Flow field display device and method based on multi-section focused schlieren imaging

Abstract

The present invention discloses a flow field display device and method based on multi-section focused schlieren imaging, which belongs to the field of focused schlieren imaging technology, solves the problem of difficulty in simultaneously acquiring multiple flow field information, resulting in low accuracy of the measured results. The device of the present invention includes an imaging device, an imaging recording device, a density field processing system and an optical system bracket; the imaging recording device is arranged at a position at least in two directions beside the imaging device, the density field processing system is connected to the imaging recording device through signal transmission, and the imaging device and the imaging recording device are respectively arranged on the optical system bracket; the method includes adjusting system parameters and setting components; the light source emits a light beam through the flow field area to be measured; the light beam is split and the position is selected for interception; the light beam is imaged, the intercepted image information is collected and transmitted to the system; the image signal is processed into flow field information and displayed. The present invention is used to measure and display the flow field density and change amount, and the result is highly accurate.

Description

Technical Field

[0001] The present invention belongs to the technical field of focused schlieren imaging, and in particular relates to a flow field display device and method based on multi-section focused schlieren imaging. Background Art

[0002] Flow field display measurement is one of the important contents in the research and testing of major national weapons and equipment and other major projects. Through flow field display means, the flow field wave system structure distribution around the test target can be directly and quickly obtained to further understand the physical and chemical change process characteristics of the flow field around ultra-high-speed aircraft and other test targets, and provide important supporting data for the optimal design of real aircraft and other targets.

[0003] Currently, conventional "Z"-shaped structure shadows and schlieren imaging are the main methods used in flow field display. The grayscale changes in the shadow image formed by the "Z"-shaped structure shadow reflect the integral value of the second-order derivative change of the light beam density along the entire test flow field, where the distance of the light beam along the test flow field can usually range from hundreds of millimeters to several meters.

[0004] The grayscale change in the Schlieren image reflects the integral value of the first-order derivative change of the density of the light beam along the entire test flow field. The Schlieren method is more sensitive than the shadow method, but the flow field information obtained by both methods is the integral effect value, and it is difficult to obtain the quantitative value of flow field parameters such as density through image processing. Therefore, the shadow and Schlieren methods are mainly used to qualitatively display the flow field structure around the model. At the same time, due to the integral effect characteristics, only the flow field structure along the upper and lower and left and right of the light beam can be judged according to the flow field, and the flow field structure at any position along the direction of the light beam cannot be accurately identified. That is, the shadow and Schlieren imaging has a two-dimensional qualitative characteristic, and the flow field structure cannot be quantitatively and three-dimensionally displayed.

[0005] The interference technology of shadow schlieren optical path can be used to carry out quantitative measurement of flow field density, but the result obtained is still the integral effect of the light beam along the entire flow field. In the past decade, other flow field methods such as background schlieren (BOS), planar laser induced fluorescence (PLIF), planar laser particle velocimetry (PIV), pressure-sensitive paint (PSP) and other technologies have been developed successively to achieve quantitative measurement of certain parameters in flow field measurement. For example, BOS technology can realize large-field flow field display and quantitatively obtain the integral value of density change along the direction of the light beam, but the spatial resolution is low, and there is a large error in the area with large density change; PLIF technology can quantitatively obtain multiple parameter values ​​of the flow field, but the optical path structure is complex, and it can only display the flow field of a certain plane at a certain moment; PIV is mainly used for quantitative measurement of low-speed and high-speed flow field velocities, but it is often not used because the particle tracking performance in ultra-high-speed flow fields cannot meet the requirements; PSP mainly measures the pressure on the surface of the model, and its surface coating will be damaged at high temperatures, affecting the test accuracy. Therefore, these technologies cannot simultaneously achieve quantitative measurement of ultra-high-speed flow field parameters and three-dimensional flow field display.

[0006] Multi-section focused schlieren imaging is currently recognized at home and abroad as one of the most effective methods for quantitative measurement and three-dimensional display of flow fields. By acquiring flow field information of different sections, three-dimensional quantitative analysis of the flow field density gradient along the direction of the light beam in the test section can be achieved.

[0007] Theoretically, the maximum number of cross-section calculations that can be achieved with existing technologies is relatively small. This is mainly limited by the size gradient of the flow field in the test section and the limitation of computer computing power. In fact, the measurement method of flow field display of multi-section measurement devices is very limited. It is difficult to obtain flow field information of multiple sections at the same time. One of the reasons is that the depth of field of the imaging lens determines the position distance of the obtainable section, and the depth of field has a certain relationship with the object distance, which makes it difficult to display enough cross-sectional flow field information; the second reason is that multiple cross-sectional imaging separations in the optical path will gradually reduce the imaging quality and resolution, and cannot fully display the transient information of different cross-sections of the flow field. The accuracy of the data results calculated by the back end is also not recognized. Summary of the invention

[0008] The objects of the present invention are:

[0009] In order to solve the problem that it is difficult to simultaneously obtain multiple flow field information when measuring multiple sections using the focused schlieren imaging technology in the prior art, resulting in low accuracy of the measurement results, a flow field display device and method based on multi-section focused schlieren imaging are provided, which can simultaneously obtain multiple different section information, thereby using the superposition of multi-section information to restore the flow field density gradient.

[0010] The technical solution adopted by the present invention is as follows:

[0011] A flow field display device based on multi-section focused schlieren imaging comprises an imaging device, an imaging recording device, a density field processing system and an optical system bracket; the imaging recording device is arranged at positions at least in two directions beside the imaging device, the density field processing system is connected to the imaging recording device through signal transmission, and the imaging device and the imaging recording device are respectively arranged on the optical system bracket;

[0012] Among them, the imaging device is used to generate an optical path for plane interception by focused schlieren imaging method through the light emitted by the light source; the imaging recording device is used to receive at least two images intercepted in the optical path formed by the imaging device, and transmit the image information to the density field processing system; the density field processing system is used to process the image information into flow field density distribution information and output a display image.

[0013] Further, the imaging device comprises a pulse light source, a Fresnel lens, a source grid, a focusing lens and a beam splitter arranged in sequence along the same straight line direction, and a group of knife-edge grids and field lenses are arranged in sequence on at least two sides of the beam splitter;

[0014] Among them, the Fresnel lens is used to control the focusing of the light emitted by the pulse light source; the source grid is used to shape the light beam passing through the Fresnel lens; the focusing lens is used to focus the light beam passing through the source grid and the flow field; the beam splitter is used to split the light beam passing through the focusing lens into at least two directions; the knife-edge grid is used to cut the light source image on the optical path; and the field lens is used to reduce the image and allow the image to enter the imaging target surface of the imaging recording device respectively.

[0015] Furthermore, a beam expander and a light softener are provided between the pulse light source and the Fresnel lens; wherein the beam expander is used to collimate the light beam emitted by the pulse light source; and the light softener is used to adjust the distribution uniformity of the light beam.

[0016] Furthermore, the imaging and recording device includes an image receiving device and a remote control transmission system, and the remote control transmission system is electrically connected to the image receiving device; wherein the image receiving device is used to receive image information at at least two sections in the optical path of the imaging device; and the remote control transmission system is used to transmit the image information obtained by the image receiving device to the density field processing system through remote control.

[0017] Furthermore, the imaging and recording device also includes a filtering and attenuation device, which is arranged between the image receiving device and the remote control transmission system, or between the image receiving device and the imaging device; wherein the filtering and attenuation device is used to filter and attenuate the light beam to remove stray light.

[0018] Furthermore, the optical system bracket includes a beam expansion bracket and an imaging bracket, and the optical path exit end of the beam expansion bracket is arranged opposite to the optical path entrance end of the imaging bracket; the bottom of the beam expansion bracket and the imaging bracket are respectively connected to a beam expansion vibration isolation platform and an imaging vibration isolation platform; wherein the beam expansion bracket and the imaging bracket are used to be respectively arranged on both sides of the flow field and respectively support the optical system elements arranged on both sides of the flow field.

[0019] The flow field display method based on multi-section focused schlieren imaging includes the following steps:

[0020] S100, inputting the information of the flow field to be measured into the multi-section focusing schlieren imaging system, the system adjusting the system parameters according to the flow field information to be measured, and adjusting the parameters and spacing of each component in the system, and setting each component of the flow field display device based on the adjusted parameters and spacing information;

[0021] S200, the system controls the light source of the imaging device of the flow field display device to emit a light beam, and after focusing the light beam, the light beam is made to pass through the flow field area to be measured in the form of a conical light path;

[0022] S300, the imaging device focuses the light beam passing through the flow field area to be measured and then splits the light beam into light beams in at least two directions, and selects positions to intercept the light beam in each direction respectively;

[0023] S400, the imaging recording device images the light beam from the imaging device, collects the intercepted image information, converts it into an image electronic signal and transmits it to the density field processing system;

[0024] S500, the density field processing system processes the received image electronic signal, processes the image electronic signal into flow field information and outputs it for display.

[0025] Furthermore, the method for adjusting the parameters and spacing of each component in the system in S100 is specifically as follows:

[0026] S101. Adjust parameters and spacing according to the system detection sensitivity required by the flow field to be measured:

[0027] ;

[0028] Among them, ε min is the sensitivity; a is the size of the uncut light source image at the knife-edge grid position; L is the distance from the source grid to the focusing lens; l is the distance from the flow field to be measured to the focusing lens; L' is the distance from the focusing lens to the knife-edge grid;

[0029] S102, according to the system resolution required by the flow field to be measured Adjust parameters and spacing:

[0030] ;

[0031] Where, l' is the distance from the focusing lens to the imaging screen; λ is the wavelength of the light source; b is the width of the bright fringe of the knife-edge grating;

[0032] S103, according to the sharp focus depth D of the flow field to be measured S , Depth of out-of-focus D U Adjustment parameters:

[0033] ;

[0034] Where A is the effective aperture of the focusing lens;

[0035] S104. Obtain the following parameters according to the parameters adjusted in S101-S103:

[0036] ; Where n is the number of bright fringes per millimeter of the knife-edge grating;

[0037] ;in, is the number of bright fringes when the light source passes through the knife-edge grating;

[0038] According to the imaging relationship satisfied by the focusing lens imaging: , where f is the focal length of the focusing lens.

[0039] Furthermore, the method for the density field processing system in S500 to process the received image electronic signal specifically comprises the following steps:

[0040] S501, the density field processing system judges the state of the schlieren images obtained through different sections, marks them as dynamic images and static images respectively, selects the schlieren images and background images of multiple sections in the dynamic images and static images respectively, and performs image preprocessing;

[0041] The image preprocessing includes determining the position consistency of the static image and the dynamic image, adjusting the consistency of the brightness in the background, and adjusting the information enhancement;

[0042] S502, the density field processing system performs image parameter calibration, image grayscale extraction and grayscale change calculation on the pre-processed dynamic image and static image respectively, and performs superposition calculation of flow field density gradient in combination with the parameters input by the system;

[0043] S503, the density field processing system calculates the flow field density and the variation according to the calculated flow field density gradient, and filters the data;

[0044] S504: The density field processing system inverts the three-dimensional information of the flow field based on the flow field density variation data obtained through S501-S503, and restores the three-dimensional graph of the flow field.

[0045] Furthermore, the method for calculating the flow field density and its variation in S503 is to use the least square method to establish a mathematical and physical model of focused schlieren imaging, input the focused schlieren image information to obtain the flow field density gradient and density value, specifically including:

[0046] Input different Schlieren imaging system parameters into the density field processing system, obtain different Schlieren imaging system expansion function values, and establish the expansion function of the Schlieren system ; Obtain the focus depth information of the schlieren imaging information, and divide the sharp focus depth area and the non-sharp focus depth area according to the interest of the information; According to the expansion function Get sharp focus depth D S Area information and non-sharp focus depth D U Area information, at sharp focus depth D S Based on the area information, remove the non-sharp focus depth D U The information of the area is used to obtain the flow field information value with improved accuracy;

[0047] Assume that the knife-edge grid is placed horizontally and the direction of the knife-edge grid is the x direction, then the direction perpendicular to the knife-edge grid is the y direction, where the relationship between the grayscale and density gradient of the image is:

[0048] ;

[0049] Wherein, ΔI is the grayscale change value of the focused schlieren image before and after the flow field is established, I0 is the grayscale value of the focused schlieren image obtained before the test, K is the Glaston-Dyer constant, L is the distance from the source grid to the focusing lens, l is the distance between the test flow field and the focusing lens, a is the size of the uncut light source image at the knife-edge grid position, and f is the focal length of the focusing lens; z1 and z2 are the positions where the light beam enters and leaves the test flow field along the optical axis, z is a value between z1 and z2, and ρ is the density value;

[0050] Use the ladder method to simulate the expansion function , we get the discontinuous step simulation expansion function, the formula is:

[0051] ;

[0052] Where N is the number of steps, β j is the amplitude of the stepped expansion function, d 聚焦 The sharp focus depth D S The focal depth value at the area, I 聚焦 is the gray value of the focused schlieren image at the focus point, j is the corresponding flow field area, d j is the focal depth value at area j, I j is the gray value of the focused schlieren image at area j;

[0053] By inputting the focused schlieren image information into the discontinuous step simulation expansion function, the flow field density gradient and density value can be obtained.

[0054] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0055] Compared with the existing focused schlieren imaging technology which only stays at the two-dimensional qualitative measurement stage and cannot obtain more accurate results through three-dimensional quantitative measurement technology, the flow field display device based on multi-section focused schlieren imaging of the present invention can realize the transient capture of multiple flow field cross-section information by focusing on the test area, splitting the light beam, and simultaneously cutting sections from different positions, reflecting the physical and chemical change process characteristics of the flow field, and three-dimensionally quantitatively inverting the three-dimensional air fluid changes of the flow field. The device greatly increases the possibility of three-dimensional inversion of flow field information, and the flow field density gradient and density results obtained according to the focused schlieren system image are more accurate.

[0056] The flow field display method based on multi-section focused schlieren imaging of the present invention inputs the information of the flow field to be measured into the multi-section focused schlieren imaging system, adjusts the system parameters and element spacing according to the flow field information to be measured, and intercepts the light beams at multiple positions in different directions respectively and then performs corresponding processing. While achieving the flow field information by simultaneously acquiring multiple section information for processing and calculation, the interference of image information with low accuracy is reduced through the processing of the density field processing system, so that the target image is distinguished from the background image, the information to be measured is enhanced after processing, and the displayed result is clearer and more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a schematic diagram of the structure and light path distribution of the flow field display device of the present invention;

[0058] Figure 2 A schematic diagram of the spacing between components in the focused schlieren imaging of the present invention;

[0059] Figure 3 It is a flow chart of the flow field display method of the present invention;

[0060] Figure 4 It is a flowchart of one processing embodiment of the density field processing system of the present invention.

[0061] In the attached figure:

[0062] L—is the distance from the source grid to the focusing lens;

[0063] l—is the distance from the test flow field to the focusing lens;

[0064] L' - the distance from the focusing lens to the knife-edge grid;

[0065] l'—the distance from the focusing lens to the imaging screen. DETAILED DESCRIPTION

[0066] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0067] The present invention is a flow field display device and method based on multi-section focused schlieren imaging, which is mainly used in aerodynamics, explosion and impact, combustion and chemical reaction and other fields to realize flow field visualization technology. For example, it can be used to display the density field in wind tunnel tests, observe the boundary layer of airflow, combustion, shock waves, and hot and cold convection in the gas, and convert the fluid density changes that are invisible to the human eye into light intensity information and record it, so that the observation of the flow field is more direct and vivid. Compared with traditional schlieren technology, multi-section focused schlieren imaging can distinguish the area of ​​interest from other areas of the flow field, so that it has the characteristics of narrow depth of field, good spatial local imaging capability, and longitudinal resolution capability, which greatly suppresses the interference of non-measurement areas and has high imaging quality. The technical solution of the present invention is specifically described below through some embodiments:

[0068] Example 1

[0069] In order to obtain information on at least two sections of the flow field in one test, and to realize quantitative measurement and three-dimensional display of the flow field through multi-section focused schlieren imaging, the imaging screen in the display device needs to be located at different positions at the same time, so that it can focus on different test areas and cut sections separately. Therefore, in this embodiment, the flow field display device places a beam splitter in front of the imaging screen, and then places an imaging screen on each of the different optical paths after beam splitting, so as to obtain different sections on multiple optical paths at the same time. Since the knife-edge grid is usually close to the imaging plane, it is difficult to place a large-sized beam splitter with a 45° inclination angle. Therefore, a beam splitter can be placed between the knife-edge grid and the focusing lens, and a knife-edge grid can be additionally added to the optical path of the beam splitting. The knife-edge grids of the two sections are preferably adjusted to the same state to more clearly describe the flow field information of different sections. Therefore, the setting of the device requires advance preparation of the design of the knife-edge grid and the debugging of the overall performance to ensure that the optical path meets the measurement conditions.

[0070] Based on the above optical path settings, such as Figure 1 As shown, it is one of the optional configuration structures of the device of the present invention. The flow field display device includes an imaging device, an imaging recording device, a density field processing system and an optical system bracket. Figure 1The diagram shows an imaging device and an imaging recording device, and the optical system bracket is mainly used to provide support for the imaging device and the imaging recording device; preferably, a vibration isolation platform can also be provided at the bottom of the optical system bracket to buffer and isolate the vibration of the external environment, thereby avoiding the influence of vibration on the measurement process.

[0071] In order to obtain the planar images at two cross sections at the same time, the imaging recording device is arranged at positions at least in two directions beside the imaging device, such as Figure 1 As shown, the beam splitter splits the light beam into two beams with directions perpendicular to each other, the density field processing system is connected to the imaging recording device through signal transmission, and the imaging device and the imaging recording device are respectively arranged on the optical system bracket; wherein the imaging device is used to generate a light path for plane interception by the focused schlieren imaging method through the light emitted by the light source; the imaging recording device is used to receive at least two images intercepted in the light path formed by the imaging device, for example Figure 1 The two locations of the knife-edge grid shown in the figure are used to transmit the image information to the density field processing system; the density field processing system is used to process the image information into flow field density distribution information and output a display image.

[0072] Based on the above-mentioned settings of various components of the flow field display device, the device passes through the test area with a conical light path and can focus on the test area. The flow field information obtained is mainly a specified focusing area of ​​the flow field, while the flow field image in the conventional schlieren reflects the integral value of the light beam along the entire flow field area. That is, the information obtained by the display device of the present invention can be used to obtain more accurate flow field parameters through image processing, such as the density gradient and density value of the flow field.

[0073] The flow field display device of this embodiment can use a wide light source. Compared with the prior art using point light and shadow, it can avoid unnecessary diffraction and interference noise in the image caused by monochromatic light, and prevent the generation of diffraction rings (caused by unclean surfaces in the light path, etc.) and interference fringes.

[0074] Example 2

[0075] On the basis of Example 1, in order to further refine the composition of each optical path element, improve the operability of the device measurement, and ensure that the optical element setting of the device can fully meet the flow field measurement display conditions of multi-section focused schlieren imaging, such as Figure 1As shown, the imaging device in this embodiment includes a pulse light source, a Fresnel lens, a source grid, a focusing lens and a beam splitter arranged in sequence along the same straight line direction, and a group of knife-edge grids and field lenses are arranged in sequence on at least two sides of the beam splitter; wherein the Fresnel lens is used to control and focus the light emitted by the pulse light source; the source grid is used to shape the light beam passing through the Fresnel lens; compared with the prior art that requires the use of large-sized optical elements related to the field of view, in this embodiment, smaller-sized optical elements such as Fresnel lenses and source grids are used to replace large-sized optical elements, which greatly reduces the volume of the imaging device part and further facilitates various operational adjustments of device parameters.

[0076] In this embodiment, the focusing lens is used to focus the light beam passing through the source grid and the flow field; the beam splitter is used to split the light beam passing through the focusing lens into at least two directions; the knife-edge grid is used to cut the light source image on the light path; and the field lens is used to reduce the image so that the image enters the imaging target surface of the imaging recording device respectively.

[0077] For imaging using a field lens, the focused schlieren method includes four imaging methods: (1) placing an imaging screen at the image plane position, and the image recording device captures the schlieren image on the imaging screen; (2) using a field lens processed by glass to reduce the image at the image plane position, and directly entering the imaging target surface of the image recording device; (3) using a field lens processed by a Fresnel lens combination to reduce the image at the image plane position, and directly entering the imaging target surface of the image recording device; (4) placing a large-format film at the image plane position, and directly sensitizing the schlieren image. Considering the high resolution and high contrast of light and dark grayscale of the imaging quality, in terms of imaging effect, film imaging has the best effect, followed by field lens imaging, and then imaging screen and combined Fresnel lens; because film is more complicated in the subsequent means of restoring flow field information, in this embodiment, a focused schlieren imaging device with a receiving method of field lens imaging is selected as the imaging means of the present invention.

[0078] Preferably, a beam expander and a light softener are provided between the pulse light source and the Fresnel lens; wherein the beam expander is used to collimate the light beam emitted by the pulse light source; the expanded laser beam can cover a larger measurement range, thereby improving the accuracy and efficiency of ranging. In particular, in optical imaging systems, the beam expander helps to improve aberrations and make imaging clearer. By reasonable beam expansion, the propagation characteristics of the light beam can be adjusted to meet the requirements of different optical elements. In complex optical systems, different optical elements have specific requirements for the size and divergence angle of the light beam, and the beam expander can adjust the light beam emitted by the light source to a suitable size and divergence state, so as to work better with other optical elements (such as lenses, reflectors, etc.). The light softener is used to adjust the uniformity of the light beam distribution. The light softener can scatter light, change the propagation direction of light, and make the light more softly distributed in a larger area.

[0079] Example 3

[0080] On the basis of Example 1, the imaging recording device includes an image receiving device and a remote control transmission system, and the remote control transmission system is electrically connected to the image receiving device; wherein the image receiving device is used to receive image information at at least two sections in the optical path of the imaging device, such as Figure 1 As shown, the image receiving device receives different image information of the two directional light beams after beam splitting; the remote control transmission system is used to transmit the image information obtained by the image receiving device to the density field processing system through remote control.

[0081] As a preferred embodiment, the imaging and recording device also includes a filtering and attenuation device, which is arranged between the image receiving device and the remote control transmission system, or between the image receiving device and the imaging device; wherein the filtering and attenuation device is used to filter and attenuate the light beam to remove stray light.

[0082] The setting of filtering and attenuation devices can protect the image receiving system from overload. Image receiving systems (such as photodetectors, CCD or CMOS sensors, etc.) usually have a range of light intensity that they can withstand. If the received light intensity is too large, it may exceed its linear working range, causing signal saturation, distortion, or even damage to the detector components. The filtering and attenuation device can reduce the light intensity entering the image receiving system to ensure that it operates within a safe working range.

[0083] The setting of filtering and attenuation devices can also reduce thermal effects. Too high light intensity may cause the image receiving system to generate too much heat. For temperature-sensitive equipment, too much heat will affect its performance, such as reducing sensitivity and increasing noise. Reducing the light intensity through filtering and attenuation devices can reduce the impact of thermal effects.

[0084] In addition, the filtering and attenuation device can optimize the image quality and adjust the contrast. In some cases, the light intensity distribution of the original image may result in poor contrast. The filtering and attenuation device can selectively attenuate light of a specific wavelength or intensity range, thereby enhancing the contrast of the area of ​​interest in the image. For example, by attenuating the background light intensity, the contrast between the image to be displayed and the unnecessary image can be made more obvious, making it easier to observe. Since there may be various stray lights in the optical path, these stray lights will interfere with the clarity and accuracy of the image. The filtering and attenuation device can use its spectral selectivity to attenuate the unwanted stray light wavelengths and only allow the target wavelength of light to pass, thereby improving the signal-to-noise ratio of the image.

[0085] Example 4

[0086] On the basis of Example 1, in order to provide stable support for the entire device, ensure the stability of the optical path setting, and reduce the measurement result error caused by the mechanical part, in this embodiment, the optical system bracket includes a beam expansion bracket and an imaging bracket, and the optical path exit end of the beam expansion bracket is arranged opposite to the optical path entrance end of the imaging bracket; the bottom of the beam expansion bracket and the imaging bracket are respectively connected to the beam expansion vibration isolation platform and the imaging vibration isolation platform; wherein, the beam expansion bracket and the imaging bracket are used to be respectively arranged on both sides of the flow field and respectively support the optical system elements arranged on both sides of the flow field.

[0087] by Figure 1 Taking the device structure shown as an example, the beam expansion bracket and the beam expansion vibration isolation platform are used to support the beam expansion device, the soft light device, the Fresnel lens and the source grid respectively; while the imaging bracket and the imaging vibration isolation platform are used to support the focusing lens, the beam splitter, two sets of knife-edge grids and two sets of image receiving screens or imaging field lens groups respectively. The two parts of the bracket and the vibration isolation platform can be separated, and the relative spacing and direction can be adjusted to each other, and the relative spacing between the beam expansion bracket and the multiple bracket components in the imaging bracket should also be adjustable to facilitate the adjustment of the optical path parameters. For example, a slide rail for the bracket to slide is set on each vibration isolation platform to move the bracket and the optical element thereon.

[0088] Example 5

[0089] This embodiment is a flow field display method based on multi-section focused schlieren imaging. Figure 3 As shown, the following steps are included:

[0090] S100, input the information of the flow field to be measured into the multi-section focusing schlieren imaging system, the system adjusts the system parameters according to the flow field information to be measured, and adjusts the parameters and spacing of each component in the system (the spacing of the relevant components is set as follows Figure 2 As shown in FIG. 1 ), setting each element of the flow field display device based on the adjusted parameters and spacing information;

[0091] S200, the system controls the light source of the imaging device of the flow field display device to emit a light beam, and after focusing the light beam, the light beam is made to pass through the flow field area to be measured in the form of a conical light path;

[0092] S300, the imaging device focuses the light beam passing through the flow field area to be measured and then splits the light beam into light beams in at least two directions, and selects positions to intercept the light beam in each direction respectively;

[0093] S400, the imaging recording device images the light beam from the imaging device, collects the intercepted image information, converts it into an image electronic signal and transmits it to the density field processing system;

[0094] S500, the density field processing system processes the received image electronic signal, processes the image electronic signal into flow field information and outputs it for display.

[0095] Preferably, the method for adjusting the parameters and spacing of each component in the system in S100 is specifically:

[0096] S101. Adjust the parameters and spacing according to the system detection sensitivity required by the flow field to be measured:

[0097] ;

[0098] Among them, ε min is the sensitivity; a is the size of the uncut light source image at the knife-edge grid position; L is the distance from the source grid to the focusing lens; l is the distance from the flow field to be measured to the focusing lens; L' is the distance from the focusing lens to the knife-edge grid;

[0099] S102, according to the system resolution required by the flow field to be measured Adjust parameters and spacing:

[0100] ;

[0101] Where, l' is the distance from the focusing lens to the imaging screen; λ is the wavelength of the light source; b is the width of the bright fringe of the knife-edge grating;

[0102] S103, according to the sharp focus depth D of the flow field to be measured S , Depth of out-of-focus D U Adjustment parameters:

[0103] ;

[0104] Where A is the effective aperture of the focusing lens;

[0105] S104. Obtain the following parameters according to the parameters adjusted in S101-S103:

[0106] ; Where n is the number of bright fringes per millimeter of the knife-edge grating;

[0107] ;in, is the number of bright fringes when the light source passes through the knife-edge grating;

[0108] According to the imaging relationship satisfied by the focusing lens imaging: , where f is the focal length of the focusing lens.

[0109] Please refer to Figure 4 The method in S500 for the density field processing system to process the received image electronic signal specifically comprises the following steps:

[0110] S501, the density field processing system judges the state of the schlieren images obtained through different sections, marks them as dynamic images and static images respectively, selects the schlieren images and background images of multiple sections in the dynamic images and static images respectively, and performs image preprocessing;

[0111] Among them, image preprocessing includes determining the position consistency of static images and dynamic images, adjusting the consistency of brightness in the background, and adjusting information enhancement;

[0112] S502, the density field processing system performs image parameter calibration, image grayscale extraction and grayscale change calculation on the pre-processed dynamic image and static image respectively, and performs superposition calculation of flow field density gradient in combination with the parameters input by the system;

[0113] S503, the density field processing system calculates the flow field density and the variation according to the calculated flow field density gradient, and filters the data;

[0114] S504: The density field processing system inverts the three-dimensional information of the flow field based on the flow field density variation data obtained through S501-S503, and restores the three-dimensional graph of the flow field.

[0115] The flow field display method based on multi-section focused schlieren imaging of the present invention inputs the information of the flow field to be measured into the multi-section focused schlieren imaging system, adjusts the system parameters and element spacing according to the flow field information to be measured, and intercepts the light beams at multiple positions in different directions respectively and then performs corresponding processing. While achieving the flow field information by simultaneously acquiring multiple section information for processing and calculation, the interference of image information with low accuracy is reduced through the processing of the density field processing system, so that the target image is distinguished from the background image, the information to be measured is enhanced after processing, and the displayed result is clearer and more accurate.

[0116] Based on the implementation scheme of the above-mentioned Example 5, there are two specific technical means for inverting flow field information:

[0117] The multi-section flow field density distribution values ​​are superimposed and calculated (least square method) to obtain a relatively complete flow field density value and the density change trend in the density value;

[0118] The multi-section flow field density distribution values ​​are spatially coupled and calculated (holographic measurement method) to achieve simultaneous recording of phase and amplitude information in one imaging, with the function of subsequent reproduction.

[0119] The two methods above extract different flow field information from the flow field, but both achieve three-dimensional quantitative measurement of flow field information. The two methods are described separately through examples below:

[0120] Example 6

[0121] On the basis of Example 5, the method for calculating the flow field density and its variation in S503 is to use the least squares method, obtain the schlieren images from different sections, perform density gradient superposition calculation through flow field density processing software, establish a mathematical and physical model of focused schlieren imaging, input the focused schlieren image information to obtain the flow field density gradient and density value, specifically including:

[0122] Input different Schlieren imaging system parameters into the density field processing system, obtain different Schlieren imaging system expansion function values, and establish the expansion function of the Schlieren system ; Obtain the focus depth information of the schlieren imaging information, and divide the sharp focus depth area and the non-sharp focus depth area according to the interest of the information; According to the expansion function Get sharp focus depth D S Area information and non-sharp focus depth D U Area information, at sharp focus depth D S Based on the area information, remove the non-sharp focus depth D U The information of the area is used to obtain the flow field information value with improved accuracy; since the information in the focused schlieren system mainly reflects a small area with a sharp focusing depth Ds, but the non-sharp focusing depth D U The area will also make a certain contribution to the flow field information. If the non-sharp focusing depth D can be subtracted from the sharp focusing depth Ds information, U If the information is obtained, the flow field information value obtained will be more accurate.

[0123] Assume that the knife-edge grid is placed horizontally and the direction of the knife-edge grid is the x direction, then the direction perpendicular to the knife-edge grid is the y direction, where the relationship between the grayscale and density gradient of the image is:

[0124] ;

[0125] Wherein, ΔI is the grayscale change value of the focused schlieren image before and after the flow field is established, I0 is the grayscale value of the focused schlieren image obtained before the test, K is the Glaston-Dyer constant, L is the distance from the source grid to the focusing lens, l is the distance between the test flow field and the focusing lens, a is the size of the uncut light source image at the knife-edge grid position, and f is the focal length of the focusing lens; z1 and z2 are the positions where the light beam enters and leaves the test flow field along the optical axis, z is a value between z1 and z2, and ρ is the density value;

[0126] Use the ladder method to simulate the expansion function , we get the discontinuous step simulation expansion function, the formula is:

[0127] ;

[0128] Where N is the number of steps, β jis the amplitude of the stepped expansion function, d 聚焦 The sharp focus depth D S The focal depth value at the area, I 聚焦 is the gray value of the focused schlieren image at the focus point, j is the corresponding flow field area, d j is the focal depth value at area j, I j is the gray value of the focused schlieren image at area j;

[0129] By inputting the focused schlieren image information into the discontinuous step simulation expansion function, the density gradient and density value of the flow field are obtained. The three-dimensional information of the flow field can be inverted by calculating the change in the flow field density through multiple schlieren images, and the three-dimensional morphology of the flow field can be restored.

[0130] According to the implementation scheme of this embodiment, the software of the density field processing system can be developed and designed, and the software can be set to be able to obtain the flow field density gradient and density value from the focused schlieren flow field images of 1 to 8 sections.

[0131] Example 7

[0132] On the basis of Example 5, as an optional implementation scheme, the method for obtaining the flow field density and its change in S503 is to use a holographic measurement method and a holographic focused schlieren technology. On the basis of the focused schlieren technology, the focused schlieren image is encrypted with the help of coherent reference light, and the amplitude and phase information of the spatial light field is obtained in the reconstruction calculation, and then the flow field information at different depth cross-sectional positions in three-dimensional space is inverted. This method overcomes the limitation of single-sheet recording of traditional focused schlieren technology.

[0133] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A flow field display device based on multi-section focused schlieren imaging, characterized in that: It comprises an imaging device, an imaging recording device, a density field processing system and an optical system bracket; the imaging recording device is arranged at positions at least in two directions beside the imaging device, the density field processing system is connected with the imaging recording device through signal transmission, and the imaging device and the imaging recording device are respectively arranged on the optical system bracket; Among them, the imaging device is used to generate an optical path for plane interception by focused schlieren imaging method through the light emitted by the light source; the imaging recording device is used to receive at least two images intercepted in the optical path formed by the imaging device, and transmit the image information to the density field processing system; the density field processing system is used to process the image information into flow field density distribution information and output a display image.

2. The flow field display device based on multi-section focused schlieren imaging according to claim 1, characterized in that: The imaging device comprises a pulse light source, a Fresnel lens, a source grid, a focusing lens and a beam splitter which are sequentially arranged along the same straight line direction, and a group of knife-edge grids and field lenses are sequentially arranged on at least two sides of the beam splitter; Among them, the Fresnel lens is used to control the focusing of the light emitted by the pulse light source; the source grid is used to shape the light beam passing through the Fresnel lens; the focusing lens is used to focus the light beam passing through the source grid and the flow field; the beam splitter is used to split the light beam passing through the focusing lens into at least two directions; the knife-edge grid is used to cut the light source image on the optical path; and the field lens is used to reduce the image and allow the image to enter the imaging target surface of the imaging recording device respectively.

3. The flow field display device based on multi-section focused schlieren imaging according to claim 2, characterized in that: A beam expander and a light softener are also provided between the pulse light source and the Fresnel lens; Wherein, the beam expansion device is used to collimate the light beam emitted by the pulse light source; and the light softening device is used to adjust the distribution uniformity of the light beam.

4. The flow field display device based on multi-section focused schlieren imaging according to claim 1, characterized in that: The imaging and recording device comprises an image receiving device and a remote control transmission system, wherein the remote control transmission system is electrically connected to the image receiving device; The image receiving device is used to receive image information at at least two sections in the optical path of the imaging device; and the remote control transmission system is used to transmit the image information acquired by the image receiving device to the density field processing system through remote control.

5. The flow field display device based on multi-section focused schlieren imaging according to claim 4, characterized in that: The imaging recording device further comprises a light filtering and attenuation device, wherein the light filtering and attenuation device is arranged between the image receiving device and the remote control transmission system, or between the image receiving device and the imaging device; The filtering and attenuating device is used to filter and attenuate the light beam to remove stray light.

6. The flow field display device based on multi-section focused schlieren imaging according to claim 1, characterized in that: The optical system bracket comprises a beam expansion bracket and an imaging bracket, wherein the optical path exit end of the beam expansion bracket is arranged opposite to the optical path entrance end of the imaging bracket; the bottoms of the beam expansion bracket and the imaging bracket are respectively connected to a beam expansion vibration isolation platform and an imaging vibration isolation platform; The beam expansion bracket and the imaging bracket are used to be respectively arranged on both sides of the flow field and respectively support the optical system components arranged on both sides of the flow field.

7. A flow field display method based on multi-section focused schlieren imaging, characterized in that: The steps include: S100, inputting the information of the flow field to be measured into the multi-section focusing schlieren imaging system, the system adjusting the system parameters according to the flow field information to be measured, and adjusting the parameters and spacing of each component in the system, and setting each component of the flow field display device based on the adjusted parameters and spacing information; S200, the system controls the light source of the imaging device of the flow field display device to emit a light beam, and after focusing the light beam, the light beam is made to pass through the flow field area to be measured in the form of a conical light path; S300, the imaging device focuses the light beam passing through the flow field area to be measured and then splits the light beam into light beams in at least two directions, and selects positions to intercept the light beam in each direction respectively; S400, the imaging recording device images the light beam from the imaging device, collects the intercepted image information, converts it into an image electronic signal and transmits it to the density field processing system; S500, the density field processing system processes the received image electronic signal, processes the image electronic signal into flow field information and outputs it for display.

8. The flow field display method based on multi-section focused schlieren imaging according to claim 7, characterized in that: The method for adjusting the parameters and spacing of each component in the system in S100 is specifically as follows: S101. Adjust the parameters and spacing according to the system detection sensitivity required by the flow field to be measured: ； Among them, ε min is the sensitivity; a is the size of the light source image not cut at the knife-edge grid position; L is the distance from the source grid to the focusing lens; l is the distance from the flow field to be measured to the focusing lens; L' is the distance from the focusing lens to the knife-edge grid; S102, according to the system resolution required by the flow field to be measured Adjust parameters and spacing: ； Where, l' is the distance from the focusing lens to the imaging screen; λ is the wavelength of the light source; b is the width of the bright fringe of the knife-edge grating; S103, according to the sharp focus depth D of the flow field to be measured S , Depth of out-of-focus D U Adjustment parameters: ； Where A is the effective aperture of the focusing lens; S104. Obtain the following parameters according to the parameters adjusted in S101-S103: ; Where n is the number of bright fringes per millimeter of the knife-edge grating; ;in, is the number of bright fringes when the light source passes through the knife-edge grating; According to the imaging relationship satisfied by the focusing lens imaging: , where f is the focal length of the focusing lens.

9. The flow field display method based on multi-section focused schlieren imaging according to claim 7, characterized in that: The method in S500 for the density field processing system to process the received image electronic signal specifically comprises the following steps: S501, the density field processing system judges the state of the schlieren images obtained through different sections, marks them as dynamic images and static images respectively, selects the schlieren images and background images of multiple sections in the dynamic images and static images respectively, and performs image preprocessing; The image preprocessing includes determining the position consistency of the static image and the dynamic image, adjusting the consistency of the brightness in the background, and adjusting the information enhancement; S502, the density field processing system performs image parameter calibration, image grayscale extraction and grayscale change calculation on the pre-processed dynamic image and static image respectively, and performs superposition calculation of flow field density gradient in combination with the parameters input by the system; S503, the density field processing system calculates the flow field density and the variation according to the calculated flow field density gradient, and filters the data; S504: The density field processing system inverts the three-dimensional information of the flow field based on the flow field density variation data obtained through S501-S503, and restores the three-dimensional graph of the flow field.

10. The flow field display method based on multi-section focused schlieren imaging according to claim 9, characterized in that: The method for calculating the flow field density and its variation in S503 is to use the least square method to establish a mathematical and physical model of focused schlieren imaging, input the focused schlieren image information to obtain the flow field density gradient and density value, specifically including: Input different Schlieren imaging system parameters into the density field processing system, obtain different Schlieren imaging system expansion function values, and establish the expansion function of the Schlieren system ; Obtain the focus depth information of the schlieren imaging information, and divide the sharp focus depth area and the non-sharp focus depth area according to the interest of the information; According to the expansion function Get a sharp focus depth D S Area information and non-sharp focus depth D U Area information, at sharp focus depth D S Based on the area information, remove the non-sharp focus depth D U The information of the area is obtained to obtain the flow field information value with improved accuracy; assuming that the knife-edge grid is placed horizontally and the direction of the knife-edge grid is the x direction, then the direction perpendicular to the knife-edge grid is the y direction, where the relationship between the grayscale and density gradient of the image is: ； Wherein, ΔI is the grayscale change value of the focused schlieren image before and after the flow field is established, I0 is the grayscale value of the focused schlieren image obtained before the test, K is the Glaston-Dyer constant, L is the distance from the source grid to the focusing lens, l is the distance between the test flow field and the focusing lens, a is the size of the uncut light source image at the knife-edge grid position, and f is the focal length of the focusing lens; z1 and z2 are the positions where the light beam enters and leaves the test flow field along the optical axis, z is a value between z1 and z2, and ρ is the density value; Use the ladder method to simulate the expansion function , we get the discontinuous step simulation expansion function, the formula is: ； Where N is the number of steps, β j is the amplitude of the stepped expansion function, d 聚焦 The sharp focus depth D S The focal depth value at the area, I 聚焦 is the gray value of the focused schlieren image at the focus point, j is the corresponding flow field area, d j is the focal depth value at area j, I j is the gray value of the focused schlieren image at area j; By inputting the focused schlieren image information into the discontinuous step simulation expansion function, the flow field density gradient and density value can be obtained.

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