A rear-mounted holographic focusing schlieren imaging method and system

Abstract

The present invention discloses a post-type holographic focusing schlieren imaging method: after the laser emits a laser beam, it is divided into a main light source and a holographic reference light; the main light source sequentially passes through a multi-stage beam expander, a soft light screen, a Fresnel lens, and a source grid to obtain a light beam of alternating light and dark, which is then irradiated onto a flow field; the light beam passing through the flow field is focused by a focusing lens, and the image of the source grid is formed at a knife-edge grid through the focusing lens; the light beam passing through the flow field is modulated by the knife-edge grid, so that the image of the flow field cross section is formed behind the knife-edge grid; the light beam modulated by the knife-edge grid passes through a field lens and is recorded together with the holographic reference light to obtain a holographic focusing schlieren image; digital reconstruction is performed to obtain three-dimensional flow field position information; based on the imaging principle of focused schlieren, the three-dimensional density gradient and density value of the flow field are calculated. The present invention also discloses a post-type holographic focusing schlieren imaging system. The method and system can realize the measurement of ultra-high-speed flow fields, meeting the requirements of ultra-high-speed aircraft tests for refined quantitative measurement of three-dimensional flow fields.

Description

Technical Field

[0001] The present invention belongs to the field of high-resolution quantitative measurement of three-dimensional flow fields, and in particular relates to a method and system for post-type holographic focusing schlieren imaging. Background Art

[0002] Research on hypersonic flight technology is of great significance. Developing advanced hypersonic vehicles is crucial for enhancing my country's national defense, aerospace capabilities, and the manufacturing of major transportation equipment. Hypersonic ground testing is the most effective and direct means of developing hypersonic vehicles. These tests are characterized by short-duration flow fields around the vehicle model, often accompanied by complex physical and chemical phenomena, and generating strong self-luminescence. During testing, issues such as boundary layer transition, aero-optical effects, and heterogeneous aerodynamic drag reduction can arise. Boundary layer transition can adversely affect high-speed vehicles by increasing friction, reducing vehicle stability, and exponentially increasing heat flux. Research on transition mechanisms, as well as transition prediction and control, has significant engineering implications for the design of aircraft and engines. Aero-optical effects can hinder the ability of high-speed missiles to search, identify, and track targets. Aero-optical effect testing can generate high-quality data, enabling effective suppression, correction, and compensation of these effects. Aerodynamic drag causes significant energy losses in high-speed trains, hindering further speed increases. Deepening research on the carbody boundary layer mechanism and analyzing complex flow fields can optimize structural design to achieve aerodynamic drag reduction. To address these issues, high-resolution three-dimensional flow field display and measurement technology are needed to obtain key test data to support optimized design solutions.

[0003] Currently, the development of high-resolution, three-dimensional, quantitative measurement methods for ultrahigh-speed flow fields faces numerous challenges. Ultrahigh-speed flow fields often operate under extreme conditions, such as high Mach numbers, high temperatures, high pressures, transient states, and even intense self-luminescence. This poses significant challenges to measurement system layout and signal reception. Real flow fields are three-dimensionally coupled, and traditional flow field visualization methods suffer from integration effects, making three-dimensional, quantitative measurement difficult. Furthermore, models used in ground-based wind tunnel tests of ultrahigh-speed flow fields are typically multi-scale, encompassing both large-scale flow structures and small-scale vortex structures, resulting in diverse measurement scenarios.

[0004] There are three main types of flow field measurement methods at home and abroad. Shadow and schlieren techniques based on the principle of light deflection characterize the structural characteristics of the flow field by measuring the density changes in the flow field area. They have the advantages of simple optical path structure, good imaging effect, and intuitive results. However, in terms of measurement results, these methods have limitations such as two-dimensionality, qualitativeness, and interference from imaging integral effects. Holographic interferometry and schlieren interferometry measurement methods based on interference theory quantitatively obtain the density integral value along the measured flow field area through flow field density difference analysis, but they need to be combined with multi-view tomography or rotate the measured object to realize three-dimensional spatial flow field density value analysis, and their three-dimensional measurement capabilities are limited. Planar laser induced fluorescence, planar laser particle velocimetry, background schlieren and other technologies use tracer particles or characteristic points to measure the flow field to obtain parameters such as flow field velocity and characteristic component distribution, but they belong to the two-dimensional scale category. The tracking problem of tracer particles in ultra-high-speed flow fields also limits the measurement. Summary of the Invention

[0005] The purpose of the present invention is to provide a method and system based on post-type holographic focusing schlieren imaging, which can achieve a breakthrough from two-dimensional qualitative to three-dimensional quantitative measurement of ultra-high-speed flow fields, and perform three-dimensional transient flow field measurement with high time and high spatial resolution to meet the needs of ultra-high-speed aircraft tests for refined quantitative measurement of three-dimensional flow fields.

[0006] In order to solve the above technical problems, the present invention adopts the following specific technical solutions:

[0007] A post-type holographic focusing schlieren imaging method, the method comprising the following steps:

[0008] (1) After the laser emits a laser beam, it is divided into transmitted light and reflected light. The transmitted light serves as the main light source of the focused schlieren optical path, and the reflected light serves as the holographic reference light.

[0009] (2) The main light source passes through a multi-stage beam expander, a soft light screen, a Fresnel lens, and a source grid in sequence to obtain a light beam with alternating light and dark, which is then irradiated onto the test flow field;

[0010] (3) The light beam passing through the flow field is focused by the focusing lens, and the image of the source grid is formed at the knife-edge grid after being modulated by the focusing lens. The light beam passing through the flow field is modulated by the knife-edge grid, so that the image of the flow field cross section is formed behind the knife-edge grid;

[0011] (4) The light beam modulated by the knife-edge grating passes through the field lens and is recorded together with the holographic reference light to obtain a holographic focused schlieren pattern;

[0012] (5) Digitally reconstructing the holographic focused schlieren pattern to obtain focused schlieren reconstructed images of different flow field sections and obtain three-dimensional flow field position information;

[0013] (6) Based on the imaging principle of focused schlieren, a mathematical model is established to represent the relationship between the light intensity change of the holographic focused schlieren image and the density gradient of the flow field, and the three-dimensional density gradient and density value of the flow field are calculated.

[0014] In step (1), the laser emits highly monochromatic, uniform, and coherent light. The holographic reference light can also be called the reference light of holographic interference.

[0015] In step (1), the laser is a pulsed laser or a continuous laser.

[0016] In step (2), the light beam alternating between bright and dark is a high-quality light beam whose diameter gradually decreases along the optical axis, and is uniformly irradiated on the flow field disturbance area.

[0017] In step (3), the imaging formula of the focusing lens is the same as that of an ordinary lens, and its imaging formula is:

[0018]

[0019] Where f is the focal length of the focusing lens, u is the distance between the object and the lens, and v is the distance between the lens and the image.

[0020] In step (3), the image of the source grid and the flow field cross section is formed in the rear area of ​​the focusing lens. The specific placement position of each optical element and the focus position of the test area can be calculated based on the imaging formula of the focusing lens.

[0021] In step (3), the knife-edge grid is located at the same position as the source grid image plane, and the black and white grids of the knife-edge grid and the light and dark stripes of the source grid image are offset from each other.

[0022] In step (3), the light beam is modulated by the knife-edge grating, and the focusing schlieren system parameters are quantitatively analyzed as the system sensitivity ε min , sharp focus depth DS, imaging resolution w and the number of source grating hybrid line pairs Φ used for imaging; where ε min It represents the minimum brightness change of the image that can be detected, DS is used to judge the ability of the two-dimensional "slice" of the flow field, and Φ is used to judge the smoothness of the image.

[0023] When the results of the quantitative analysis meet the technical indicators, the beam modulation is determined to be complete; among which, the technical indicators are that the field of view, aberration, imaging resolution, system sensitivity and sharp focus depth in the test center area can be maintained within the corresponding range.

[0024] The relationship between the focusing schlieren system parameters and the optical design parameters is:

[0025]

[0026]

[0027]

[0028]

[0029]

[0030] Where a is the size of the uncut light source image at the knife-edge grating position, L is the distance from the source grating to the focusing lens, L' is the image distance of the source grating, l is the distance from the flow field section to the focusing lens, l' is the image distance of the flow field section, λ is the wavelength of the light source, b is the width of the knife-edge grating bright fringes, A is the focusing lens aperture, and n is the number of bright fringes per millimeter of the knife-edge grating.

[0031] In step (4), the holographic focusing schlieren pattern is formed by the interference of the object light passing through the three-dimensional flow field and the coherent reference light. The interference fringes record the complex amplitude and light intensity information of the object light. The complex amplitude of the holographic focusing schlieren pattern is:

[0032] U(x0,y0)=O(x0,y0)+R(x0,y0) (7)

[0033] Among them, (x0, y0) is the coordinate system of the holographic focusing schlieren recording image, U(x0, y0) is the complex amplitude; O is the conjugate of the object light; and R is the conjugate of the reference light.

[0034] The light intensity of the holographic focused schlieren recording image is:

[0035] I(x0,y0)=|U(x0,y0)| 2 =|R| 2 +|O| 2 +R * O+RO * (8)

[0036] Among them, I(x0,y0) is the light intensity; O * is the conjugate of the object light; R * is the conjugate of the reference light.

[0037] In step (5), the method for digitally reconstructing the holographic focused schlieren pattern may be convolution reconstruction, angular spectrum reconstruction, Fresnel approximation reconstruction, wavelet reconstruction, fractional Fourier transform reconstruction, etc.

[0038] Taking the angular spectrum reconstruction method as an example, it is expressed as:

[0039] The angular spectrum relationship is:

[0040]

[0041] Among them, A(f x ,fy ) is the angular spectrum corresponding to the complex amplitude U(x,y) of the holographic focusing schlieren reconstruction image to be solved, A0(f x ,f y ) is the angular spectrum corresponding to the complex amplitude U(x0,y0) of the holographic focused schlieren recording pattern, and Δz is the reconstruction interval of the digital hologram.

[0042] According to the angular spectrum relationship, the complex amplitude of the holographic focusing schlieren reconstruction image is obtained as:

[0043] U(x,y)=F -1 [A(f x ,f y )]

[0044] =F -1 [A0(f x ,f y )·H(f x ,f y )] (10)

[0045] =F -1 {F[U(x0,y0)·H(f x ,f y )]}

[0046] Among them, the transfer function F, F -1 They represent the forward and inverse Fourier transform, respectively.

[0047] Therefore, the light intensity of the holographic focused schlieren reconstruction image is:

[0048]

[0049] In step (6), the mathematical model of the light intensity variation and flow field density gradient of the holographic focusing schlieren reconstruction image is specifically as follows:

[0050]

[0051] Where, △I(x,y)=I(x,y)-I(x0,y0); a is the size of the light source image not cut at the knife-edge position; f is the focal length of the imaging lens, i.e., the focusing lens. L is the distance from the source grid to the focusing lens, L' is the image distance of the source grid, l is the distance from the flow field section to the focusing lens, and l' is the image distance of the flow field section.

[0052] When the knife-edge grid and source grid can simultaneously maintain the stripes in the horizontal direction, this direction is referred to as "x" and the direction perpendicular to the stripes is "y". When the stripes can simultaneously maintain the vertical direction, this direction is referred to as "y" and the direction perpendicular to the knife-edge is referred to as "x". The optical axis direction is always "z", and the z1 and z2 beams enter and exit the test flow field along the optical axis.

[0053] In step (6), the density gradient of the flow field is expressed as:

[0054]

[0055] Among them, the transfer function σ(z) is quantized, j represents different flow field regions, and d j is the thickness of different regions, β j is the amplitude of each step, which is between 0 and 1, N is the number of steps, and ρ is the density.

[0056] In step (6), the density value of the flow field is calculated as follows: Assuming that the density value of point 0 in the undisturbed area of ​​the same flow field section is the starting value ρ0, the density difference between point 0 and point 1 on the section can be obtained according to the density gradient and three-dimensional position. Therefore, ρ1 can be calculated, and the density difference between point n-1 and point n on the section can be obtained as ρ n-1 -ρ n Calculate ρ n , and so on to obtain the density value of each flow field section.

[0057] The present invention also provides a rear-mounted holographic focusing schlieren imaging system, the system comprising:

[0058] The light source modulation unit includes a laser, a beam splitter, a multi-stage beam expander, a soft screen, a Fresnel lens, and a source grid in sequence. The laser emits a laser beam, which is split by the beam splitter into two beams, serving as the main light source of the focused schlieren light path and the holographic reference light, respectively. The main light source beam is expanded by the multi-stage beam expander and modulated by the soft screen, the Fresnel lens, and the source grid.

[0059] The reference light unit includes a reflector, a spatial filter, a collimating lens, a polarizer, and an attenuator. The reflector adjusts the propagation direction of the holographic reference light, expands and filters the light through the spatial filter and collimating lens, and adjusts the intensity and quality of the light through the polarizer and attenuator. The reflector then directs the light beam into the main optical path.

[0060] The imaging and recording unit includes a focusing lens, a polarizer, an attenuation plate, a knife-edge grating, a field lens group, and a camera in sequence. The light beam passing through the flow field is focused by the focusing lens, modulated by the polarizer and attenuation plate, "cut" by the knife-edge grating, converged by the field lens group, and merged into the camera for recording together with the holographic reference light through a beam splitter to obtain a holographic focused schlieren pattern.

[0061] Furthermore, in the light source modulation unit, the laser is a pulsed laser or a continuous laser, which emits a high-energy and highly coherent laser beam to ensure the intensity of the light field passing through the flow field, and its wavelength is 400nm to 600nm; the beam splitter splits the laser beam, the transmitted light serves as the main light source of the focused schlieren, and the reflected light serves as the holographic reference light; the multi-stage beam expansion component filters and multi-stage expands the narrow beam of laser light to form a relatively uniform parallel collimated beam; the soft light screen diffusely reflects the light beam, and each particle on its surface can be regarded as a point light source with uniform spatial distribution and extremely dense distribution, and each point light source The emitted light is superimposed on each other at any point in space in the transmission direction to form a uniform illuminance distribution, making the light beam more uniform. Generally, a phase modulator with higher transmission energy and more stable divergence angle is used as a soft light screen with a light transmittance of more than 90%; the Fresnel lens is used to achieve collimation and small-amplitude convergence of the light beam, so that the light source evenly illuminates the test area; the source grid is the core component of focused schlieren imaging, which consists of a series of alternating light and dark stripes, so that the light beam passing through is alternating light and dark in the radial direction, with high fringe spacing accuracy and fringe straightness, and the diameter is matched with the field of view diameter, generally in the range of 100mm to 800mm.

[0062] Furthermore, in the reference light unit, the reflected light obtained after the light emitted by the laser source is split is used as the reference light, and passes through a reflector, a spatial filter, and a collimating lens to form a uniform parallel light with a large beam diameter; the polarizer is used to adjust the light intensity; and the attenuation plate is used to filter stray light.

[0063] Furthermore, the reference light unit includes a first reflector, a spatial filter, a collimating lens, a polarizer, an attenuation plate and a second reflector in sequence; the first reflector adjusts the propagation direction of the holographic reference light, expands and filters and collimates the light through the spatial filter and the collimating lens, adjusts the intensity and quality of the light beam through the polarizer and the attenuation plate, and the second reflector merges the light beam into the main light path.

[0064] Furthermore, when the reference light beam is transmitted over a long distance back to the main optical path of the system, a light-guiding arm or optical fiber or fiber coupler needs to be added to the unit to guide the light, so that the optical path is less disturbed to ensure the quality of the reference light.

[0065] Furthermore, in the imaging and recording system, the focusing lens focuses the light beam passing through the source grid and the flow field, so that the source grid is imaged at the knife-edge grid and the flow field cross section is imaged behind the knife-edge grid; the knife-edge grid is also the core component of the focused schlieren imaging, which is used to modulate the light beam and is placed at the conjugate image of the source grid. The refractive index change caused by the disturbance of the flow field display area will interfere with the propagation of the light beam, and the changed light beam will be blocked or pass through the knife-edge grid, which is converted into light and dark changes in the imaging on the imaging screen, forming a focused schlieren effect; the focal length and aperture of the focusing lens are determined by the field of view size and the focused schlieren system parameters; the knife-edge grid and the source grid are matched, and the stripe width and number of the stripes of the two are selected according to the field of view size and the focused schlieren system parameter requirements, see formulas (2) to (6); the field lens group shrinks the modulated large-diameter light beam to facilitate the collection of the light beam by the camera target surface; the high-resolution camera is used to record the system object light and reference light that are irradiated together.

[0066] In the present invention, the beam splitter may be a semi-transparent semi-reflective mirror; the multi-stage beam expander may also be referred to as a multi-stage beam expansion component or a multi-stage beam expander.

[0067] The post-type holographic focusing schlieren imaging method and system provided by the present invention can test ultra-high-speed flow fields, and are mostly used for measuring conditions related to wind tunnel tests. Compared with other flow field measurement methods, this technical method can measure three-dimensional flow fields, and has high three-dimensional spatial resolution, meeting the needs of refined structural inversion of three-dimensional flow fields. Flow field density field information can be obtained based on the reconstructed information of the hologram, and has the characteristics of high flexibility, wide measurement range, and high measurement accuracy. The present invention provides a more powerful measurement method for the study of ultra-high-speed flow fields, and is of great significance for solving key technical problems in the fields of national defense, aerospace, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 Schematic diagram of the focused schlieren transfer function;

[0069] Figure 2 Schematic diagram of the optical path of holographic focused schlieren imaging;

[0070] exist Figure 1 Among them, 1. Laser; 2. First beam splitter; 3. Coherent optical path; 4. Multi-stage beam expander; 5. Soft light screen; 6. Fresnel lens; 7. Source grid; 8. First reflector; 9. Spatial filter; 10. Collimating lens; 11. First polarizer; 12. Attenuator; 13. Second reflector; 14. Focusing lens; 15. Second polarizer; 16. Adjustable attenuator; 17. Knife-edge grid; 18. Field lens group; 19. Second beam splitter; 20. Camera. DETAILED DESCRIPTION

[0071] The specific implementation of the present invention will be further described below through examples and in conjunction with the accompanying drawings.

[0072] The present invention provides a post-type holographic focusing schlieren imaging method comprising the following steps:

[0073] (1) The laser emits a laser beam, which is split into two beams by a semi-transparent and semi-reflective mirror. The transmitted light serves as the main light source of the focused schlieren light path, and the reflected light serves as the holographic reference light.

[0074] (2) The main light source of the focused Schlieren light path is filtered, multi-stage beam expanded, and collimated, and modulated with a soft light screen to obtain a uniform beam;

[0075] (3) When the uniform light beam passes through the Fresnel lens, its axial diameter is reduced, and the source grid modulates the reduced light beam to obtain a light beam that alternates between bright and dark in the radial direction, which illuminates the flow field in the test area;

[0076] (4) The focusing lens is placed after the flow field to focus the light beam. The image of the source grid is modulated by the focusing lens and formed at the knife-edge grid. The light beam passing through the flow field is modulated by the knife-edge grid so that the image of the flow field cross section is formed behind the knife-edge grid. The position of each optical element and the focus position of the test area can be calculated based on the imaging formula of the focusing lens;

[0077] (5) The light beam modulated by the knife-edge grating passes through the field lens and is recorded by a high-resolution camera together with the holographic reference light to obtain a holographic focused schlieren pattern;

[0078] (6) Using digital holographic technology, digitally reconstruct the holographic focused schlieren pattern to obtain the focused schlieren reconstructed images of different flow field sections;

[0079] (7) According to the imaging principle of focused schlieren, a mathematical model of the image plane light intensity change and flow field density gradient of the holographic focused schlieren image is established, and the density gradient and density value of the flow field are calculated.

[0080] In step (7), the imaging principle of focused schlieren is that when a light beam passes through a transparent test medium, the light beam will be deflected in the direction of density change. The specific expression is as follows:

[0081] In the focused schlieren, assuming that the test area along the optical axis is z1 to z2 and the knife edge direction is the x direction, the relative light intensity change caused by the light deflection on the imaging surface is expressed as the density change information:

[0082]

[0083] Where △I = I-I0, which is the change in image intensity caused by the density gradient change in the test area; K is the Gladstone-Dale constant; f is the focal length of the focusing lens; a is the size of the uncut light source image at the knife-edge position; L is the distance between the source grid and the focusing lens, l is the distance from the flow field section to the focusing lens; σ(z) is the transfer function; and ρ is the density.

[0084] The holographic focused schlieren reconstruction image is based on the focused schlieren. The mathematical model of the single-section light intensity change and the flow field density gradient is:

[0085]

[0086] Where, △I(x,y)=I(x,y)-I(x0,y0); a is the size of the light source image not cut at the knife-edge position; f is the focal length of the imaging lens, i.e., the focusing lens. For this technique, the knife-edge grid and source grid can be positioned while maintaining the stripes in the horizontal direction, with this direction designated as "x" and the direction perpendicular to the stripes as "y." Alternatively, the stripes can be positioned while maintaining the stripes in the vertical direction, with this direction designated as "y" and the direction perpendicular to the knife-edge as "x." The optical axis is always oriented as "z," with the z1 and z2 beams entering and exiting the test flow field along the optical axis.

[0087] like Figure 1 As shown in the figure, the sharp focusing depth DS and the non-sharp focusing depth DU are represented in the coordinate system. The transfer function in the model is a gradual function, reaching its maximum value in the center of the focus position and gradually decreasing in the non-sharp focusing plane away from this position. Because it is not a constant value, to facilitate data calculation, the transfer function σ(z) is stepped. This means that the entire focus depth is divided into seven distinct regions, each with a different thickness, and the transfer function in each region is a constant value.

[0088] After quantifying / simulating the transfer function, the mathematical model of the image plane intensity change and flow field density gradient of the holographic focusing schlieren reconstruction image is rewritten as:

[0089]

[0090] Among them, j represents different flow field areas, d j The thickness of different areas.

[0091] After subtracting the contribution of the density gradient in the non-sharply focused depth from the density gradient change information in the sharply focused area, the mathematical model is rewritten as:

[0092]

[0093] Among them, β j is the amplitude of each step, which is between 0 and 1. Figure 1 As shown; N is the number of steps, according to Figure 1 So N is 7. According to model (17), after obtaining the holographic focusing schlieren patterns in N adjacent regions, the density field in the central region can be obtained.

[0094] By combining this mathematical model with holographic focusing schlieren images of multiple sections, the flow field density gradient can be obtained.

[0095] Through this mathematical model, a corresponding flow field section object distance l can be used to obtain a corresponding flow field area density gradient. Assume that the distance between the starting section and the focusing lens is l0, and the image distance is l0'; ...; the distance between the nth test section and the focusing lens is l n , the image distance is l n '. l0' can be obtained according to the placement of the optical element, then:

[0096]

[0097] Then the density gradient of n flow field sections is expressed as:

[0098]

[0099] For the flow field density value, assuming that the density value of point 0 in the undisturbed area of ​​the same flow field section is the starting value ρ0, the density difference between point 0 and point 1 on the section can be obtained according to the density gradient and three-dimensional position as ρ0-ρ1, so ρ1 can be calculated, and the density difference between point n-1 and point n on the section can be obtained as ρ n-1 -ρ n Calculate ρ n , and so on to obtain the density value of each flow field section.

[0100] The embodiment of the present invention also provides a rear-mounted holographic focusing schlieren imaging system, such as Figure 2 As shown, it includes three parts: light source modulation unit, reference light unit, and imaging recording unit.

[0101] (1) Light source modulation unit: The laser 1 emits a laser beam, and the first beam splitter 2 (semi-transparent and semi-reflective mirror) splits the beam into two beams, which serve as the main light source of the focusing schlieren light path and the holographic reference light respectively. The beam serving as the main light source passes through the coherent light path 3, is expanded by the multi-stage beam expander 4, and is modulated by the soft light screen 5, the Fresnel lens 6, and the source grid 7.

[0102] Specifically, in this embodiment, laser 1 is used to generate a high-energy, highly coherent laser beam with a wavelength of 532 nm and a uniformity of better than 85% after beam expansion to 100 mm. The coherent optical path 3, consisting of four reflectors, is used to compensate for the optical path difference caused by the reference beam. The soft light screen 5, using a phase modulator, has a light transmittance of over 90%. The source grid 7 is composed of a black and white grid, whose diameter and stripe width are determined by the field of view and equations (2) to (6).

[0103] (2) Reference light unit: The first reflector 8 adjusts the propagation direction of the light beam, and the light beam is expanded and collimated by the spatial filter 9 and the collimating lens 10. The light beam intensity and quality are adjusted by the first polarizer 11 and the attenuation plate 12. The light beam is then merged into the main light path by the second reflector 13.

[0104] (3) Imaging recording unit: the light beam passing through the measurement area is focused by the focusing lens 14, modulated by the second polarizer 15 and the adjustable attenuation plate 16, "cut" by the knife-edge grid 17, converged by the field lens group 18, and merged into the camera 20 for recording together with the reference light through the second beam splitter 19 to obtain a holographic focused schlieren pattern.

[0105] Specifically, in this embodiment, the focal length and aperture of the focusing lens 14, and the diameter and stripe width of the knife-edge grid 17 are determined by the field of view size and formulas (2) to (6).

[0106] The above is a detailed description of the contents of the present invention in combination with the embodiments, but the implementation methods of the present invention are not limited to the above embodiments. Any other changes, replacements, combination simplifications, etc. made under the core guiding idea of ​​the patent of the present invention are included in the scope of protection of the patent of the present invention.

Claims

1. A post-type holographic focusing schlieren imaging method, characterized in that: The method comprises the following steps: (1) After the laser emits a laser beam, it is divided into transmitted light and reflected light. The transmitted light serves as the main light source of the focused schlieren optical path, and the reflected light serves as the holographic reference light. (2) The main light source passes through a multi-stage beam expander, a soft light screen, a Fresnel lens, and a source grid in sequence to obtain a light beam with alternating light and dark, which is then irradiated onto the test flow field; (3) The light beam passing through the flow field is focused by the focusing lens, and the image of the source grid is formed at the knife-edge grid after being modulated by the focusing lens. The light beam passing through the flow field is modulated by the knife-edge grid, so that the image of the flow field cross section is formed behind the knife-edge grid; (4) The light beam modulated by the knife-edge grating passes through the field lens and is recorded together with the holographic reference light to obtain a holographic focused schlieren pattern; (5) Digitally reconstructing the holographic focused schlieren pattern to obtain focused schlieren reconstructed images of different flow field sections and obtain three-dimensional flow field position information; (6) Based on the imaging principle of focused schlieren, a mathematical model is established to represent the relationship between the light intensity change of the holographic focused schlieren image and the density gradient of the flow field, and the three-dimensional density gradient and density value of the flow field are calculated.

2. The rear-mounted holographic focusing schlieren imaging technology according to claim 1, characterized in that: In step (3), the knife-edge grid is located at the same position as the source grid image plane, and the black and white grids of the knife-edge grid and the light and dark stripes of the source grid image are offset from each other.

3. The post-type holographic focusing schlieren imaging method according to claim 1, characterized in that: In step (3), the light beam is modulated by a knife-edge grating and quantitatively analyzed based on the focusing schlieren system parameter; the focusing schlieren system parameter is the system sensitivity ε min , sharp focus depth DS, imaging resolution w and the number of source grating hybrid line pairs Φ used for imaging; where ε min It represents the minimum brightness change of the image that can be detected, DS is used to judge the ability of the two-dimensional slice of the flow field, and Φ is used to judge the smoothness of the image.

4. The post-type holographic focusing schlieren imaging method according to claim 3, characterized in that: The relationship between the focusing schlieren system parameters and the optical design parameters is: Where a is the size of the uncut light source image at the knife-edge grating position, L is the distance from the source grating to the focusing lens, L' is the image distance of the source grating, l is the distance from the flow field section to the focusing lens, l' is the image distance of the flow field section, λ is the wavelength of the light source, b is the width of the knife-edge grating bright fringes, A is the focusing lens aperture, and n is the number of bright fringes per millimeter of the knife-edge grating.

5. The post-type holographic focusing schlieren imaging method according to claim 1, characterized in that: In step (6), the mathematical model of the light intensity variation and flow field density gradient of the holographic focusing schlieren reconstruction image is specifically as follows: Where, △I(x,y)=I(x,y)-I(x0,y0); a is the size of the light source image not cut at the knife-edge position; f is the focal length of the imaging lens, i.e., the focusing lens. L is the distance from the source grid to the focusing lens, L' is the image distance of the source grid, l is the distance from the flow field section to the focusing lens, and l' is the image distance of the flow field section; When the knife-edge grating and the source grating simultaneously keep the stripes in the horizontal direction, this direction is taken as x, and the direction perpendicular to the stripes is y; when the knife-edge grating and the source grating simultaneously keep the stripes in the vertical direction, this direction is taken as y, and the direction perpendicular to the knife edge is x; the optical axis direction is always z, and the z1 and z2 light beams enter and leave the test flow field along the optical axis.

6. The post-type holographic focusing schlieren imaging method according to claim 5, characterized in that: The density gradient of the flow field is expressed as: Among them, the transfer function σ(z) is quantized, j represents different flow field regions, and d j is the thickness of different regions, β j is the amplitude of each step, which is between 0 and 1, N is the number of steps, and ρ is the density.

7. The post-type holographic focusing schlieren imaging method according to claim 1, characterized in that: The density value of the flow field is calculated as follows: Assuming that the density value of point 0 in the undisturbed area of ​​the same flow field section is the starting value ρ0, the density difference between point 0 and point 1 on the section can be obtained as ρ0-ρ1 according to the density gradient and three-dimensional position, so ρ1 can be calculated, and the density difference between point n-1 and point n on the section can be obtained as ρ n-1 -ρ n Calculate ρ n , and so on to obtain the density value of each flow field section.

8. A rear-mounted holographic focusing schlieren imaging system, characterized in that: The system comprises: The light source modulation unit includes a laser, a beam splitter, a multi-stage beam expander, a soft screen, a Fresnel lens, and a source grid in sequence. The laser emits a laser beam, which is split by the beam splitter into two beams, serving as the main light source of the focused schlieren light path and the holographic reference light, respectively. The main light source beam is expanded by the multi-stage beam expander and modulated by the soft screen, the Fresnel lens, and the source grid. The reference light unit includes a reflector, a spatial filter, a collimating lens, a polarizer, and an attenuator. The reflector adjusts the propagation direction of the holographic reference light, expands and filters the light through the spatial filter and collimating lens, and adjusts the intensity and quality of the light through the polarizer and attenuator. The reflector then directs the light beam into the main optical path. The imaging and recording unit includes a focusing lens, a polarizer, an attenuation plate, a knife-edge grating, a field lens group, and a camera in sequence. The light beam passing through the flow field is focused by the focusing lens, modulated by the polarizer and attenuation plate, cut by the knife-edge grating, converged by the field lens group, and merged into the camera for recording together with the holographic reference light through a beam splitter to obtain a holographic focused schlieren pattern.

9. The rear-mounted holographic focusing schlieren imaging system according to claim 8, characterized in that: The laser is a pulse laser or a continuous laser with a wavelength ranging from 400nm to 600nm.

10. The rear-mounted holographic focusing schlieren imaging system according to claim 8, characterized in that: The focal length and aperture of the focusing lens are determined by the field size and the focusing schlieren system parameters; the source grid and the knife-edge grid are matched, and the stripe width and number of the stripes of the two are determined according to the field size and the focusing schlieren system parameters.

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