Rear-mounted holographic focusing schlieren imaging method and system

Abstract

The invention discloses a rear-mounted holographic focusing schlieren imaging method. The method comprises the following steps: after a laser emits a laser beam, the laser beam is divided into a main light source and holographic reference light; a main light source sequentially passes through a multi-stage beam expander, a soft light screen, a Fresnel lens and a source grating to obtain light beams with alternate brightness and darkness, and the light beams irradiate a flow field; the light beam penetrating through the flow field is focused by the focusing lens, an image of the source grating is formed at the knife edge grating through the focusing lens, and the light beam penetrating through the flow field is modulated by the knife edge grating, so that an image of the section of the flow field is formed behind the knife edge grating; the light beam modulated by the knife edge grid passes through the field lens and is recorded together with the holographic reference light to obtain a holographic focusing schlieren map; performing digital reconstruction to obtain three-dimensional flow field position information; and calculating the three-dimensional density gradient and the density value of the flow field based on the imaging principle of focusing schlieren. The invention further discloses a rear-mounted holographic focusing schlieren imaging system. According to the method and the system, the ultra-high-speed flow field can be measured, and the requirement of an ultra-high-speed aircraft test on fine quantitative measurement of the three-dimensional flow field is met.

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. The development of advanced hypersonic aircraft is crucial to improving my country's national defense strength, aerospace capabilities and the manufacturing level of major transportation equipment. Hypersonic ground tests are the most effective and direct means of developing hypersonic aircraft. Their characteristics are that the flow field around the aircraft model is short-lasting, often accompanied by complex physical and chemical phenomena, and produces strong self-luminescence. During the test, there are problems such as boundary layer transition, aerodynamic optical effect and heterogeneous aerodynamic drag reduction. The boundary layer transition problem may cause adverse effects on high-speed aircraft, such as increased friction, reduced aircraft stability, and multiple increases in heat flow. Research on transition mechanisms and transition prediction and control has great engineering practical significance for the design of aircraft and engines. Aerodynamic optical effects will restrict the search, identification and tracking capabilities of high-speed missiles for targets. Aerodynamic optical effect testing can obtain high-quality aerodynamic optical effect data, thereby effectively suppressing, correcting and compensating for aerodynamic optical effects. Aerodynamic drag will cause significant energy loss in high-speed trains, restricting further speed increases. In-depth research on the boundary layer mechanism of the vehicle body and complex flow field analysis can optimize the structural design to achieve aerodynamic drag reduction. To solve related problems, it is necessary to use high-resolution three-dimensional flow field display and measurement technology to obtain key test data to support the optimization design plan.

[0003] At present, the development of high-resolution three-dimensional quantitative measurement methods for ultra-high-speed flow fields faces many difficulties. Ultra-high-speed flow fields are mostly under extreme conditions such as high Mach number, high temperature, high pressure, transient and even strong self-luminescence, which poses great challenges to the layout of the measurement system and signal reception; the real flow field is three-dimensionally coupled, and the traditional flow field display method has an integral effect, making it difficult to achieve three-dimensional and quantitative measurement; the models of ground wind tunnel tests of ultra-high-speed flow fields are generally cross-scale, with both large-scale flow structures and small-scale vortex structures, and the measurement scenarios are diverse.

[0004] There are three main types of flow field measurement methods at home and abroad. The shadow and schlieren techniques based on the principle of light deflection characterize the flow field structure characteristics 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-dimensional, qualitative, and interference from imaging integral effects. Holographic interference and schlieren interference 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 the three-dimensional spatial flow field density value analysis, and the three-dimensional measurement capability is 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, and 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, meeting 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 light 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 the light beam is 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 cross 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 strong 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 light 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] Among them, 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 focusing 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 a knife-edge grating, and the focusing schlieren system parameter is quantitatively analyzed as the system sensitivity ε min , sharp focusing depth DS, imaging resolution w and the number of source grating hybrid line pairs Φ used for imaging; where ε min It indicates 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 result of quantitative analysis reaches the technical index, it is determined that the beam modulation is completed; the technical index is that the field of view, aberration, imaging resolution, system sensitivity and sharp focusing 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] Among them, 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 bright stripes of the knife-edge grating, A is the aperture of the focusing lens, and n is the number of bright stripes 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, and 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 image, 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 follows:

[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:

[0050]

[0051] Among them, △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 keep the stripes in the horizontal direction at the same time, this direction is taken as "x", and the direction perpendicular to the stripes is "y"; when the stripes can be kept in the vertical direction at the same time, 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 beams enter and leave 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 the 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 light screen, a Fresnel lens, and a source grid in sequence; the laser emits a laser beam, and the beam splitter splits the laser beam into two beams, which are respectively used as the main light source of the focusing schlieren light path and the holographic reference light; the light beam as the main light source is expanded by the multi-stage beam expander and modulated by the soft light 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 attenuation plate. The reflector adjusts the propagation direction of the holographic reference light, expands the beam and performs filtering and collimation through the spatial filter and the collimating lens, adjusts the intensity and quality of the beam through the polarizer and the attenuation plate, and the reflector merges the beam into the main light 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 the 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, and the laser 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, and the transmitted light is used as the main light source of the focused schlieren, and the reflected light is used as the holographic reference light; the multi-stage beam expansion component filters and multi-stage expands the narrow beam of laser 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, 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, and the light transmittance is more than 90%; the Fresnel lens is used to realize the collimation and small-amplitude convergence of the light beam, so that the light source can evenly illuminate the test area; the source grid is the core element of focused schlieren imaging, which is composed 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 stripe spacing accuracy and stripe 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 is formed into uniform parallel light with a large beam diameter through a reflector, a spatial filter, and a collimating lens; 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 the beam and performs filtering and collimation through the spatial filter and the collimating lens, adjusts the beam intensity and quality through the polarizer and the attenuation plate, and the second reflector merges the beam into the main light path.

[0064] Furthermore, when the reference light beam is transmitted back to the main optical path of the system through a long distance, a light guide arm or optical fiber or optical 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 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 element of 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 passed through the knife-edge grid, which is converted into light and dark changes in the imaging on the imaging screen to form 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 stripe number 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 and semi-reflective mirror; the multi-stage beam expander may also be referred to as a multi-stage beam expander assembly 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 wind tunnel test-related conditions. 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. The 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 It is a schematic diagram of the focusing schlieren transfer function;

[0069] Figure 2 This is a schematic diagram of the optical path of holographic focused schlieren imaging;

[0070] exist Figure 1 In the figure, 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 technology of the present invention is further described below through examples and in conjunction with the accompanying drawings.

[0072] A post-type holographic focusing schlieren imaging method provided by the present invention comprises 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) filtering, multi-stage beam expansion, and collimation are performed on the main light source of the focused Schlieren light path, and modulated with a soft light screen to obtain a uniform light 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 light and dark in the radial direction, which irradiates the flow field in the test area;

[0076] (4) A focusing lens is placed after the flow field to focus the light beam. The image of the source grid is formed at the knife-edge grid after the light beam is 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. The positions 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 focused schlieren reconstructed images of different flow field cross sections;

[0079] (7) According to the imaging principle of focused schlieren, a mathematical model of the image plane light intensity change of the holographic focused schlieren image and the flow field density gradient 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 the light beam passes through the 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 from 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 by the density change information as follows:

[0082]

[0083] Among them, △I=I-I0, which is the change of image plane light intensity caused by the change of density gradient 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 grid 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; 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] Among them, △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 placed in the horizontal direction while keeping the stripes, with this direction as "x" and the direction perpendicular to the stripes as "y"; they can be placed in the vertical direction while keeping the stripes, with this direction as "y" and the direction perpendicular to the knife-edge as "x". The optical axis direction is always "z", and the z1 and z2 beams enter and leave the test flow field along the optical axis.

[0087] like Figure 1 As shown, the sharp focusing depth DS and the non-sharp focusing depth DU are expressed in the coordinate system. For the transfer function in the model, it is a progressive function, which is at the maximum value in the central area of ​​the focusing position and gradually decreases on the non-sharp focusing surface away from the position. Since it is not a constant value, in order to facilitate data calculation, the transfer function σ(z) is stepped, that is, the entire focusing depth is divided into 7 different areas, each area has a different thickness, and the transfer functions of different areas are all a constant value.

[0088] After quantifying / simulating the transfer function, the mathematical model of the image plane light 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 images in N adjacent regions, the density field in the central region can be obtained.

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

[0095] Through this mathematical model, a corresponding flow field section object distance l can 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 thus 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: a light source modulation unit, a reference light unit, and an imaging recording unit.

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

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

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

[0104] (3) Imaging and 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 with the reference light through the second beam splitter 19 into the camera 20 for recording, thereby obtaining a holographic focused schlieren pattern.

[0105] Specifically, in this embodiment, the focal length and aperture of the focusing lens 14, 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, and any other changes, replacements, combined simplifications, etc. made under the core guiding idea of ​​the patent of the present invention are included in the protection scope 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 light 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 the light beam is 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 cross 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 is 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 according to the focusing schlieren system parameter; the focusing schlieren system parameter is the system sensitivity ε min , sharp focusing depth DS, imaging resolution w and the number of source grating hybrid line pairs Φ used for imaging; where ε min It indicates the minimum brightness change of the image that can be detected, DS is used to judge the ability of two-dimensional slicing 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: Among them, 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 bright stripes of the knife-edge grating, A is the aperture of the focusing lens, and n is the number of bright stripes 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: Among them, △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 keep the stripes in the horizontal direction at the same time, this direction is taken as x, and the direction perpendicular to the stripes is y; when the knife-edge grating and the source grating keep the stripes in the vertical direction at the same time, this direction is taken as y, and the direction perpendicular to the knife edge is x; the direction of the optical axis 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 the three-dimensional position, so ρ1 can be calculated, and thus 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 light screen, a Fresnel lens, and a source grid in sequence; the laser emits a laser beam, and the beam splitter splits the laser beam into two beams, which are respectively used as the main light source of the focusing schlieren light path and the holographic reference light; the light beam as the main light source is expanded by the multi-stage beam expander and modulated by the soft light 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 attenuation plate. The reflector adjusts the propagation direction of the holographic reference light, expands the beam and performs filtering and collimation through the spatial filter and the collimating lens, adjusts the intensity and quality of the beam through the polarizer and the attenuation plate, and the reflector merges the beam into the main light 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 the 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 stripes of the two are determined according to the field size and the focusing schlieren system parameters.

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