CN116413164A - Density layered liquid density field measurement method and system based on background guide schlieren - Google Patents
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Abstract
The application relates to a density layered liquid density field measurement method and system based on background guide schlieren, wherein the method comprises the following steps: cross-correlating the first reference image under the uniform clear water flow field with the second reference image under the air field to determine a calibration field; remapping the background pattern image and the calibration field to obtain a mapped background pattern image; cross-correlating the mapped background pattern image with a second reference image to obtain a displacement field; adopting Radon inverse transformation aiming at the displacement field to obtain a reconstructed displacement field; determining refractive index distribution according to the reconstructed displacement field; the density field is determined from the refractive index profile. According to the method, the background schlieren technology is utilized in natural heat convection (air) to research that natural ventilation is easy to be interfered by surrounding air flow, the background schlieren technology in the existing gas flow field is expanded to the liquid flow field, the background schlieren technology is combined with a brine experiment, experimental errors are reduced, and accuracy is guaranteed.
Description
Technical Field
The application relates to the technical field of density field measurement, in particular to a density field measurement method and a density field measurement system for density layered liquid based on background guide schlieren.
Background
In the research of indoor hot and humid environment, ventilation, fire smoke and other fields, the adoption of salt water experiment to simulate the air movement in real environment is one of important experimental means, and tracer dye is added to observe the flowing state, so that the movement development of the air in the actual building is analyzed according to the similar theory. Common brine model measurement methods are classified into contact type and non-contact type. The contact measurement generally adopts a thermocouple, and the contact measurement can generate interference on the flow of a flow field, and a sensor downstream of the flow field can be influenced by an upstream sensor, so that the contact measurement is more favored in experiments. The current non-contact measurement method is to measure the density distribution of the flow field by reading the gray level of the acquired image, and the method has the problems that the diffusion of trace dye affects the accuracy of experimental data, the speed field cannot be read, the motion details of plumes in layering cannot be directly observed by human eyes, and the like.
Schlieren imaging is one of the important non-contact measurement techniques, and the ability to observe changes in air density enables researchers to capture phenomena in compressible flow, thermal convection, and chemical mixing processes. Background guided schlieren (BOS) is a new technique for flow visualization of fluid density gradients using Gladstone-Dale relationships between gas density and refractive index, which belongs to a synthetic schlieren method. BOS uses a light source to project a textured background (typically a random dot pattern) on one side of the test chamber onto a camera sensor on the other side. The first image (referred to as the reference image) is recorded by a stagnant fluid of uniform density. The light deflection is caused by the refractive index change of the light in the flow field area, the distortion of corresponding pixels in the background schlieren pattern is caused, the displacement is quantized by an optical flow or Particle Image Velocimetry (PIV) method, and a virtual displacement field proportional to the refractive index derivative is provided. Currently, most BOS studies focus on the direct placement of heating sources in homogeneous gaseous (air) media, while the lack of practical measurement and quantitative analysis for liquid flow fields.
Disclosure of Invention
In order to overcome at least one defect in the prior art, the application provides a density field measurement method and a system for density layered liquid based on background guide schlieren.
In a first aspect, a method for measuring a density field of a density layered liquid based on a background guided schlieren is provided, comprising:
under a uniform clear water flow field, a first reference image of a background pattern is obtained;
acquiring a second reference image of the background pattern in an air field;
obtaining a background pattern image under a density gradient flow field;
performing cross-correlation on the first reference image and the second reference image to determine a calibration field;
remapping the background pattern image and the calibration field to obtain a mapped background pattern image;
cross-correlating the mapped background pattern image with the second reference image to obtain a displacement field;
adopting Radon inverse transformation for the displacement field to obtain a reconstructed displacement field;
determining refractive index distribution according to the reconstructed displacement field;
a density field is determined from the refractive index profile.
In one embodiment, determining the refractive index profile from the reconstructed displacement field comprises:
wherein, delta n is the refractive index of the displacement point (delta x ', delta y') of the reconstructed displacement field, K is the reciprocal of the influence coefficient;
the refractive index of all displacement points in the reconstructed displacement field forms the refractive index distribution
In one embodiment, determining the density field from the refractive index profile comprises:
wherein,,
for density field +.>
For refractive index distribution ρ 0 Is clear water density, n 0 The refractive index of clear water, and beta is the change rate of the refractive index to the density.
In one embodiment, the method further comprises:
a fluid velocity is determined from the density field.
In a second aspect, there is provided a background guided schlieren based density layered liquid density field measurement system comprising: the device comprises an LED light source, a background pattern plate, a test liquid system, an image acquisition device and a computer;
the LED light source is used for uniformly illuminating the background pattern plate;
light rays emitted by the LED light source pass through the test liquid system through the background pattern plate and are incident to the image acquisition device;
when the test liquid system is filled with the density layering liquid, a density gradient flow field is formed; when the clean water is filled, a uniform clean water flow field is formed; when air is filled, an air field is formed;
the image acquisition device is used for acquiring a first reference image of a background pattern in the background pattern plate under a uniform clear water flow field, a second reference image of the background pattern in the background pattern plate under an air field and a background pattern image in the background pattern plate under a density gradient flow field;
the computer is used for realizing the density field measuring method of the density layered liquid based on the background guiding schlieren.
In one embodiment, the LED light source and the image acquisition device are respectively arranged at two sides of the test liquid system, and the central lines of the LED light source and the image acquisition device are consistent; the distance between the background pattern plate and the test liquid system is smaller than the distance between the test liquid system and the image acquisition device.
In one embodiment, the image acquisition device is a high-speed industrial camera, the distance Z between the background pattern plate and the test liquid system d A distance Z between the background pattern plate and the high-speed industrial camera of 400mm-700mm b The focal length f of the high-speed industrial camera is between 1200 and 1500mm and is between 40 and 60mm.
Compared with the prior art, the application has the following beneficial effects:
(1) The background schlieren technology in the application combines the theory of PIV technology, so that not only are the cost and the field of experimental equipment saved, but also the limitation of the size of the lens on the range of the flow field to be detected is avoided, and the measuring range is wide.
(2) According to the method, the background schlieren technology is utilized in natural heat convection (air) to research that natural ventilation is easy to be interfered by surrounding air flow, the background schlieren technology in the existing gas flow field is expanded to the liquid flow field, the background schlieren technology is combined with a brine experiment, experimental errors are reduced, and accuracy is guaranteed.
(3) The method avoids placing the colored dye into the liquid field in the salt water experiment, so that the state of the flow field to be detected is not interfered, the optical reaction is rapid, and the flow field real-time detection can be realized; the method has high measurement accuracy and can obtain quantitative measurement data of the flow field.
(4) The offset of the background pattern spots shot by the industrial camera is determined, so that the density distribution in the liquid field is quantitatively obtained, and the device is simple in structure, convenient to operate and high in precision.
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The present application may be better understood by reference to the following description taken in conjunction with the accompanying drawings, which are incorporated in and form a part of this specification, together with the following detailed description. In the drawings:
FIG. 1 shows a schematic structural diagram of a background guided schlieren-based density layered liquid density field measurement system according to an embodiment of the present application;
fig. 2 shows a flow diagram of a background guided schlieren based density layered liquid density field measurement method according to an embodiment of the present application.
Detailed Description
Exemplary embodiments of the present application will be described hereinafter with reference to the accompanying drawings. In the interest of clarity and conciseness, not all features of an actual embodiment are described in the specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions may be made to achieve the developers' specific goals, and that these decisions may vary from one implementation to another.
It should 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 not greatly related to the present application are omitted.
It is to be understood that the present application is not limited to the described embodiments due to the following description with reference to the drawings. In this context, embodiments may be combined with each other, features replaced or borrowed between different embodiments, one or more features omitted in one embodiment, where possible.
Fig. 1 shows a schematic structural diagram of a background guided schlieren-based density layered liquid density field measurement system according to an embodiment of the present application, see fig. 1, the system comprising: the device comprises 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 pickup device are sequentially disposed, and the image pickup device may employ a high-speed industrial camera;
the LED light source uniformly illuminates the background pattern plate; the LED light source is characterized in that the front of the LED light source is provided with a sulfuric acid paper as a diffusion screen, thousands of spots with diameters of 0.5 millimeter are randomly distributed on a background pattern plate to be used as background patterns of schlieren; according to the speckle non-overlapping condition, a Gao Sisan speckle image is generated to serve as a background pattern, the size of a background speckle in the image is about 3 pixels, the distance between the points is between 2 and 4 pixels, and the shape of the background speckle has little influence on the density field calculation. The experiment adopts a white background and black dot contrast mode to improve the image contrast and ensure the high quality of image processing.
Light emitted by the LED light source passes through the test liquid system through the background pattern plate and is incident to the image acquisition device.
The test liquid system comprises a clear water tank, wherein a simulated building environment system is arranged in the clear water tank, the density layered liquid can be saline water, and when the saline water enters the simulated building environment system, density difference is caused to simulate heated air flow in a building; a reduced scale building model is made in the simulated building environment system according to a similar theory, and the movement condition of the hot buoyancy air flow in the ambient air is researched by utilizing the movement of the brine in the clear water. When the test liquid system is filled with the density layered liquid, a density gradient flow field is formed; when the clean water is filled, a uniform clean 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 a background pattern in the background pattern plate under the uniform clear water flow field, a second reference image of the background pattern in the background pattern plate under the air field and a background pattern image in the background pattern plate under the density gradient flow field.
When brine with a certain concentration is added into the uniform clear water tank, the density distribution of the water tank is changed to form a density gradient flow field, when light rays from the background pass through the flow fields with different densities (or concentrations) in the clear water tank, deflection occurs because of the change of refractive index, the position of the light rays entering an image receiving surface is changed, a distorted picture is recorded by a high-speed industrial camera, the computer receives the image and then carries out image processing, a reference image and a measured image are imported, the multi-frame image is processed based on a digital image correlation algorithm, the image is finally reconstructed, and the purpose of real-time visualization of the flow fields is achieved, so that the density field is finally obtained.
In order to ensure the image shot by the high-speed industrial camera, the test liquid system and the background pattern plate are placed in a darkroom by using a shade, and only the fixed light source illumination in the room is reserved.
Before the test starts, the clear water tank is filled with clear water, the interface is stable, the simulated building environment system is placed in the clear water tank, the LED light source and the high-speed industrial camera are respectively arranged on two sides of the clear water tank, the central lines are kept consistent, the optical axis of the camera is ensured to be perpendicular to the center of the background pattern plate, the center of the lens is equal in height and parallel to the center of the simulated building environment system, the high-speed industrial camera is turned on, the screen display pattern in the high-speed industrial camera is positioned in the center of the whole view finding frame, and the edge image quality loss caused by lens distortion is avoided. The distance between the background pattern plate and the test liquid system is smaller than the distance between the test liquid system and the image acquisition device.
Further, the camera focal length f, the distance Z between the background pattern plate and the clear water tank d Distance Z between background pattern plate and high-speed industrial camera b The highest resolution mode is employed. The quantitative relation that camera focal length satisfies is:
wherein z is i Representing the distance between the camera target surface and the lens.
From the geometric relationship, the image point displacement in the y-direction is l:
l＝Z d α y f/Z b
wherein alpha is y For deflection angle, Z d 、Z b Both f are factors affecting the refractive index sensitivity of the background schlieren system, in the system of the present application Z d The value of (C) is 400mm-700mm, Z b The value of (2) is 1200-1500mm; the focal length f of the industrial camera is 40-60mm under the existing laboratory condition. The background schlieren device can be ensured to obtain a clearer schlieren image in the parameter range, and meanwhile, the safe performance of an experiment is ensured.
The following describes in detail a specific flow of a method for measuring a density field of a density layered liquid based on a background guided schlieren according to an embodiment of the present application, and fig. 2 shows a flow chart of a method for measuring a density field of a density layered liquid based on a background guided schlieren according to an embodiment of the present application, where the method includes:
step S1, under a uniform clear water flow field, acquiring a first reference image of a background pattern; here, when the clear water tank is filled with clear water, the camera collects an average value of the image of the first 5min as a first reference image P of the background pattern under the condition of uniform clear water flow field water (x,y)。
Step S2, under the air field, obtaining a second reference image P of the background pattern air (x, y); here, when the clear water tank is filled with air, the camera photographs the background pattern plate, and a second reference image of the background pattern is acquired.
Step S3, obtaining a background pattern image P under a density gradient flow field salt (x, y); here, according to the experimental scheme, the prepared brine with a certain concentration is sent into a high-level brine storage tank in a test liquid system, and openings with corresponding quantity and size of a simulated building environment system are opened; and (3) opening a valve, enabling brine in the high-order brine storage tank to sequentially flow through the small filter, the rotameter and the plume nozzle and directly enter the simulated building environment system, so that density gradient is generated in the simulated building environment system, and simultaneously, the camera shoots and collects background images to obtain background pattern images.
Step S4, for the first reference image P water (x, y) and a second reference image P air (x, y) cross-correlating to determine a calibration field;
step S5, background pattern image P salt (x, y) remapping with the calibration field to obtain a mapped background pattern image;
step S6, the mapped background pattern image and the second reference image P air (x, y) cross-correlating to obtain a displacement field;
s7, adopting Radon inverse transformation aiming at the displacement field to obtain a reconstructed displacement field;
and S8, determining refractive index distribution according to the reconstructed displacement field.
Specifically, first, the refractive index of the displacement point may be determined using the following formula:
wherein, delta n is the refractive index of the displacement point (delta x ', delta y') of the reconstructed displacement field, K is the reciprocal of the influence coefficient;
then, the refractive index of all displacement points in the reconstructed displacement field forms a refractive index distribution
Step S9, determining a density field according to the refractive index distribution.
Specifically, the density field may be determined using the following formula:
wherein,,
for density field +.>
For refractive index distribution ρ 0 Is clear water density, n 0 Refractive index of clear water, beta is the change rate of refractive index to density, +.>
n is the refractive index corresponding to each displacement point, and ρ is the density corresponding to each displacement point.
Further, after the density field is obtained, the fluid velocity may also be obtained according to the following functional relationship:
wherein K1 is an isentropic index, c is the speed of sound, oneGenerally take 340m/s, v x For 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.
In summary, the present application has the following technical effects:
(1) The background schlieren technology in the application combines the theory of PIV technology, so that not only are the cost and the field of experimental equipment saved, but also the limitation of the size of the lens on the range of the flow field to be detected is avoided, and the measuring range is wide.
(2) According to the method, the background schlieren technology is utilized in natural heat convection (air) to research that natural ventilation is easy to be interfered by surrounding air flow, the background schlieren technology in the existing gas flow field is expanded to the liquid flow field, the background schlieren technology is combined with a brine experiment, experimental errors are reduced, and accuracy is guaranteed.
(3) The method avoids placing the colored dye into the liquid field in the salt water experiment, so that the state of the flow field to be detected is not interfered, the optical reaction is rapid, and the flow field real-time detection can be realized; the method has high measurement accuracy and can obtain quantitative measurement data of the flow field.
(4) The offset of the background pattern spots shot by the industrial camera is determined, so that the density distribution in the liquid field is quantitatively obtained, and the device is simple in structure, convenient to operate and high in precision.
The foregoing is merely various embodiments of the present application, but the scope of the present application is not limited thereto, and any person skilled in the art can easily think about changes or substitutions within the technical scope of the present application, and the changes and substitutions are intended to be covered in the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (7)
1. A method for measuring a density-layered liquid density field based on background-oriented schlieren, comprising:
under a uniform clear water flow field, a first reference image of a background pattern is obtained;
acquiring a second reference image of the background pattern in an air field;
obtaining a background pattern image under a density gradient flow field;
performing cross-correlation on the first reference image and the second reference image to determine a calibration field;
remapping the background pattern image and the calibration field to obtain a mapped background pattern image;
cross-correlating the mapped background pattern image with the second reference image to obtain a displacement field;
adopting Radon inverse transformation for the displacement field to obtain a reconstructed displacement field;
determining refractive index distribution according to the reconstructed displacement field;
a density field is determined from the refractive index profile.
2. The method of claim 1, wherein determining the refractive index profile from the reconstructed displacement field comprises:
wherein, delta n is the refractive index of the displacement point (delta x ', delta y') of the reconstructed displacement field, K is the reciprocal of the influence coefficient;
the refractive index of all displacement points in the reconstructed displacement field forms the refractive index distribution
3. The method of claim 1, wherein determining a density field from the refractive index profile comprises:
wherein,,
for density field +.>
For refractive index distribution ρ 0 Is clear water density, n 0 The refractive index of clear water, and beta is the change rate of the refractive index to the density.
4. The method of claim 1, wherein the method further comprises:
a fluid velocity is determined from the density field.
5. A background guided schlieren-based density layered liquid density field measurement system, comprising: the device comprises an LED light source, a background pattern plate, a test liquid system, an image acquisition device and a computer;
the LED light source is used for uniformly illuminating the background pattern plate;
light rays emitted by the LED light source pass through the test liquid system through the background pattern plate and are incident to the image acquisition device;
when the test liquid system is filled with the density layering liquid, a density gradient flow field is formed; when the clean water is filled, a uniform clean water flow field is formed; when air is filled, an air field is formed;
the image acquisition device is used for acquiring a first reference image of a background pattern in the background pattern plate under a uniform clear water flow field, a second reference image of the background pattern in the background pattern plate under an air field and a background pattern image in the background pattern plate under a density gradient flow field;
the computer is configured to implement the background guided schlieren-based density layered liquid density field measurement method of any one of claims 1-4.
6. The system of claim 5, wherein the LED light source and the image acquisition device are disposed on either side of the test fluid system, respectively, with a center line that remains consistent; the distance between the background pattern plate and the test liquid system is smaller than the distance between the test liquid system and the image acquisition device.
7. The system of claim 5, wherein the image acquisition device is a high-speed industrial camera, the distance Z between the background pattern plate and the test liquid system d A distance Z between the background pattern plate and the high-speed industrial camera of 400mm-700mm b The focal length f of the high-speed industrial camera is between 1200 and 1500mm and is between 40 and 60mm.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117233126A (en) * | 2023-09-18 | 2023-12-15 | 中国测试技术研究院 | Background schlieren measurement calibration device and method |
| CN117347228A (en) * | 2023-09-18 | 2024-01-05 | 中国科学院力学研究所 | Background schlieren-based method and system for measuring liquid density layering |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117233126A (en) * | 2023-09-18 | 2023-12-15 | 中国测试技术研究院 | Background schlieren measurement calibration device and method |
| CN117347228A (en) * | 2023-09-18 | 2024-01-05 | 中国科学院力学研究所 | Background schlieren-based method and system for measuring liquid density layering |
| CN117233126B (en) * | 2023-09-18 | 2024-05-28 | 中国测试技术研究院 | Background schlieren measurement calibration device and method |
| CN117347228B (en) * | 2023-09-18 | 2024-06-07 | 中国科学院力学研究所 | Background schlieren-based method and system for measuring liquid density layering |
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