine adjustment bracket (not shown) is provided to ensure that the grating 402 is located at the focal plane of the concentrated xenon light emitted by the light source. The first concave spherical mirror 403 is provided with an angle adjusting device (not shown in the figure) to ensure that the light beam smoothly passes through the compression corner flow test model 5, and the diameters of the first concave spherical mirror 403 and the second concave spherical mirror 406 are 50-500 mm, preferably 300mm. In one embodiment of the present invention, the high speed camera 409 model is phantom@v2512, has a body memory of 60G, a photographing frame frequency of 30000fps at a maximum resolution of 1280×800, a photographing frame frequency of 700000fps at a minimum resolution of 128×8, and in one embodiment of the present invention, the high speed camera sampling frame frequency is set to 20kHz (20000 fps), and the exposure time is set to 1 to 500 μs, preferably 1 μs. The high-speed camera 409 is electrically connected to the image post-processing device 4010.
The invention also provides a supersonic separation area measurement method based on high-speed schlieren, which comprises the following steps: when the wind tunnel is started, the supersonic separation area measuring device based on high-speed schlieren starts to work, at this moment, the xenon lamp 401 emits high-intensity converging light beams, the light beams are irradiated to the first concave spherical reflecting mirror 403 after being regulated by the grating 402, the light beams are reflected to the first optical glass 404 through the first concave spherical reflecting mirror 403 and enter the compressed corner flow test model 5, therefore, the supersonic flow field structure is imaged on the second concave spherical reflecting mirror 406 and reflected to the plane mirror 407 and then projected to the high-speed camera 409, part of light source images are cut off by the knife edge 408, light signals containing the supersonic flow field structure are collected by the high-speed camera 409 and transmitted to the image processing device 4010, and the image processing device 4010 processes the image signals to realize visual quantitative measurement of the supersonic flow separation area. The region between the two pieces of optical glass is the supersonic flow separation test region. The angles between the optical axes of the xenon lamp 401 and the first concave spherical mirror 403 and the optical axes of the first concave spherical mirror 403 and the second concave spherical mirror 406 are not more than 30 degrees, and as small as possible, the angles between the optical axes of the first concave spherical mirror 403 and the second concave spherical mirror 406 and the optical axes of the second concave spherical mirror 406 and the plane mirror 407 are not more than 30 degrees, and as small as possible.
In one embodiment of the present invention, the flat panel model 201 and ramp 202 are bolted to form a compressed corner structure under supersonic incoming flow conditions. Compression corner flow in the corner region flow separation may occur due to the shock induced back pressure gradient.
Provided include, but are not limited to, the following steps: when the measurement is started, the separation area measuring device is required to be installed in the supersonic flow field test area so as to perform the measurement.
The image post-processing device 4010 processes the schlieren image of the compressed corner supersonic flow field structure, and mainly comprises:
1. schlieren video acquisition
The embedded video acquisition system is arranged in the high-speed camera, so that multiple functions such as shearing transplanting, multitasking parallelism, real-time acquisition and the like can be realized; based on PC high-performance processing, the front-end video data can be transmitted to the PC end through a high-speed local area network for acquisition. The PC acquisition system realizes video data sharing through a high-speed local area network, and the PC end also comprises a data analysis microcomputer, so that test grain video can be watched in real time after authorization. This technique is well known to those skilled in the art and will not be described in detail.
2. Video framing operation and image selection
The PC end post-processing software comprises video framing operation, can output video clips at different moments, can select the size of output image data according to requirements, and finally derives, for example, jpg format pictures. This technique is well known to those skilled in the art and will not be described in detail.
3. Refractive index gradient calculation
The light beam passes through the compressed corner flow test area in the form of a parallel light beam, and the refractive index gradient of the light beam changes due to the air flow disturbance of the smooth test area, and the refractive index gradient solving method of the light beam in any gradient refractive index medium comprises an Euler method, a Dragon-Kutta method and a Taylor series expansion method, which are all numerical methods for tracking the light transmission in the medium (Feng Dinghua, pan Sha, wang Wenlong, li Hua. Simulation and analysis of light tracking in any gradient refractive index medium [ J ]. Computer simulation. 2010, 27 (2): 135). In the course of ray tracing in a medium with discrete refractive index distribution, the refractive index of the required spatial point and its gradient are solved by using distance weighted interpolation and Barron gradient operator (Feng Dinghua, pan Sha, wang Wenlong, li Hua. Simulation and analysis of ray tracing in a medium with arbitrary gradient refractive index [ J ]. Computer simulation 2010, 27 (2): 135). The result of the solution is a digital gray matrix, and finally, a black-and-white image containing flow field structural features is displayed on a high-speed camera imaging, and the technology is well known to those skilled in the art and is not described again.
The steps 1 to 3 are all technical means well known to those skilled in the art, and need not be described in detail, and the following steps 4 to 6 are technical features of the present invention, as described in detail below.
Rms Convergence determination
Further convergence analysis is performed on the image intensities (gray values) from which the sequence of successive schlieren images was obtained. FIG. 5 is a diagram of a supersonic flow field I with a compression angle of 30 degrees mean And I RMS Residual evolution, I mean And I RMS The mean value of the image intensity and the root mean square of the image intensity are respectively given. The data volume comprises S samples, S is equal to 10000, for example, the residual volume
Which is defined as the image intensity field average value +.>
Mean value of the schlieren intensity field +.>
Absolute value of the difference.
Similar root mean square value of schlieren intensity field
Root mean square value of intensity field of schlieren with N-1->
Absolute value of difference +.>
Wherein (i, j) represents the ith (i) in the horizontal direction and the vertical direction of the image, respectively th ) And j (j) th ) Pixel values. In the present invention, determination is made
And->
The residual quantity is not more than 0.1, which is the convergence.
5. Computing root mean square distribution of schlieren intensity field
On the basis that the root mean square residual error of the schlieren intensity field in the step 4 has convergence, the root mean square I of each pixel position of the schlieren intensity field is further calculated RMS Is S.
The mathematical meaning of the above formula is the pulsation level of the image intensity, and the corresponding physical meaning indicates the pulsation level of the flow field structure corresponding to each region. It should be noted that only proof I (I, j) RMS With convergence, the above formula has physical meaning, otherwise it is meaningless.
6. Separation zone size and location determination
Root mean square I of schlieren intensity field obtained according to step 5 RMS Is to draw I RMS The flow field structure can be further locked according to the distribution of the pulsation intensity in the cloud image, obvious boundary characteristics can appear in different pulsation intensity areas in the cloud image, and obvious equivalent areas appear near compression corners, so that the flow field structure corresponding to the areas can be identified as separation areas. The location and size of the separation zone can be determined from the boundaries of the pulsating intensity at the compression corners in the cloud image.
And determining the position and the size of the separation region according to the pulse level distribution difference, and realizing the schlieren visual quantitative measurement of the supersonic compression corner flow separation region. Although schlieren technology is a qualitative flow field diagnostic device, with the rapid development of schlieren technology, such as low exposure, short pulse light source, high frame rate acquisition, etc. A large amount of real-time schlieren data can be obtained, and the flow field structure with high density gradient, such as shock wave, can be identified through schlieren images. But still cannot identify separation zones, shear layers, and similar low density gradient flow field structures. By statistical analysis of the schlieren intensity field, i.e. checking the mean intensity field and the root mean square intensity field, by iteration of a large number of schlieren data, it is ensured that these two parameters are convergent, and the schlieren intensity field root mean square I is calculated RMS The pulsation level in the flow field can be determined, the flow field structure similar to the low density gradient is locked according to the difference of the pulsation level, and the flow field structure has obvious boundary characteristics.
Taking supersonic compression corner flow as an example, as shown in fig. 4, fig. 4 (a) to (f) respectively show transient flow field structures with ramp angles of 20 °, 22 °, 24 °, 26 °, 28 °, 30 °. It is evident that with increasing angle, the separation shock is forced to move upstream, indicating that the separation zone gradually increases in size, but the separation zone cannot be represented in the original schlieren.
Because the high-speed schlieren technique can collect a plurality of continuous transient structures in the flow field, the high-speed schlieren technique can provide a large number of transient continuous structure samples, thereby ensuring the statistical convergence of the schlieren intensity field, as shown in fig. 5. Fig. 5 illustrates that the maximum difference between adjacent schlieren intensities calculated at each step is convergent in the overall trend, and the larger the number of samples, the smaller the residual. The method can well embody and capture the dynamic change rule of the flow structure.
In supersonic flow fields, although different flow field structures have different pulsation levels, some features cannot be distinguished directly by the original schlieren (e.g., shear layer, separation zone, etc.). The root mean square distribution of the schlieren intensity field reflects the pulsatile characteristics of the global flow field, from which the distribution of the separation zone in the flow field is established laterally, as shown in fig. 6. Because of the interaction of the shock wave with the boundary layer, the boundary layer I after being excited by separation RMS The increase is significant.
From fig. 6 it can be seen that the level of the pulse downstream of the separation shock is divided into two distinct parts: one part is positioned on the flat plate, the other part is positioned on the slope, and the pulsation intensity on the slope is larger. Further carefully observe the whole area I RMS Distribution, it was found that a region where the pulsation intensity was relatively stable was present near the compression corner, and the pulsation intensity of this region was significantly weaker than the boundary was significant.
The size of the partial area is increased along with the increase of the compression angle, because the pulsation intensity of the separation bubbles is weaker than that of the shear layer and is positioned below the shear layer, the flow structure corresponding to the partial area is judged to be the separation bubbles, and the size and the specific position of the separation area are obtained. By obtaining the pulsation horizontal distribution of the supersonic flow field structure, the visual quantitative measurement of the size of the separation area and the judgment of the position are realized.
Claims (1)
1. The method for processing the schlieren image of the compressed corner supersonic flow field structure is characterized by comprising the following steps:
1. schlieren video acquisition
The high-speed camera 409 internally comprises an embedded video acquisition system, so that a plurality of functions such as shearing transplanting, multitasking parallelism, real-time acquisition and the like are realized; based on PC high-performance processing, the front-end video data is transmitted to the PC end through a high-speed local area network for acquisition; the PC acquisition system realizes video data sharing through a high-speed local area network, and the PC end also comprises a data analysis microcomputer, so that test grain video can be watched in real time after authorization;
2. video framing operation and image selection
The PC end post-processing software comprises video framing operation, outputs video clips at different moments, can select the size of output image data according to the needs, and finally derives a picture file;
3. refractive index gradient calculation
The light beam passes through the compressed corner flow test area in a parallel light beam mode, the refractive index gradient of the light beam changes due to air flow disturbance of the smooth test area, and in the light ray tracing process of the refractive index discrete distribution medium, the refractive index and gradient of a required space point are solved by adopting a distance weighted interpolation and Barron gradient operator; the solving result is a digital gray matrix, and finally, a black-and-white image containing flow field structural features is displayed on the imaging of a high-speed camera;
rms Convergence determination
Further performing convergence analysis on the image intensity of the obtained continuous schlieren image sequence; i mean And I RMS Respectively an image intensity average value and an image intensity root mean square; the data volume contains S samples, residual volume
Which is defined as the image intensity field average value +.>
Mean value of the schlieren intensity field +.>
The absolute value of the difference;
root mean square value of schlieren intensity field
Root mean square value of intensity field of schlieren with N-1->
Absolute value of difference between
Wherein (i, j) represents the ith (i) in the horizontal direction and the vertical direction of the image, respectively th ) And j (j) th ) A pixel value; determination of
And->
The residual quantity is not more than 0.1, namely convergence;
5. computing root mean square distribution of schlieren intensity field
On the basis that the root mean square residual error of the schlieren intensity field in the step 4 has convergence, the root mean square I of each pixel position of the schlieren intensity field is further calculated RMS Is S;
the mathematical meaning of the formula is the pulsation level of the image intensity, and the corresponding physical meaning represents the pulsation level of the flow field structure corresponding to each region; it should be noted that only proof I (I, j) RMS Having convergence, the above formula has physical meaning, otherwise nonsensical;
6. separation zone size and location determination
Root mean square I of schlieren intensity field obtained according to step 5 RMS Is to draw I RMS The contour cloud picture is marked with contour lines, the flow field structure can be further locked according to the distribution of the pulsation intensity in the cloud picture, obvious boundary characteristics can appear in different pulsation intensity areas in the cloud picture, and obvious contour areas appear near compression corners, so that the flow field structure corresponding to the areas can be identified as separation areas; the location and size of the separation zone can be determined from the boundaries of the pulsating intensity at the compression corners in the cloud image.
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