Method and System for Measuring Density Field of Density-Stratified Liquid Based on Background-Oriented Schlieren

Abstract

The present application relates to a method and system for measuring the density field of density-stratified liquids based on background-oriented schlieren. The method includes: performing cross-correlation on a first reference image under a uniform clear water flow field and a second reference image under an air field to determine a calibration field; remapping a background pattern image with the calibration field to obtain a remapped background pattern image; performing cross-correlation on the remapped background pattern image and the second reference image to obtain a displacement field; applying an inverse Radon transform to the displacement field to obtain a reconstructed displacement field; determining a refractive index distribution based on the reconstructed displacement field; and determining a density field based on the refractive index distribution. The present application uses the background schlieren technique in natural thermal convection (air) to study that natural ventilation is easily disturbed by the surrounding environmental airflows. It extends the existing background schlieren technique in gas flow fields to liquid flow fields, combines the background schlieren technique with saline experiments, reduces experimental errors, and ensures accuracy.

Description

Technical Field

[0001] The present application relates to the technical field of density field measurement. Specifically, it relates to a method and system for measuring the density field of density-stratified liquids based on background-oriented schlieren. Background Art

[0002] In the research of indoor thermal and humid environment, ventilation, fire smoke and other fields, using salt water experiments to simulate air movement under real conditions is one of the important experimental means. Tracer dyes are added to observe the flow state, and then the movement and development of air in actual buildings are analyzed according to the similarity theory. The commonly used measurement methods for salt water models are divided into contact type and non-contact type. Contact measurement generally uses thermocouples. Contact measurement will interfere with the flow of the flow field, and the sensors downstream of the flow field will be affected by the sensors upstream. Therefore, non-contact measurement is more preferred in experiments. Currently, the commonly used non-contact measurement method is to measure the density distribution of the flow field by reading the gray level of the collected images. This method has problems such as the diffusion of tracer dyes affecting the accuracy of experimental data, being unable to read the velocity field, and the movement details of the internal plume in the stratified layer often being unable to be directly observed by the human eye.

[0003] Schlieren imaging, as one of the important non-contact measurement techniques, the ability to observe changes in air density enables researchers to photograph phenomena in compressible flows, thermal convection, and chemical mixing processes. Background-oriented schlieren technology (BOS) belongs to the synthetic schlieren method and is a new technology for visualizing fluid density gradients using the Gladstone-Dale relationship between gas density and refractive index. BOS uses a light source to project a texture background (usually a random dot pattern) on one side of the test chamber onto the camera sensor on the other side. The first image (referred to as the reference image) is recorded through a stagnant fluid with uniform density. Utilizing the change in the refractive index of light in the flow field region causes light deflection, resulting in distortion of the corresponding pixels in the background schlieren pattern. The displacement is quantified using optical flow or particle image velocimetry (PIV) methods, and a virtual displacement field proportional to the refractive index derivative is provided. Currently, most BOS research focuses on directly placing a heating heat source in a uniform gas (air) medium, while there is a lack of actual measurement and quantitative analysis for liquid flow fields. Summary of the Invention

[0004] To overcome at least one deficiency in the prior art, the present application provides a method and system for measuring the density field of density-stratified liquids based on background-oriented schlieren.

[0005] In a first aspect, a method for measuring the density field of density-stratified liquids based on background-oriented schlieren is provided, including:[

[0006] Obtaining a first reference image of the background pattern under a uniform clear water flow field;

[0007] Under an air field, obtain a second reference image of the background pattern;

[0008] Under a density gradient flow field, obtain an image of the background pattern;

[0009] Perform cross-correlation on the first reference image and the second reference image to determine a calibration field;

[0010] Remap the background pattern image with the calibration field to obtain a remapped background pattern image;

[0011] Perform cross-correlation on the remapped background pattern image and the second reference image to obtain a displacement field;

[0012] Perform an inverse Radon transform on the displacement field to obtain a reconstructed displacement field;

[0013] Determine the refractive index distribution according to the reconstructed displacement field;

[0014] Determine the density field according to the refractive index distribution.

[0015] In one embodiment, determining the refractive index distribution according to the reconstructed displacement field includes:

[0016]

[0017] Wherein, Δn is the refractive index of the displacement point (Δx', Δy') in the reconstructed displacement field, and K is the reciprocal of the influence coefficient;

[0018] The refractive indices of all displacement points in the reconstructed displacement field constitute the refractive index distribution

[0019] In one embodiment, determining the density field according to the refractive index distribution includes:

[0020]

[0021] Wherein, is the density field, is the refractive index distribution, ρ0 is the density of clear water, n0 is the refractive index of clear water, and β is the rate of change of the refractive index with respect to the density.

[0022] In one embodiment, the method further includes:

[0023] Determine the fluid velocity according to the density field.

[0024] In a second aspect, there is provided a density stratified liquid density field measurement system based on background-oriented schlieren, including: an LED light source, a background pattern board, a test liquid system, an image acquisition device, and a computer;

[0025] The LED light source is used to uniformly illuminate the background pattern board;

[0026] The light emitted by the LED light source passes through the test liquid system through the background pattern board and is incident on the image acquisition device;

[0027] When the test liquid system is filled with density-stratified liquid, a density gradient flow field is formed; when filled with clear water, a uniform clear water flow field is formed; when filled with air, an air field is formed;

[0028] The image acquisition device is used to acquire a first reference image of the background pattern in the background pattern board under a uniform clear water flow field, a second reference image of the background pattern in the background pattern board under an air field, and a background pattern image in the background pattern board under a density gradient flow field;

[0029] The computer is used to implement the above-mentioned method for measuring the density field of density-stratified liquid based on background-oriented schlieren.

[0030] In one embodiment, the LED light source and the image acquisition device are respectively arranged on both sides of the test liquid system, and the center lines are kept consistent; the distance between the background pattern board and the test liquid system is less than the distance between the test liquid system and the image acquisition device.

[0031] In one embodiment, the image acquisition device is a high-speed industrial camera, and the distance Z d between the background pattern board and the test liquid system is 400 mm - 700 mm, and the distance Z b between the background pattern board and the high-speed industrial camera is 1200 - 1500 mm, and the focal length f of the high-speed industrial camera is 40 - 60 mm.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] (1) In the present application, the background schlieren technology combines the theory of PIV technology, which not only saves the cost of experimental equipment and the site, but also avoids the limitation of the lens size on the range of the flow field to be measured, and has a wide measurement range.

[0034] (2) In the present application, the background schlieren technology is used to study that natural ventilation is easily disturbed by the surrounding environmental airflow in natural thermal convection (air), and the existing background schlieren technology in the gas flow field is extended to the liquid flow field, and the background schlieren technology is combined with the salt water experiment to reduce experimental errors and ensure accuracy.

[0035] (3) The method of the present application avoids putting colored dyes into the liquid field in the brine experiment, so as not to interfere with the state of the flow field to be measured. The optical reaction is rapid and real-time detection of the flow field can be achieved; this method has high measurement accuracy and can obtain quantitative measurement data of the flow field.

[0036] (4) The present application determines the offset of the spots of the background pattern photographed by the industrial camera, so as to quantitatively obtain the density distribution in the liquid field. The structure is simple, the operation is convenient, and the accuracy is high. Description of the Drawings

[0037] The present application can be better understood by referring to the description given below in conjunction with the drawings. The drawings, together with the following detailed description, are included in this specification and form a part of this specification. In the drawings:

[0038] Figure 1 shows a schematic structural diagram of a density-stratified liquid density field measurement system based on background-oriented schlieren according to an embodiment of the present application;

[0039] Figure 2 shows a flowchart of a density-stratified liquid density field measurement method based on background-oriented schlieren according to an embodiment of the present application. Detailed Embodiments

[0040] Hereinafter, exemplary embodiments of the present application will be described in conjunction with the drawings. For clarity and conciseness, not all features of the actual embodiments are described in the specification. However, it should be understood that many specific decisions specific to the embodiments can be made during the development of any such actual embodiment in order to achieve the specific goals of the developer, and these decisions may vary with different embodiments.

[0041] Here, it should also be noted that in order to avoid obscuring the present application with unnecessary details, only the device structures closely related to the solution according to the present application are shown in the drawings, and other details less related to the present application are omitted.

[0042] It should be understood that the present application is not limited to the described embodiments only due to the following description with reference to the drawings. In this document, where feasible, embodiments can be combined with each other, features can be replaced or borrowed between different embodiments, and one or more features can be omitted in one embodiment.

[0043] Figure 1 shows a schematic structural diagram of a density-stratified liquid density field measurement system based on background-oriented schlieren according to an embodiment of the present application, see Figure 1The system includes: an LED light source, a background pattern plate, a test liquid system, an image acquisition device and a computer; Here, the LED light source, the background pattern plate, the test liquid system and the image acquisition device are arranged in sequence, and the image acquisition device can adopt a high-speed industrial camera;

[0044] The LED light source evenly illuminates the background pattern plate; sulfuric acid paper is used as a diffusion screen in front of the LED light source, and thousands of spots with a diameter of 0.5 mm are randomly distributed on the background pattern plate as the background pattern of the schlieren; according to the speckle non-overlapping condition, a Gaussian speckle image is generated as the background pattern. The size of the background spots in the image is about 3 pixels, and the spacing between points is between 2-4 pixels. The shape of the background spots has little effect on the density field. The experiment uses a contrast form of black dots on a white background to improve the image contrast and ensure the high quality of image processing.

[0045] The light emitted by the LED light source passes through the test liquid system via the background pattern plate and is incident on the image acquisition device.

[0046] The test liquid system includes a clean water tank, in which a simulated building environment system is set. The density stratified liquid here can be salt water. When the salt water enters the simulated building environment system, it causes density differences, which are used to simulate the heated airflow inside the building. In the simulated building environment system, a scaled building model is made according to the similarity theory, and the movement of the salt water in the clean water is used to study the movement of the thermal buoyancy airflow in the ambient air. When the density stratified liquid is filled into the test liquid system, a density gradient flow field is formed; when it is filled with clean water, a uniform clean water flow field is formed; when it is filled with air, an air field is formed.

[0047] The image acquisition device is used to acquire a first reference image of the background pattern in the background pattern plate under a uniform clean water flow field, a second reference image of the background pattern in the background pattern plate under an air field, and an image of the background pattern in the background pattern plate under a density gradient flow field.

[0048] When a certain concentration of salt water is added to the uniform clean water tank of this system, the density distribution of the water tank changes, forming a density gradient flow field. When the light from the background passes through the flow field with different densities (or concentrations) in the clean water tank, it is deflected due to the change in refractive index, and the position of the incident image receiving surface will change. The distorted picture is recorded by a high-speed industrial camera. After receiving the image, the computer performs image processing, imports the reference image and the measurement image, processes multiple frames of images based on the digital image correlation algorithm, and finally reconstructs the image to achieve the purpose of real-time visualization of the flow field and finally obtain the density field.

[0049] In order to ensure the image captured by the high-speed industrial camera, the high-speed industrial camera, the test liquid system, and the background pattern plate are placed in a dark room using a blackout curtain, leaving only a fixed light source in the room for illumination.

[0050] Before the experiment starts, fill the water tank with clear water. Wait until the interface is stable, then place the simulated building environment system in the water tank. The LED light source and the high-speed industrial camera are respectively arranged on both sides of the water tank, and their center lines are kept consistent. Ensure that the optical axis of the camera is perpendicular to the center of the background pattern board, and the center of the lens is at the same height and parallel to the center of the simulated building environment system. Turn on the high-speed industrial camera. The pattern displayed on the screen of the high-speed industrial camera is located at the center of the entire viewfinder to avoid image quality loss at the edges caused by lens distortion. The distance between the background pattern board and the test liquid system is less than the distance between the test liquid system and the image acquisition device.

[0051] Furthermore, the camera focal length f, the distance Z between the background pattern board and the water tank d , the distance Z between the background pattern board and the high-speed industrial camera b , adopt the highest resolution mode. The quantitative relationship satisfied by the camera focal length is:

[0052]

[0053] where z i represents the distance between the camera target surface and the lens.

[0054] From the geometric relationship, the image point displacement in the y direction is l:

[0055] l = Z d α y f / Z b

[0056] In the formula, α y is the deflection angle, and Z d , Z b , and f are all factors affecting the refractive index sensitivity of the background schlieren system. In the system of this application, the value of Z d is 400 mm - 700 mm, and the value of Z b is 1200 - 1500 mm; the lens focal length f of the industrial camera is 40 - 60 mm under the existing laboratory conditions. Within the above parameter range, it can ensure that the background schlieren device can obtain relatively clear schlieren images and ensure the safe progress of the experiment.

[0057] The following details the specific process of the density stratified liquid density field measurement method based on background-oriented schlieren in the embodiments of this application. Figure 2 The flowchart of the density stratified liquid density field measurement method based on background-oriented schlieren according to the embodiments of this application is shown. The method includes:

[0058] Step S1, obtain the first reference image of the background pattern under a uniform clear water flow field. Here, when the clear water tank is filled with clear water, under the condition of a uniform clear water flow field, the camera collects the average value of the images in the initial 5 minutes as the first reference image P of the background pattern water (x,y).

[0059] Step S2, obtain the second reference image P of the background pattern under an air flow field air (x,y); here, when the clear water tank is filled with air, the camera takes pictures of the background pattern board to collect the second reference image of the background pattern

[0060] Step S3, obtain the background pattern image P under a density gradient flow field salt (x,y); here, according to the experimental scheme, send the prepared brine with a certain concentration into the high-level brine storage tank in the test liquid system, and open the corresponding number and size of openings in the simulated building environment system; open the valve, and the brine in the high-level brine storage tank flows through the small filter, rotameter and plume nozzle in sequence and directly enters the inside of the simulated building environment system, so that a density gradient is generated inside the simulated building environment system, and at the same time the camera takes pictures of the background image to collect and obtain the background pattern image

[0061] Step S4, perform cross-correlation on the first reference image P water (x,y) and the second reference image P air (x,y) to determine the calibration field

[0062] Step S5, remap the background pattern image P salt (x,y) with the calibration field to obtain the remapped background pattern image

[0063] Step S6, perform cross-correlation on the remapped background pattern image and the second reference image P air (x,y) to obtain the displacement field

[0064] Step S7, perform the Radon inverse transform on the displacement field to obtain the reconstructed displacement field

[0065] Step S8, determine the refractive index distribution according to the reconstructed displacement field

[0066] Specifically, first, the refractive index of the displacement point can be determined by the following formula

[0067]

[0068] where, Δn is the refractive index of the displacement point (Δx', Δy') in the reconstructed displacement field, and K is the reciprocal of the influence coefficient

[0069] Then, the refractive indices of all displacement points in the reconstructed displacement field constitute the refractive index distribution

[0070] Step S9: Determine the density field according to the refractive index distribution.

[0071] Specifically, the following formula can be used to determine the density field:

[0072]

[0073] where, is the density field, is the refractive index distribution, ρ0 is the density of clear water, n0 is the refractive index of clear water, β is the change rate of refractive index with respect to density, n is the refractive index corresponding to each displacement point, and ρ is the density corresponding to each displacement point.

[0074] Further, after obtaining the density field, the fluid velocity can be obtained according to the following functional relationship:

[0075]

[0076] where K1 is the isentropic exponent, c is the speed of sound, generally taken as 340 m / s, and v x is the fluid velocity corresponding to each displacement point, ρ is the density corresponding to each displacement point, and the density field is composed of the densities corresponding to each displacement point.

[0077] In summary, the present application has the following technical effects:

[0078] (1) In the present application, the background schlieren technique combines the theory of PIV technique, which not only saves the cost of experimental equipment and the site, but also avoids the limitation of the lens size on the range of the flow field to be measured, and has a wide measurement range.

[0079] (2) In the present application, the background schlieren technique is used to study that natural ventilation is easily interfered by the surrounding environmental airflow in natural thermal convection (air). The background schlieren technique in the existing gas flow field is extended to the liquid flow field, and the background schlieren technique is combined with the brine experiment to reduce experimental errors and ensure accuracy.

[0080] (3) The method of the present application avoids putting colored dyes into the liquid field in the brine experiment, so as not to interfere with the state of the flow field to be measured. The optical reaction is rapid and real-time detection of the flow field can be achieved; the method has high measurement accuracy and can obtain quantitative measurement data of the flow field.

[0081] (4) The present application determines the offset of the spots in the background pattern captured by the industrial camera, so as to quantitatively obtain the density distribution in the liquid field. The structure is simple, the operation is convenient, and the accuracy is high.

[0082] As described above, these are only various embodiments of the present application. However, the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of changes or substitutions, which should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims described above.

Claims

1. A method for measuring the density field of density-stratified liquid based on background-oriented schlieren, characterized in that, Including: Under a uniform clear water flow field, obtaining a first reference image of the background pattern; Under an air field, obtaining a second reference image of the background pattern; Under a density gradient flow field, obtaining a background pattern image; Performing cross-correlation on the first reference image and the second reference image to determine a calibration field; Remapping the background pattern image with the calibration field to obtain a remapped background pattern image; Performing cross-correlation on the remapped background pattern image and the second reference image to obtain a displacement field; Performing an inverse Radon transform on the displacement field to obtain a reconstructed displacement field; Determining a refractive index distribution according to the reconstructed displacement field; Determining a density field according to the refractive index distribution; Wherein, determining a refractive index distribution according to the reconstructed displacement field includes: Among them, is the displacement point in the reconstructed displacement field refractive index, is the reciprocal of the influence coefficient; The refractive indices of all displacement points in the reconstructed displacement field constitute the refractive index distribution ; Wherein, determining a density field according to the refractive index distribution includes: wherein, is the density field, is the refractive index distribution, is the density of clear water, is the refractive index of clear water, and β is the change rate of refractive index with respect to density.

2. The method according to claim 1, wherein The method further includes: Determining a fluid velocity according to the density field.

3. A density stratified liquid density field measurement system based on background-oriented schlieren, characterized in that, Including: An LED light source, a background pattern board, a test liquid system, an image acquisition device and a computer; The LED light source is used for uniformly illuminating the background pattern board; The light emitted by the LED light source passes through the background pattern board and then through the test liquid system and is incident on the image acquisition device; When the test liquid system is filled with a density-stratified liquid, a density gradient flow field is formed; when filled with clear water, a uniform clear water flow field is formed; when filled with air, an air field is formed; The image acquisition device is used for acquiring a first reference image of the background pattern in the background pattern board under a uniform clear water flow field, a second reference image of the background pattern in the background pattern board under an air field, and a background pattern image in the background pattern board under a density gradient flow field; The computer is used for implementing the density field measurement method of density-stratified liquid based on background-oriented schlieren according to any one of claims 1-2.

4. The system according to claim 3, wherein The LED light source and the image acquisition device are respectively arranged on both sides of the test liquid system, and their center lines are kept consistent; the distance between the background pattern board and the test liquid system is less than the distance between the test liquid system and the image acquisition device.

5. The system according to claim 3, wherein The image acquisition device is a high-speed industrial camera, and the distance between the background pattern board and the test liquid system Z d is 400 mm - 700 mm, and the distance between the background pattern board and the high-speed industrial camera Z b is 1200 - 1500 mm, and the focal length f of the high-speed industrial camera is 40 - 60 mm.

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