Single mirror coaxial schlieren imaging method for observing vortex structure in shock tube

Abstract

The single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube belongs to the technical field of fluid flow experiment test, and relates to a single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube. The method integrates a point light source and a blade to construct an integrated coaxial light source, and uses a schlieren mirror, a shock tube and a high-speed camera to build a single-mirror coaxial schlieren imaging system. In the system, the incident light of the point light source is reflected by the schlieren mirror, passes through the flow field in the shock tube and is focused on the tip of the blade. The blade is used to interfere with the focal point. Because the airflow density in the shock tube is different, the refractive index of the light is different, different schlieren gray images with different light and dark degrees are formed, and the high-speed camera is used to record the change of the vortex structure in the shock tube. The method is easy to debug and saves a lot of experimental debugging time. On the basis of ensuring the light intensity, the influence of ghosting and astigmatism is reduced, and the schlieren imaging of the vortex structure in the shock tube is accurate and clear.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fluid flow experimental testing, and relates to a single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube. BACKGROUND

[0002] A shock tube is an important device for studying wave motion and high-speed gas flow. High-pressure gas drives high-speed turbulence inside the shock tube, and the schlieren imaging technology is used outside the shock tube to obtain a gray-scale image for accurately reflecting the vortex structure characteristics. In order to meet the demand for clearly and accurately describing the flow state of high-speed gas flow in the shock tube, it is required to improve the imaging quality of the schlieren image. Three kinds of light paths widely used in the current schlieren imaging method for observing the flow characteristics of the flow channel are: single-mirror off-axis light path, single-mirror coaxial light path, and double-mirror Z-shaped light path. In the system using the single-mirror off-axis light path, there is an angle between the center line of the camera lens and the center line of the schlieren mirror, which causes the primary image of the gas flow in the schlieren mirror and the secondary gray-scale image after the schlieren refraction to be recorded together, resulting in the problem of ghosting in the final imaging, which interferes with the resolution accuracy of the target observation image. In the system using the single-mirror coaxial light path, a beam splitter is used to avoid the occurrence of ghosting phenomenon, but after the light is split twice by the beam splitter, 75% of the light intensity is lost, and the illuminance provided by the point light source is low, which causes the viewfinder area to be dark when the camera is shooting at a high frame rate, and it is impossible to form a high-contrast schlieren image of high-speed gas flow. The double-mirror Z-shaped light path increases the difficulty of installation and adjustment, and the off-axis of the point light source and the camera and the schlieren mirror also causes the occurrence of astigmatism, so that the focal point cannot be interfered, and finally the contrast of the schlieren image is small, and the imaging effect is not good.

[0003] The patent with the patent publication number CN114964713A and the inventors of Zhou Yiwei et al. discloses an invention patent "A schlieren experimental method for visual imaging of large-scale flow field", which uses a double-mirror Z-shaped light path with a condenser lens to solve the problem of astigmatism and obtain the schlieren image of the flow field. However, the use of the condenser lens increases the difficulty of adjusting the light path and increases the cost of experimental adjustment time. The patent with the patent publication number CN114199721A and the inventors of Yang Lijun et al. discloses an invention patent "Schlieren optical system and two-dimensional density distribution measurement method of flow field to be measured", which uses a single-mirror off-axis light path to obtain the two-dimensional density distribution of the flow field to be measured. However, the single-mirror off-axis light path will cause the schlieren image of the high-speed flow field to be mixed with ghosting, and this method is not suitable for obtaining the schlieren image of high-speed flow in the shock tube. SUMMARY

[0004] The present application is to overcome the shortcomings of the existing design, aiming at the poor applicability of various light paths to the schlieren imaging of vortex structure in a shock tube, a single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube is invented. The method integrates a point light source and a blade to build an integrated coaxial light source, and builds a single-mirror coaxial schlieren imaging system. The incident light rays of the point light source pass through the schlieren mirror, reflect through the flow field in the shock tube and focus on the tip of the blade. The blade interferes with the focal point. Because the refractive index of the light rays is different due to the different densities of the airflow in the shock tube, a schlieren gray scale image with different light and dark degrees is formed. The high-speed camera records the development and change of the vortex structure in the shock tube at high frequency. The method integrates the point light source and the blade at an ultra-close distance, so that the incident light cone and the reflected light cone overlap. Without losing light intensity, the off-axis degree of the point light source, the camera and the schlieren mirror is greatly reduced, the influence of the ghosting and astigmatism of the schlieren image is improved, and the accurate and clear imaging observation of the vortex structure in the shock tube is realized.

[0005] The technical scheme adopted by the present application is a single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube. The method integrates a point light source and a blade to build an integrated coaxial light source, and builds a single-mirror coaxial schlieren imaging system with a schlieren mirror, a shock tube and a high-speed camera. The incident light rays of the point light source pass through the schlieren mirror, reflect through the flow field in the shock tube and focus on the tip of the blade. The blade interferes with the focal point. Because the refractive index of the light rays is different due to the different densities of the airflow in the shock tube, a schlieren gray scale image with different light and dark degrees is formed. The high-speed camera records the development and change of the vortex structure in the shock tube at high frequency. The method further comprises the following steps:

[0006] Step one, build an integrated coaxial light source;

[0007] In the integrated coaxial light source 4, a plurality of stepped inner holes are machined in the inside of the light source shell 17, which gradually decreases from left to right. In the smallest inner hole, the light source shell 17 and the blade 15 are fixed by adhesion, and a hole 16 is left above the blade for light to pass through, so as to adhere the blade 15 and the point light source 14 together to build an integrated coaxial light source 4. The lower part of the light source shell 17 is machined with a threaded hole, and a threaded support rod 13 is connected through threads. The support rod 13 is fixed on the base 21 through a fixing bolt 18, and the base 21 is installed on the optical platform 2.

[0008] Step two, build a single-mirror coaxial schlieren imaging system;

[0009] Then the high-speed camera 3 is fixed on the optical platform 2, and the schlieren mirror 7 is fixed on the optical platform 2 opposite to the high-speed camera 3 by bolts; the position range of the integrated coaxial light source 4 is preliminarily determined according to the focal length of the schlieren mirror 7, that is, a circular arc trajectory with the center of the schlieren mirror 7 as the origin and twice the focal length as the radius, the point light source 14 is turned on, and the position of the integrated coaxial light source 4 is adjusted along the circular arc trajectory, so that the focal point of the reflected light after the schlieren mirror 7 is focused and the point light source 14 of the integrated coaxial light source 4 are on the same vertical line; the fixed bolt 18 is loosened, the height of the threaded support rod 13 is adjusted, the knife blade 15 interferes with the focal point, the fixed bolt 18 is tightened to fix the height of the threaded support rod 13; the position of the high-speed camera 3 is adjusted to ensure that all regions in the schlieren mirror 7 are bright regions in the formed image, the aperture is rotated to obtain a clear image; the palm is placed in front of the schlieren mirror 7 to obtain the weak air density change caused by the palm temperature, and the height of the fixed threaded support rod 13 is finely adjusted to change the interference degree of the knife blade 15 to the focal point, so that the height of the fixed threaded support rod 13 meeting the requirements is obtained. Finally, a single-mirror coaxial schlieren system with fixed position is obtained;

[0010] Step three, obtain the characteristics of the vortex structure in the shock tube;

[0011] On the basis of steps one and two, the shock tube 6 is placed in front of the schlieren mirror 7 and closely attached to the mirror surface, the shock tube 6 is fixed by the lifting platform 5 and the height is adjusted so that the shock tube 6 is located in the middle of the schlieren mirror 7 to obtain the maximum observation area; the nozzle 8 is connected to the shock tube 6, and the air pipe 10 is sequentially connected to the electromagnetic valve 9, the two-stage pressure regulating and filtering device 11 and the compressed air pump 1, wherein the two-stage pressure regulating and filtering device 11 is fixed to the pressure regulating device fixing platform 12 by bolts, and the pressure regulating device fixing platform 12 is fixed to the optical platform 2 by bolts;

[0012] The point light source 14 is turned on, the compressed air pump 1 is turned on, the two-stage pressure regulating and filtering device 11 is adjusted to a specified pressure, and dry and clean gas at the specified pressure is intermittently introduced into the shock tube 6 through the electromagnetic valve 9 to generate a vortex structure, while the light emitted by the point light source 14 passes through the shock tube 6, the air flow density changes due to the vortex in the tube, thereby causing different refractive indexes of the light, the refracted light is reflected by the schlieren mirror 7 and interfered by the knife blade 15, and then captured by the high-speed camera 3, so that a schlieren image with different brightness is finally formed to obtain the characteristics of the vortex structure.

[0013] The beneficial effects of the present application are: the method adopts an integrated coaxial light source, point light sources are integrated with blades at an ultra-close distance, the incident light cone and the reflected light cone are overlapped, the off-axis degree of the point light source, the camera and the schlieren mirror is greatly reduced without losing light intensity, the influence of ghosting and astigmatism of the schlieren image is improved, and accurate and clear imaging observation of the vortex structure in the shock tube is realized. The schlieren system is built by using a single mirror coaxial light path, the overall device structure is simple, debugging is convenient, and a large amount of experimental debugging time is saved. The method can reduce the influence of ghosting and astigmatism on the basis of ensuring light intensity, and realize accurate and clear schlieren imaging of the vortex structure in the shock tube. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The present application is a single mirror coaxial schlieren imaging device for observing the vortex structure in the shock tube, wherein 1 is a compression gas pump, 2 is an optical platform, 3 is a high-speed camera, 4 is an integrated coaxial light source, 5 is a lifting platform, 6 is a shock tube, 7 is a schlieren mirror, 8 is a nozzle, 9 is an electromagnetic valve, 10 is a gas pipe, 11 is a two-stage pressure regulating filtering device, and 12 is a pressure regulating device fixing platform.

[0015] Figure 2 The present application is a partial sectional view of the integrated coaxial light source, wherein A is a partial sectional enlarged portion, 4 is an integrated coaxial light source, 18 is a fixing bolt, and 21 is a base.

[0016] Figure 3 The present application is Figure 2 A is an enlarged view of the A part, wherein 13 is a threaded support rod, 14 is a point light source, 15 is a blade, 16 is an aperture, and 17 is a light source housing.

[0017] Figure 4 The present application is a light path diagram of the single mirror coaxial schlieren system, wherein 3 is a high-speed camera, 4 is an integrated coaxial light source, 7 is a schlieren mirror, 19 is an incident light cone, and 20 is a reflected light cone. DETAILED DESCRIPTION

[0018] The specific implementation of the present application will be described in detail below in combination with the drawings and technical solutions.

[0019] The single mirror coaxial schlieren imaging device for observing the vortex structure in the shock tube built based on the present application is shown in the axial measurement diagram of the present embodiment as Figure 1 The high-pressure gas in the shock tube 6 is provided by the compression gas pump 1, the generation of the vortex structure in the shock tube 6 is realized by simulating the high-pressure air inlet process through the two-stage pressure regulating filtering device 11, the electromagnetic valve 9 and the nozzle 8, the light of the integrated coaxial light source 4 used by the imaging device reaches the schlieren mirror 7 after passing through the shock tube 6, the light is reflected by the schlieren mirror and passes through the aperture 16 of the integrated coaxial light source 4 and is captured by the high-speed camera 3, and the light path diagram is shown in Figure 4 .

[0020] A single-mirror coaxial schlieren imaging method for observing vortex structure in a shock tube, characterized in that the method constructs an integrated coaxial light source by integrating a point light source and a blade, and builds a single-mirror coaxial schlieren imaging system by using a schlieren mirror, a shock tube and a high-speed camera. In the system, the incident light of the point light source is reflected by the schlieren mirror and passes through the flow field in the shock tube to focus on the tip of the blade. The blade is used to interfere with the focal point. Due to the different refractive indexes of light caused by the different densities of airflow in the shock tube, schlieren gray-scale images with different brightness levels are formed. The high-speed camera records the changes of vortex structure in the shock tube at high frequency. The high-pressure air inlet process is simulated by using a compression air pump, a two-stage pressure regulating and filtering device, a solenoid valve and a nozzle, so as to realize high-precision schlieren imaging of high-speed turbulent flow driven by high-pressure gas in the shock tube. The specific steps of the method are as follows:

[0021] Step one, construct an integrated coaxial light source.

[0022] The integrated coaxial light source is shown in the partial sectional view of Figure 2 、 Figure 3 . The light source shell 17 is connected with the threaded support rod 13 through threads. The height of the integrated coaxial light source 4 is adjusted by adjusting the height of the threaded support rod 13, and the height of the support rod 13 in the base 21 is fixed by the fixing bolt 18. The base 21 is placed on the optical platform 2. The light source shell 17 is fixed with the support rod 13 by adhesion, and a hole 16 for light passing through is left above. The point light source 14 and the blade 15 are fixed by adhesion. The axis of the schlieren mirror 7, the axis of the point light source 14 and the axis of the high-speed camera 3 are located in the same vertical plane, and the incident light cone 19 emitted by the point light source 14 and the reflected light cone 20 reflected by the schlieren mirror 7 are highly overlapped, as shown in the optical path diagram of the single-mirror coaxial schlieren system. Figure 3 Therefore, the off-axis degree of the point light source 14, the high-speed camera 3 and the schlieren mirror 7 is reduced, and the imaging is accurate and clear. The angle of the blade 15 affects the interference effect of the blade on the focal point. The smaller the angle of the blade 15, the less the interference of the blade 15 on the light at the non-focal point, and the better the uniformity and integrity of the bright part in the schlieren image.

[0023] Step two, build a single-mirror coaxial schlieren imaging system.

[0024] The schlieren mirror 7 is fixed on the optical platform 2 by bolts, the position range of the integrated coaxial light source 4 is preliminarily determined according to the focal length of the schlieren mirror 7, the center of the schlieren mirror 7 is taken as the origin, and a circular arc track with a radius of double focal length is taken as the position range of the integrated coaxial light source 4, the point light source 14 is turned on, the position of the integrated coaxial light source 4 is adjusted along the circular arc track, so that the focal point of the light reflected by the schlieren mirror 7 and the point light source 14 of the integrated coaxial light source 4 are on the same vertical line, the height of the threaded support rod 13 is adjusted so that the knife blade 15 interferes with the focal point, the height of the threaded support rod 13 is fixed by using the fixing bolt 18, the high-speed camera 3 is placed behind the integrated coaxial light source, the position of the high-speed camera 3 is adjusted to ensure that all the areas in the schlieren mirror 7 are bright areas in the formed image, the aperture is rotated to obtain a clear image, and thus the position of the high-speed camera 3 is determined. The palm is placed in front of the schlieren mirror 7, and the weak air density change caused by the palm temperature is required to form a clear schlieren image, the height of the threaded support rod 13 is finely adjusted, the interference degree of the knife blade 15 on the focal point is changed, and the height of the threaded support rod 13 meeting the requirements is obtained. Finally, the single-mirror coaxial schlieren system with fixed position is obtained, Figure 4 It is a light path diagram of the single-mirror coaxial schlieren system.

[0025] Step three, obtaining the characteristics of the vortex structure in the shock tube;

[0026] On the basis of steps one and two, the shock tube 6 is placed in front of the schlieren mirror 7 and closely attached to the mirror surface, the shock tube 6 is fixed by the lifting platform 5 and the height is adjusted so that the shock tube 6 is located in the middle of the schlieren mirror 7 to obtain the maximum observation area, the nozzle 8 is connected to the shock tube 6, and the air pipe 10 is sequentially connected to the electromagnetic valve 9, the two-stage pressure regulating and filtering device 11 and the compressed air pump 1, wherein the two-stage pressure regulating and filtering device 11 is fixed to the pressure regulating device fixing platform 12 by bolts, and the pressure regulating device fixing platform 12 is fixed to the optical platform 2 by bolts. On this basis, the point light source 14 is turned on, the compressed air pump 1 is turned on, the two-stage pressure regulating and filtering device 11 is adjusted to a specified pressure, and dry and clean gas with the specified pressure is intermittently introduced into the shock tube 6 through the electromagnetic valve 9 to generate a vortex structure, at the same time, the light emitted by the point light source 14 passes through the shock tube 6, the air flow density changes due to the vortex in the tube, and thus the refractive index of the light changes, the refracted light is reflected by the schlieren mirror 7, interfered by the knife blade 15 and captured by the high-speed camera 3, and finally a schlieren image with different brightness is formed to obtain the characteristics of the vortex structure.

[0027] The present application can effectively ensure the accurate and clear imaging of the single-mirror schlieren system based on the ingenious design, greatly reduces the off-axis degree of the point light source, the camera and the schlieren mirror without losing light intensity, reduces the influence of ghosting and astigmatism, and realizes high-precision schlieren imaging of the flow state in the shock tube. The overall device has simple structure and convenient debugging, and the imaging is accurate and clear, which plays a key role in obtaining more accurate and clear schlieren images and researching the vortex structure characteristics in the shock tube.

Claims

1. A single mirror co-axial schlieren imaging method for observing vortex structures in a shock tube, characterized in that, The method constructs an integrated coaxial light source by integrating a point light source with a blade, and builds a single-mirror coaxial schlieren imaging system by using a schlieren mirror, a shock tube and a high-speed camera. In the system, the incident light of the point light source passes through the schlieren mirror, is reflected and focused on the tip of the blade through the flow field in the shock tube. The blade interferes with the focal point. Due to the different air densities in the shock tube, the refractive index of the light is different, forming a schlieren grayscale image with different light and dark degrees. The high-speed camera records the changes of the vortex structure in the shock tube at a high frequency. Then, the high-pressure air inlet process is simulated by using a compression air pump, a two-stage pressure regulating filter device, an electromagnetic valve and a nozzle, so that the high-precision schlieren imaging of high-speed turbulent flow driven by high-pressure gas in the shock tube is realized. The specific steps of the method are as follows: Step one, constructing an integrated coaxial light source; In the integrated coaxial light source (4), a plurality of stepped inner holes are processed in the inside of the light source shell (17), which gradually decreases from left to right. In the smallest inner hole, the light source shell (17) is fixed with the blade (15) by adhesion, and a hole (16) for light passing is left above the blade, so that the blade (15) and the point light source (14) are bonded together to form an integrated coaxial light source (4). The lower part of the light source shell (17) is processed with a threaded hole, and the threaded support rod (13) is connected with the threaded hole through threads. The support rod (13) is fixed on the base (21) through the fixing bolt (18), and the base (21) is installed on the optical platform (2); Step two, building a single-mirror coaxial schlieren imaging system; The high-speed camera (3) is installed and fixed on the optical platform (2), and the schlieren mirror (7) is fixed on the optical platform (2) opposite to the high-speed camera (3) through a bolt. According to the focal length of the schlieren mirror (7), the circular arc track with the center of the schlieren mirror (7) as the origin and the double focal length as the radius is preliminarily determined as the position range of the integrated coaxial light source (4); The point light source (14) is turned on, and the position of the integrated coaxial light source (4) is adjusted along the circular arc track, so that the focal point focused after being reflected by the schlieren mirror (7) is on the same vertical line with the point light source (14) of the integrated coaxial light source (4). The fixing bolt (18) is loosened, the height of the threaded support rod (13) is adjusted, the blade (15) interferes with the focal point, and then the fixing bolt (18) is tightened to fix the height of the threaded support rod (13). The position of the high-speed camera (3) is adjusted to ensure that all regions in the schlieren mirror (7) are bright regions in the imaging, and the aperture is rotated to obtain a clear image. The palm is placed in front of the schlieren mirror (7) to obtain the weak air density change caused by the temperature of the palm, and the clear schlieren image is formed. The height of the threaded support rod (13) is finely adjusted to change the interference degree of the blade (15) to the focal point, and the required height of the threaded support rod (13) is obtained. Finally, a single-mirror coaxial schlieren system with fixed position is obtained; Step three, obtaining the vortex structure characteristics in the shock tube; On the basis of step one and two, the shock tube (6) is placed in front of the schlieren mirror (7) and closely attached to the mirror surface. The shock tube (6) is fixed by the lifting platform (5) and the height is adjusted so that the shock tube (6) is located in the middle of the schlieren mirror (7) to obtain the maximum observation area. The nozzle (8) is connected to the shock tube (6), and the air pipe (10) is connected to the electromagnetic valve (9), the secondary pressure regulating filter device (11) and the compressed air pump (1) in sequence. The secondary pressure regulating filter device (11) is fixed to the pressure regulating device fixed platform (12) by bolts, and the pressure regulating device fixed platform (12) is fixed to the optical platform (2) by bolts; The point light source (14) is turned on, the compressed air pump (1) is started, the secondary pressure regulating filter device (11) is adjusted to the specified pressure, and the dry and clean gas reaching the specified pressure is intermittently introduced into the shock tube (6) through the electromagnetic valve (9) to generate a vortex structure. At the same time, the light emitted by the point light source (14) passes through the shock tube (6), and the change of air flow density caused by the vortex in the tube leads to different refractive indexes of the light, so that the refracted light is reflected by the schlieren mirror (7) and interfered by the blade (15) and captured by the high-speed camera (3), and finally a schlieren image with different brightness is formed to obtain the vortex structure characteristics.

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