CN114383668B - Variable background-based flow field measuring device and method - Google Patents

Abstract

The application provides a flow field measurement device and method based on a variable background, which relates to the technical field of quantitative measurement by schlieren method, including: LED parallel backlight, liquid crystal panel, camera, image processing module and background pattern generation module; LED parallel The backlight source is used to emit parallel light sources to the LCD panel; the background pattern generation module is used to generate the optimal background pattern according to the camera resolution and field of view based on the preset background pattern generation algorithm, and send the optimal background pattern to the LCD panel display; the camera is used to image the flow field to be measured before the optimal background pattern and the flow field to be measured before the optimal background pattern, respectively, to obtain the first image and the second image; the image processing module is used to The first image and the second image are processed according to a preset algorithm to obtain the density and temperature of the flow field to be measured. The application changes the size and style of the background pattern according to the measurement requirements, thereby improving the measurement accuracy of the flow field to be measured.

Description

A flow field measurement device and method based on variable background

technical field

The present application relates to the technical field of schlieren quantitative measurement, and in particular, to a flow field measurement device and method based on a variable background.

Background technique

When using the background schlieren method for quantitative measurement, it is usually necessary to capture the dynamic process, so the light source is required to have sufficient intensity to support high frame rate measurement. When the background pattern is larger, the light source needs to be distributed over a larger area, so the light intensity is limited.

In the existing background schlieren method, if a large field of view is measured, the time resolution will be sacrificed, such as using a halogen lamp to illuminate a large background pattern; if the flow field to be measured is reconstructed with high time resolution, the size of the field of view will be sacrificed. , such as using a high-intensity laser to illuminate a small background pattern. Both of these methods have certain limitations, and the background patterns are usually printed and cannot be replaced according to actual needs. Therefore, how to establish a measuring device that can not only ensure a large enough field of view, but also quickly change the background pattern is one of the problems faced by the current background schlieren method.

SUMMARY OF THE INVENTION

In view of this, the present application provides a flow field measurement device and method based on a variable background, which can solve the technical problems in the prior art that the background pattern cannot be changed and the sampling frame rate is insufficient.

On the one hand, an embodiment of the present application provides a flow field measurement device based on a variable background, including: an LED parallel backlight, a liquid crystal panel, a camera, an image processing module, and a background pattern generation module;

The LED parallel backlight is used for emitting parallel light sources to the liquid crystal panel;

The background pattern generation module is used to perform the following steps:

Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera;

Step S2: Calculate the actual size of the background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel;

Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the square side length a;

Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern;

Step S5: generate the histogram of the background pattern, thereby calculating the grayscale average value of the background pattern;

Step S6: determine whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold, if so, if the average gray level is greater than 0.5, then reduce the sampling radius R, if the average gray level is less than 0.5, then increase the sampling radius R, Go to step S4; otherwise, go to step S7;

Step S7: using the generated background pattern as the optimal background pattern, and sending the optimal background pattern to the liquid crystal panel;

The liquid crystal panel is used to display an optimal background pattern;

The camera is used to image the flow field to be measured before the optimal background pattern is not placed and the flow field to be measured is placed before the optimal background pattern to obtain a first image and a second image;

The image processing module is used to process the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured.

Further, a parallel light film is arranged on the LED parallel backlight source.

Further, the size of the LED parallel backlight source and the liquid crystal panel is determined according to the size of the flow field to be measured.

Further, the image processing module is specifically used for:

obtain the first image and the second image;

Perform preprocessing on the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image;

Select the cross-correlation window, perform cross-correlation processing on the pre-processed first image and the pre-processed second image, and obtain the offset of each pixel of the background pattern of the entire flow field area to be measured, thereby obtaining the entire to-be-measured flow field. The offset distribution of the background pattern in the flow field region;

Obtain the relative refractive index distribution of the flow field to be measured according to the offset distribution of the background pattern in the entire flow field to be measured;

According to the ambient refractive index and the relative refractive index distribution of the flow field to be measured, the refractive index distribution of the flow field to be measured is calculated;

Calculate the density distribution of the flow field to be measured according to the relative refractive index distribution of the flow field to be measured;

According to the density distribution of the flow field to be measured, the temperature distribution of the flow field to be measured is calculated.

Further, after obtaining the offset of each pixel of the background pattern of the entire flow field area to be measured, it includes:

From the background pattern of the flow field region to be measured, select a region of interest that needs parameter processing or a region of interest with axis symmetry, where the number of rows of the region of interest is t, and the number of columns is q;

Starting from the first line of the area of interest, extract the positions of the maximum and minimum values of the offset in the first line of data, calculate the average value of the abscissa of the two positions, and take the integer part of the average value of the abscissa as the first line The initial value m of the symmetry axis of the data;

Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius;

The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [m-b, m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. Cross-correlation coefficient, a total of 2b+1 cross-correlation coefficients are obtained;

作为第一行的对称轴位置；Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row;

；Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

;

的平均值，取平均值的整数部分作为整个待测流场区域对称轴的横坐标。calculate

The average value of , and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field area to be measured.

On the other hand, an embodiment of the present application provides a method for measuring a flow field based on a variable background, which is applied to the device for measuring a flow field based on a variable background in the embodiment of the present application, including:

The background pattern generation module performs the following steps:

Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera;

Step S2: Calculate the actual size of the background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel;

Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the square side length a;

Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern;

Step S5: generate the histogram of the background pattern, thereby calculating the grayscale average value of the background pattern;

Step S6: determine whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold, if so, if the average gray level is greater than 0.5, then reduce the sampling radius R, if the average gray level is less than 0.5, then increase the sampling radius R, Go to step S4; otherwise, go to step S7;

Step S7: using the generated background pattern as the optimal background pattern, and sending the optimal background pattern to the liquid crystal panel;

using a camera to image the optimal background pattern displayed by the liquid crystal panel to obtain a first image;

Place the flow field to be measured between the liquid crystal panel and the camera, and use the camera to image the flow field to be measured and the optimal background pattern to obtain a second image;

The image processing module processes the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured.

Further, the method also includes:

Set the camera resolution and capture frame rate;

Input the resolution of the camera and the size of the flow field to be measured into the background pattern generation module.

Further, before using the camera to image the optimal background pattern displayed by the liquid crystal panel, it includes:

Adjust the LED parallel backlight to the maximum brightness and project the light on the LCD panel;

Adjust the height and level of the camera so that its field of view completely covers the flow field area to be measured and the optimal background pattern behind the flow field area to be measured.

Further, the image processing module processes the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured: including:

obtain the first image and the second image;

Perform preprocessing on the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image;

Select the cross-correlation window, perform cross-correlation processing on the pre-processed first image and the pre-processed second image, and obtain the offset of each pixel of the background pattern of the entire flow field area to be measured, thereby obtaining the entire to-be-measured flow field. The offset distribution of the background pattern in the flow field region;

Obtain the relative refractive index distribution of the flow field to be measured according to the offset distribution of the background pattern in the entire flow field to be measured;

According to the ambient refractive index and the relative refractive index distribution of the flow field to be measured, the refractive index distribution of the flow field to be measured is calculated;

Calculate the density distribution of the flow field to be measured according to the relative refractive index distribution of the flow field to be measured;

According to the density distribution of the flow field to be measured, the temperature distribution of the flow field to be measured is calculated.

Further, after obtaining the offset of each pixel of the background pattern of the entire flow field area to be measured, it includes:

From the background pattern of the flow field region to be measured, select a region of interest that needs parameter processing or a region of interest with axis symmetry, where the number of rows of the region of interest is t, and the number of columns is q;

Starting from the first line of the area of interest, extract the positions of the maximum and minimum values of the offset in the first line of data, calculate the average value of the abscissa of the two positions, and take the integer part of the average value of the abscissa as the first line The initial value m of the symmetry axis of the data;

Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius;

The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [m-b, m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. Cross-correlation coefficient, a total of 2b+1 cross-correlation coefficients are obtained;

作为第一行的对称轴位置；Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row;

Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

的平均值，取平均值的整数部分作为整个待测流场区域对称轴的横坐标。calculate

The average value of , and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field area to be measured.

In this embodiment of the present application, the side lengths of the black square sampling points in the background pattern are determined according to the resolution of the camera and the size of the background area captured by the camera, and the sampling radius of the Poisson disk sampling method is determined according to the gray average value of the background pattern, In this way, the size and style of the background pattern can be changed according to the measurement requirements, which improves the measurement accuracy of the flow field to be measured.

Description of drawings

In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the accompanying drawings that need to be used in the description of the specific embodiments or the prior art will be briefly introduced below. The drawings are some embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

1 is a structural composition diagram of a flow field measurement device based on a variable background provided by an embodiment of the present application;

2 is a flow chart of generating an optimal background pattern by a background pattern generation module provided in an embodiment of the present application;

Fig. 3 is the schematic diagram that the flow field to be measured is a propane-air premixed flame;

4 is a schematic diagram of an optimal background pattern provided by an embodiment of the present application;

5 is a schematic diagram of a specific implementation process of an image processing module provided by an embodiment of the present application;

6 is an offset distribution diagram of the background pattern of the entire flow field area drawn by the embodiment of the present application;

Fig. 7 is the flame temperature distribution cloud map drawn by the embodiment of the application;

FIG. 8 is a flowchart of a method for measuring a flow field based on a variable background provided by an embodiment of the present application.

Icons: 101-LED parallel backlight; 102-liquid crystal panel; 103-flow field to be measured; 104-imaging lens; 105-image sensor; 106-image processing device; 107-background pattern generation device.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments It is a part of the embodiments of the present application, but not all of the embodiments. The components of the embodiments of the present application generally described and illustrated in the drawings herein may be arranged and designed in a variety of different configurations.

Thus, the following detailed description of the embodiments of the application provided in the accompanying drawings is not intended to limit the scope of the application as claimed, but is merely representative of selected embodiments of the application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

First, the design ideas of the embodiments of the present application are briefly introduced.

The background schlieren method determines the parameters of the flow field to be measured based on the deflection angle of the light passing through the flow field to be measured: place a specific background pattern behind the flow field to be measured, and use a camera to measure the flow field with and without the flow field to be measured. The two background patterns are captured, and the cross-correlation algorithm is used to extract the background offset to solve the deflection angle of the light after passing through the flow field to be measured. Compared with the traditional schlieren method, the advantage of this method is that the measurement field of view is no longer limited. As long as the size of the background pattern is selected reasonably, the measurement field of view can be infinite. The shock waves generated by the aircraft are qualitatively captured.

In the existing background schlieren method, if a large field of view is measured, the time resolution will be sacrificed, such as using a halogen lamp to illuminate a large background pattern; if the flow field to be measured is reconstructed with high time resolution, the size of the field of view will be sacrificed. , such as using a high-intensity laser to illuminate a small background pattern. Both of these methods have certain limitations, and the background patterns are usually printed and cannot be replaced according to actual needs. Therefore, how to establish a measuring device that can not only ensure a large enough field of view, but also quickly change the background pattern is one of the problems faced by the current background schlieren method.

In order to solve the above-mentioned technical problems, the embodiment of the present application provides a flow field measurement device based on a variable background. Based on the principle of the refractive index gradient reconstruction of the density field in the background schlieren method, an LED parallel backlight with a parallel light film is used as the background schlieren method. The parallel light source, the liquid crystal panel as the background pattern display device, and the high-speed camera as the image acquisition device can measure the area with high temporal and spatial resolution of the large field of view, and a background pattern generation algorithm is given, which can combine the camera parameters and the waiting area. The size of the field of view produces an optimal background pattern and should have a uniform grayscale distribution overall.

The measuring device of the embodiment of the present application can measure the instantaneous temperature of the flow field to be measured on the millisecond time scale, and can change the size of the LED parallel backlight and the size and style of the background pattern according to the measurement requirements, so as to ensure the best spatial resolution rate and measurement accuracy. Without reducing the measurement accuracy, it overcomes the defects that the original experimental device cannot change the background pattern in real time and the sampling frame rate is insufficient. And the light path arrangement is shortened by the LED parallel backlight source, which saves space.

After introducing the application scenarios and design ideas of the embodiments of the present application, the technical solutions provided by the embodiments of the present application are described below.

As shown in FIG. 1 , an embodiment of the present application provides a flow field measurement device based on a variable background. The device includes: an LED parallel backlight 101 , a liquid crystal panel 102 , an imaging lens 104 , an image sensor 105 , and an image processing device 106 and a background pattern generating device 107, wherein the imaging lens 104 and the image sensor 105 form a camera. The flow field to be measured 103 is placed between the liquid crystal panel 102 and the imaging lens 104, while ensuring that the centers of the LED parallel backlight 101, the liquid crystal panel 102, the flow field to be measured 103 and the camera are on the same horizontal plane. The image processing device 106 and the camera are connected by wire, and the liquid crystal panel 102 and the background pattern generating device 107 are connected by wire. At the same time, it is ensured that the image processing device 106 and the background pattern generating device 107 have a sufficient distance from the optical path system, and the heat dissipation of the devices will not affect the optical path system.

Preferably, the surface of the LED parallel backlight source 101 is covered with a parallel light film; the background pattern generation device 107 is an electronic device configured with a background pattern generation module and can be connected to the liquid crystal panel 102; the imaging lens 104 selects a 105mm macro lens; the image sensor 105 A high-speed camera sensor is used; the image processing device 106 is an electronic device configured with a background pattern generation module. The LED parallel backlight 101 , the liquid crystal panel 102 and the flow field to be measured 103 are supported by an optical table, and the imaging lens 104 and the image sensor 105 are supported by a tripod.

The LED parallel backlight source 101 is used to emit parallel light sources to the liquid crystal panel; the background pattern generation module is used to generate an optimal background pattern based on a preset background pattern generation algorithm according to the camera resolution and field of view, and the optimal background pattern is The pattern is sent to the liquid crystal panel; the liquid crystal panel 102 is used to display the optimal background pattern; the camera is used to image the flow field to be measured before the optimal background pattern and the flow field to be measured before the optimal background pattern, respectively, to obtain the first an image and a second image; the image processing module is used to process the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured.

The sizes of the LED parallel backlight 101 and the liquid crystal panel 102 are determined according to the size of the flow field to be measured.

Because the liquid crystal panel 102 used can change the background pattern in real time and has a higher resolution, compared with the traditional background schlieren device, it can provide a high-precision background pattern for identification. Parts of the background pattern should have sufficient features, and the whole should present a uniform grayscale distribution.

As shown in Figure 2, the process of generating the optimal background pattern by the background pattern generation module is as follows:

Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera;

Step S2: Calculate the actual size of the background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel;

Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the side length of the square;

Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern;

Among them, the Poisson disk sampling method is used to generate multiple Poisson disk sampling points, which meet the following conditions:

The distance between any two sampling points is greater than the given sampling radius R;

The sampling area is completely covered by all the sampling discs, among which, there is only one sampling point in one sampling disc; the radius of the sampling disc is the sampling radius R.

Step S5: generating a histogram of the background pattern, thereby calculating the grayscale average value of the background pattern;

Step S6: Determine whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold. If yes, reduce the sampling radius R if the average gray level is greater than 0.5, and increase the sampling radius R if the average gray level is less than 0.5. Go to step S4; otherwise, go to step S7;

Among them, the threshold is a small number, which can be defined according to the convergence speed; the purpose is to hope that the gray average value of the generated background pattern is close to 0.5.

Step S7: using the generated background pattern as the optimal background pattern, and sending the optimal background pattern to the liquid crystal panel.

As an application example, the adopted flow field to be measured is a propane-air premixed flame, as shown in FIG. 3 , and the placement area is the area of the flow field to be measured 103 in FIG. 1 . At the beginning of the measurement, place it in the measurement position. The optimal background pattern generated by the background pattern generation module is shown in Figure 4. The local features of the pattern are obvious and the overall grayscale is uniform.

As a possible implementation manner, FIG. 5 is an image processing process performed by an image processing module. Specifically, when the flow field to be measured is an axisymmetric flow field, the image processing module is specifically used for:

Step A1: acquiring the first image and the second image;

Step A2: Preprocessing the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image;

Step A3: Select a cross-correlation window, perform cross-correlation processing on the preprocessed first image and the preprocessed second image, and obtain the offset of each pixel of the background pattern of the entire flow field area to be measured, thereby obtaining: The offset distribution of the background pattern in the entire flow field area to be measured;

Among them, the cross-correlation window is a small square, which can cover the pixels in p*p images. When the flow field to be measured is a flame, its offset distribution is shown in Figure 6.

This step is followed by:

The measurement of the center-symmetric flow field requires the selection of the symmetry axis of the entire flow field area to be measured, so as to perform subsequent tomographic reconstruction, including:

From the background pattern of the flow field area to be measured, select the area of interest that needs to be processed by parameters or the area of interest with axis symmetry, the number of rows of the area of interest is t, and the number of columns is q;

Starting from the first row of the area of interest, since the offsets on both sides of the symmetry axis are opposite to each other, first extract the positions of the maximum and minimum offsets in the first row of data, and calculate the abscissas of the two positions Average value, take its integer part m as the initial value of the symmetry axis of the first row of data;

Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius, which is selected according to the spatial resolution of the processed data;

个互相关系数；The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [mb,m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. cross-correlation coefficient, we get a total of

a cross-correlation coefficient;

作为第一行的对称轴位置；Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row;

，取平均值的整数部分作为整个待测流场区域对称轴的横坐标。Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

, and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field region to be measured.

Step A4: Calculate the light deflection angle distribution according to the offset distribution of the background pattern in the entire flow field area to be measured, and integrate it along the ordinate axis to obtain the relative refractive index distribution of the flow field to be measured;

Step A5: Calculate the refractive index distribution of the flow field to be measured according to the refractive index of the environment and the relative refractive index distribution of the flow field to be measured;

Step A6: Calculate the density distribution of the flow field to be measured according to the refractive index distribution and the Glaston-Doyle coefficient of the flow field to be measured;

Step A7: Calculate the temperature distribution of the flow field to be measured based on the density distribution of the flow field to be measured and the equation of state of the ideal gas;

Step A8: According to the temperature distribution of the flow field to be measured, draw a cloud map of the temperature distribution of the flow field to be measured;

When the flow field to be measured is a flame, its temperature distribution cloud diagram is shown in Figure 7.

Based on the measurement device provided by the above embodiment of the present application, as shown in FIG. 8 , the embodiment of the present application provides a method for measuring a flow field based on a variable background, including the following steps:

Step 201: the background pattern generation module generates an optimal background pattern according to the camera resolution and field of view based on a preset background pattern generation algorithm, and sends the optimal background pattern to the liquid crystal panel;

Step 202: use a camera to image the optimal background pattern displayed by the liquid crystal panel to obtain a first image;

Step 203: place the flow field to be measured between the liquid crystal panel and the camera, and use the camera to image the flow field to be measured and the optimal background pattern to obtain a second image;

Step 204: The image processing module processes the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured.

As a possible implementation manner, before step 201, it further includes:

Set the camera resolution and capture frame rate;

Input the resolution of the camera and the size of the flow field to be measured into the background pattern generation module.

As a possible implementation manner, the background pattern generation module generates an optimal background pattern according to the camera resolution and field of view based on a preset background pattern generation algorithm; including:

Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera;

Step S2: calculating the actual size of the actual background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel;

Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the side length of the square;

Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern;

Step S5: generate the histogram of the background pattern, thereby calculating the grayscale average value of the background pattern;

Step S6: determine whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold, if so, if the average gray level is greater than 0.5, then reduce the sampling radius R, if the average gray level is less than 0.5, then increase the sampling radius R, Go to step S4; otherwise, go to step S7;

Step S7: take the generated background pattern as the optimal background pattern, and send the optimal background pattern to the liquid crystal panel.

As a possible implementation manner, before step 202, it further includes:

Adjust the LED parallel backlight to the maximum brightness and project the light on the LCD panel;

Adjust the height and level of the camera so that its field of view completely covers the flow field area to be measured and the optimal background pattern behind the area to be measured.

As a possible implementation manner, the image processing module processes the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured: including:

obtain the first image and the second image;

Perform preprocessing on the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image;

Select a cross-correlation window, perform cross-correlation processing on the preprocessed first image and the preprocessed second image, and obtain the offset distribution of the background pattern in the entire flow field area;

Obtain the relative refractive index distribution of the flow field to be measured according to the offset distribution of the background pattern in the entire flow field area;

According to the ambient refractive index and the relative refractive index distribution of the flow field to be measured, the refractive index distribution of the flow field to be measured is calculated;

Calculate the density distribution of the flow field to be measured according to the relative refractive index distribution of the flow field to be measured;

According to the density distribution of the flow field to be measured, the temperature distribution of the flow field to be measured is calculated.

As a possible implementation manner, after obtaining the offset of each pixel of the background pattern of the entire flow field area to be measured, it includes:

From the background pattern of the flow field region to be measured, select a region of interest that needs parameter processing or a region of interest with axis symmetry, where the number of rows of the region of interest is t, and the number of columns is q;

Starting from the first line of the area of interest, extract the positions of the maximum and minimum values of the offset in the first line of data, calculate the average value of the abscissa of the two positions, and take the integer part of the average value of the abscissa as the first line The initial value m of the symmetry axis of the data;

Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius;

The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [m-b, m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. Cross-correlation coefficient, a total of 2b+1 cross-correlation coefficients are obtained;

作为第一行的对称轴位置；Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row;

；Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

;

的平均值，取平均值的整数部分作为整个待测流场区域对称轴的横坐标。calculate

The average value of , and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field area to be measured.

It should be noted that although several units or sub-units of the apparatus are mentioned in the above detailed description, this division is merely exemplary and not mandatory. Indeed, according to embodiments of the present application, the features and functions of two or more units described above may be embodied in one unit. Conversely, the features and functions of one unit described above may be further subdivided to be embodied by multiple units.

Furthermore, although the operations of the methods of the present application are depicted in the figures in a particular order, this does not require or imply that the operations must be performed in the particular order, or that all illustrated operations must be performed to achieve desirable results. Additionally or alternatively, certain steps may be omitted, multiple steps may be combined to be performed as one step, and/or one step may be decomposed into multiple steps to be performed.

While the preferred embodiments of the present application have been described, additional changes and modifications to these embodiments may occur to those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be construed to include the preferred embodiment and all changes and modifications that fall within the scope of this application.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: The technical solutions described in the foregoing embodiments can still be modified, or some or all of the technical features thereof can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the embodiments of the present application. scope.

Claims (10)

1. a flow field measurement device based on variable background, is characterized in that, comprises: LED parallel backlight, liquid crystal panel, camera, image processing module and background pattern generation module; The LED parallel backlight is used for emitting parallel light sources to the liquid crystal panel; The background pattern generation module is used to perform the following steps: Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera; Step S2: Calculate the actual size of the background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel; Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the side length a; Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern; Step S5: generate the histogram of the background pattern, thereby calculating the grayscale average value of the background pattern; Step S6: judge whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold, if so, if the average gray level is greater than 0.5, then reduce the sampling radius R, if the average gray level is less than 0.5, then increase the sampling radius R, Go to step S4; otherwise, go to step S7; Step S7: using the generated background pattern as the optimal background pattern, and sending the optimal background pattern to the liquid crystal panel; The liquid crystal panel is used to display an optimal background pattern; The camera is used to image the flow field to be measured before the optimal background pattern is not placed and the flow field to be measured is placed before the optimal background pattern to obtain a first image and a second image; The image processing module is used to process the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured. 2 . The variable background-based flow field measurement device according to claim 1 , wherein a parallel light film is arranged on the LED parallel backlight. 3 . 3 . The variable background-based flow field measurement device according to claim 1 , wherein the size of the LED parallel backlight and the liquid crystal panel is determined according to the size of the flow field to be measured. 4 . 4. The flow field measurement device based on variable background according to claim 1, wherein the image processing module is specifically used for: obtain the first image and the second image; Perform preprocessing on the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image; Select the cross-correlation window, perform cross-correlation processing on the preprocessed first image and the preprocessed second image, and obtain the offset of each pixel of the background pattern of the entire flow field area to be measured, thereby obtaining the entire to-be-measured flow field. The offset distribution of the background pattern in the flow field region; Obtain the relative refractive index distribution of the flow field to be measured according to the offset distribution of the background pattern in the entire flow field to be measured; According to the ambient refractive index and the relative refractive index distribution of the flow field to be measured, the refractive index distribution of the flow field to be measured is calculated; Calculate the density distribution of the flow field to be measured according to the relative refractive index distribution of the flow field to be measured; According to the density distribution of the flow field to be measured, the temperature distribution of the flow field to be measured is calculated. 5. The flow field measurement device based on variable background according to claim 4, characterized in that, after obtaining the offset of each pixel of the background pattern of the entire flow field region to be measured, it comprises: From the background pattern of the flow field region to be measured, select a region of interest that needs parameter processing or a region of interest with axis symmetry, where the number of rows of the region of interest is t, and the number of columns is q; Starting from the first line of the area of interest, extract the positions of the maximum and minimum values of the offset in the first line of data, calculate the average value of the abscissa of the two positions, and take the integer part of the average value of the abscissa as the first line The initial value m of the symmetry axis of the data; Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius; The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [m-b, m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. Cross-correlation coefficient, a total of 2b+1 cross-correlation coefficients are obtained; Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row; Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

; calculate

The average value of , and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field area to be measured. 6. A flow field measurement method based on variable background, characterized in that, applied to the flow field measurement device based on variable background according to any one of claims 1-5, comprising: The background pattern generation module performs the following steps: Step S1: obtaining the resolution of the camera and the size of the background area captured by the camera; Step S2: Calculate the actual size of the background pattern area corresponding to one pixel of the camera, thereby obtaining the actual side length of the pixel; Step S3: Set the side length a of the black square sampling point to be 2 times or 3 times the actual side length of the pixel, and the initial value of the sampling radius R is 1.5 times the side length a; Step S4: using the side length a and the sampling radius R as parameters, using the Poisson disk sampling method to randomly generate a plurality of black square sampling points on a white background as a background pattern; Step S5: generate the histogram of the background pattern, thereby calculating the grayscale average value of the background pattern; Step S6: determine whether the absolute value of the difference between the average gray level and 0.5 is greater than the threshold, if so, if the average gray level is greater than 0.5, then reduce the sampling radius R, if the average gray level is less than 0.5, then increase the sampling radius R, Go to step S4; otherwise, go to step S7; Step S7: using the generated background pattern as the optimal background pattern, and sending the optimal background pattern to the liquid crystal panel; using a camera to image the optimal background pattern displayed by the liquid crystal panel to obtain a first image; Place the flow field to be measured between the liquid crystal panel and the camera, and use the camera to image the flow field to be measured and the optimal background pattern to obtain a second image; The image processing module processes the first image and the second image according to a preset algorithm to obtain the density and temperature of the flow field to be measured. 7. The method for measuring flow field based on variable background according to claim 6, wherein the method further comprises: Set the camera resolution and capture frame rate; Input the resolution of the camera and the size of the flow field to be measured into the background pattern generation module. 8. The method for measuring a flow field based on a variable background according to claim 6, characterized in that before using the camera to image the optimal background pattern displayed by the liquid crystal panel, the method comprises: Adjust the LED parallel backlight to the maximum brightness and project the light on the LCD panel; Adjust the height and level of the camera so that its field of view completely covers the flow field area to be measured and the optimal background pattern behind the flow field area to be measured. 9 . The flow field measurement method based on variable background according to claim 6 , wherein the image processing module processes the first image and the second image according to a preset algorithm to obtain the density of the flow field to be measured. 10 . and temperature, including: obtain the first image and the second image; Perform preprocessing on the first image and the second image respectively; the preprocessing includes: removing noise interference of the image and smoothing the grayscale histogram of the image; Select a cross-correlation window, perform cross-correlation processing on the preprocessed first image and the preprocessed second image, and obtain the offset distribution of the background pattern in the entire flow field area; Obtain the relative refractive index distribution of the flow field to be measured according to the offset distribution of the background pattern in the entire flow field to be measured; According to the ambient refractive index and the relative refractive index distribution of the flow field to be measured, the refractive index distribution of the flow field to be measured is calculated; Calculate the density distribution of the flow field to be measured according to the relative refractive index distribution of the flow field to be measured; According to the density distribution of the flow field to be measured, the temperature distribution of the flow field to be measured is calculated. 10. The flow field measurement method based on variable background according to claim 9, characterized in that, after obtaining the offset of each pixel of the background pattern of the entire flow field region to be measured, comprising: From the background pattern of the flow field region to be measured, select a region of interest that needs parameter processing or a region of interest with axis symmetry, where the number of rows of the region of interest is t, and the number of columns is q; Starting from the first line of the area of interest, extract the positions of the maximum and minimum values of the offset in the first line of data, calculate the average value of the abscissa of the two positions, and take the integer part of the average value of the abscissa as the first line The initial value m of the symmetry axis of the data; Determine the traversal range of the symmetry axis as [m-b, m+b], where b is the traversal radius; The absolute values of the q offsets in the first row of the area of interest are formed into set B. For each possible symmetry axis in the [m-b, m+b] interval, the data on the left and right sides of the symmetry axis are calculated from the data in set B. Cross-correlation coefficient, a total of 2b+1 cross-correlation coefficients are obtained; Compare the magnitudes of 2b+1 cross-correlation coefficients, and use the coordinate of the symmetry axis corresponding to the largest cross-correlation coefficient

as the position of the symmetry axis of the first row; Follow the above steps to traverse other t-1 lines in the area of interest to obtain the symmetry axis coordinates of the t-1 line

; calculate

The average value of , and the integer part of the average value is taken as the abscissa of the symmetry axis of the entire flow field area to be measured.

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