An experimental correction method and system for optical distortion at density interfaces in density stratified fluids

Abstract

The application discloses a kind of experimental correction method and system of optical deformation of density interface in density stratified fluid, method steps: S1, obtains the continuous original image of spherical particle cross-interface, control adjacent image particle displacement is not more than average pixel radius, set particle falling direction as y axis positive direction;S2, select the image i before particle enters interface, get y coordinate by profile, build the mapping array of corrected y' and original y;S3, select the i+n (n≥1, indicate that it is processed n image) image, get y coordinate and particle center by profile, determine particle center correction position according to mapping array, obtain its corrected y' by formula for specific line, add mapping array;S4, repeat S3 to obtain complete mapping array;S5, according to complete mapping array, correct subsequent image.The application constructs mapping array in stages to realize accurate correction, and can improve the reliability of related experimental image measurement data.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of density stratified fluid, in particular, to a method and system for correcting optical distortion of density interface in density stratified fluid. BACKGROUND

[0002] Density stratification of fluid is a common phenomenon in nature, such as atmospheric stratification, halocline and thermocline in the ocean. Density stratification changes the settling characteristics of immersed particles, mainly manifested as increased particle resistance, slowed down settling velocity, and intensified aggregation. Many large-scale environmental problems are closely related to the density stratification of fluid, such as the diffusion of haze, the aggregation of ocean snow, the spread and diffusion of marine microplastics, and the accumulation of sediment at river estuaries. Configuring stratified fluid in an experimental environment and conducting related measurements is an important means to understand and master the rules of particle settling and aggregation in stratified fluid.

[0003] In experiments, solutions containing different concentrations of media are generally used to prepare density stratified fluid, such as sodium chloride solution (brine), glycerol and water, and a mixture of water and alcohol. The change in solution density also brings about a difference in optical refractive index, causing optical distortion of the image of the density interface object, which seriously reduces the measurement accuracy of particle position and velocity.

[0004] When particles pass through the density interface, they will experience severe optical distortion, making it difficult to measure their position directly from the image. There is a lack of methods to correct such optical distortion in existing literature and patents. The existing methods mainly have two kinds, one is to use a ruler containing scales, which is placed in the vertical plane through which the particles settle before the experiment, and an image of the ruler is taken, then the scale pixel coordinates are extracted for the actual particle space position calibration. This method not only increases the disturbance of the flow field, which can accelerate the mixing and diffusion of the density interface, but also can only calibrate the real coordinates of the scale position, with limited measurement accuracy of millimeter level. The other way is not to directly deal with optical distortion, but to use three or more solutes to adjust the optical refractive index of the fluid, so that the fluid density changes while maintaining the same refractive index. This method not only increases the experimental cost, but also limits the density range of the fluid particles, reduces the parameter space for particle measurement, and easily brings about large changes in viscosity, affecting the experimental control. The present application provides an optical correction method based on particle image, which can achieve pixel-level calibration accuracy and does not bring additional disturbance to the flow field, and can be effectively applied to planar PIV and PTV measurement of stratified fluid. SUMMARY

[0005] The present application aims at the image distortion problem caused by optical refractive index change in density stratified flow, and provides a method and system for position correction by using the size characteristics of particles, which can effectively correct the distorted particle image without adding additional disturbance, and improve the calculation accuracy of particle position and velocity.

[0006] The present application is implemented by adopting the following technical solutions:

[0007] An experimental correction method for optical distortion of density interface in density stratified flow, comprising the following steps:

[0008] S1, obtaining a plurality of continuous original images of a spherical particle passing through a density interface process, the displacement of the spherical particle between adjacent original images being not more than the average pixel radius of the spherical particle, and the falling direction of the spherical particle being taken as the positive direction of the y-axis;

[0009] S2, from the continuous original images, taking an image i before the spherical particle enters the density interface, extracting the spherical particle contour to obtain the y coordinates of all rows of the spherical particle and the center of the spherical particle in the image i, and constructing a mapping array of the y-axis pixel coordinates y' of the corrected spherical particle and .

[0010] S3, taking an i+n image, n≥1, representing n processed images, extracting the spherical particle contour to obtain the y coordinates of all rows of the spherical particle and the center of the spherical particle in the image i+n, and obtaining the center correction position of the spherical particle in the image i+n according to the mapping array in S2; for the rows in the image i+n whose y coordinates are greater than the center coordinate of the spherical particle but less than the bottom position coordinate of the spherical particle, the corresponding row corrected y-axis coordinates y' are obtained through the correction formula, and the y' obtained in S3 and the corresponding y are added to the mapping array.

[0011] S4, repeatedly processing the remaining images in S3 to obtain a complete mapping array of the original image y-axis pixel position and the corrected position y'.

[0012] S5, correcting the subsequent images in the density stratified flow based on the mapping array obtained in S4.

[0013] Further, in S1, the average pixel radius of the spherical particle is obtained through the original image of the undeformed spherical particle.

[0014] Further, in S2, the mapping array of the y-axis pixel coordinates y' of the corrected spherical particle and is specifically:

[0015] The spherical particles in the image i and the images before the image i are not deformed, and the mapping relationship is y'=y. .

[0016] Further, in S3, the center acquisition method of the spherical particle is:

[0017] The pixel width corresponding to all rows of the spherical particle in the image i+n is obtained, and the maximum pixel width is the center of the spherical particle.

[0018] Further, in S3, the corrected y-axis coordinate y' corresponding to the row is obtained through the correction formula, and the specific formula is:

[0019] y'= ;

[0020] wherein, is the center correction position of the spherical particle in the image i+n, r is the average pixel radius of the spherical particle, is the pixel width corresponding to the y row in the image i+n.

[0021] An experimental correction system for optical deformation of a density interface in a density stratified fluid is used to implement any of the described methods.

[0022] Further, the system comprises:

[0023] An image acquisition module, which is used to acquire a plurality of continuous images of the spherical particle passing through the density interface through the image acquisition module;

[0024] A data processing and storage module, which is used to process the images and store a mapping array through the data processing and storage module, so as to correct subsequent images in the density stratified fluid.

[0025] Further, the image acquisition module adopts a high-speed camera, and the data processing and storage module adopts a computer.

[0026] Beneficial effects:

[0027] The present application corrects the optical deformation of the particle after entering the density interface by using the shape characteristics of the spherical particle itself and the correction formula, so that accurate particle position information is obtained, and the operation is simple. BRIEF DESCRIPTION OF DRAWINGS

[0028] The present application will be further described below in combination with the drawings and examples;

[0029] Figure 1 is a correction principle diagram of an embodiment of the present application, wherein the black solid line profile represents the actual photographed deformed particle profile, and the blue dashed line represents the corrected particle profile, and the mapping relationship between the original position and the corrected position can be established according to the diagram.

[0030] Figure 2is the original particle image collected in an embodiment of the present application, and the particles near the density interface are obviously deformed.

[0031] Figure 3 is the particle contour extracted from the original image in an embodiment of the present application, which is used for particle pixel position and width statistics.

[0032] Figure 4 is the particle horizontal pixel width in an embodiment of the present application The graph of the change of y with the vertical direction pixel position.

[0033] Figure 5 is the mapping relationship between the original pixel coordinate y and the corrected pixel coordinate y' of the particle obtained in an embodiment of the present application.

[0034] Figure 6 is the corrected particle image established according to the mapping relationship in an embodiment of the present application. DETAILED DESCRIPTION

[0035] The present application will be further described below in conjunction with the drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the present application, but not to limit the present application. In addition, it should be noted that, for the convenience of description, only the parts related to the present application are shown in the drawings, but not all the process steps, structures and experimental results.

[0036] A specific embodiment, an experimental correction method of optical deformation of density interface in a density stratified fluid, comprising the following steps:

[0037] S1, obtaining a plurality of continuous original images of the process of spherical particles passing through the density interface, the displacement of the spherical particles between adjacent original images is not more than the average pixel radius of the spherical particles, and the falling direction of the spherical particles is taken as the positive direction of y axis;

[0038] S2, from the continuous original images, taking the image i before the spherical particles enter the density interface, extracting the spherical particle contour, obtaining the y coordinates of all rows of the spherical particles, and constructing the mapping array of the y axis pixel coordinate y' of the corrected spherical particles and ;

[0039] S3, taking the i+n image, n≥1, indicating that n images are processed, extracting the spherical particle contour, obtaining the y coordinates of all rows of the spherical particles and the center of the spherical particles in the image i+n, and obtaining the center correction position of the spherical particles in the image i+n according to the mapping array in S2; for the rows whose y coordinates are greater than the center coordinate of the spherical particles but less than the bottom position coordinate of the spherical particles in the image i+n, the corresponding row corrected y axis coordinate y' is obtained through the correction formula, and the y' obtained by S3 and the corresponding y are added to the mapping array.

[0040] S4, repeat S3 to process the remaining images in sequence to obtain a complete mapping array of the original image y-axis pixel position and the corrected position y';

[0041] S5, correct the subsequent images in the density stratified fluid based on the mapping array obtained in S4.

[0042] Example 1:

[0043] Step one: take pictures of the process of spherical particles passing through the density interface to obtain a series of continuous particle images, as shown in FIG. 1. Ensure that the particle displacement between adjacent images does not exceed the particle radius. Due to the change in optical refractive index, there is obvious optical deformation in the particle image within the density interface. In this example, the image resolution is 2560*1600 pixels, and the vertical direction pixel coordinate y takes the value range [1, 2560], increasing from top to bottom. Figure 2

[0044] Step two: count the average pixel radius of the particle image in the undeformed area, denoted as r. In this example, r = 63, indicating that the particle radius occupies 63 pixel points in width.

[0045] Step three: start processing from any image before the particle enters the density interface, assuming that the image is i. In this example, we start processing from the first image, taking i = 1. First, extract the particle contour, as shown in FIG. 2. Record the particle image position y value and the pixel width corresponding to each row Figure 3 . In this example, , as y changes, as shown in FIG. 3. The maximum width is the position of the particle center . If the maximum value is not unique, take the average y value. In this example, Figure 4 . Denote the corrected vertical direction pixel coordinate as y'. Since the particle in this image has not yet deformed, the mapping array y' = y can be directly constructed, denoted as the pixel coordinate corresponding to the bottom position of the particle at this time . Extend the mapping relationship to the range , and the mapping relationship above the density interface follows y' = y.

[0046] Step four: process the i+n image, n ≥ 1, indicating that n images have been processed, i = 1, n = 1. First, extract the particle contour y value and the pixel width corresponding to each row , determine the center coordinates of the spherical particle according to the maximum width position . Since the particle displacement on the adjacent image is less than the particle radius, there is , and ​​​​​The range of the mapping array is extended to The corrected position y' of the bottom of the particle in the image is calculated according to the width of the row The pixel coordinate corresponding to the bottom of the particle in the image is The newly calculated y' and the corresponding y are added to the mapping array, and the mapping range is extended to . Step five: repeat step four to process the remaining images, and obtain the complete mapping relationship between the pixel position and the corrected position of the original image. In this example, the mapping relationship between y and y' is shown in Table 1.

[0047] Step six: use the obtained mapping relationship to calculate the corrected position of each pixel in the original image, and perform image correction. In this example, the corrected image is shown in Table 2. Figure 5

[0048] Step six: use the obtained mapping relationship to calculate the corrected position of each pixel in the original image, and perform image correction. In this example, the corrected image is shown in Table 2. Figure 6 Figure 1 In the correction formula, δy is .

[0049] Example 2:

[0050] An experimental correction system for optical deformation of a density interface in a density stratified fluid, used to implement the method of example 1.

[0051] The image acquisition module is used to acquire a plurality of continuous images of the spherical particles passing through the density interface process, and the data processing and storage module is used to process the images and store the mapping array, which is used to correct the subsequent images in the density stratified fluid. The image acquisition module uses a high-speed camera, and the data processing and storage module uses a computer.

[0052] The above is only a preferred embodiment of the present application, and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation, or direct or indirect application in other related technical fields, as described in the specification and drawings of the present application, are also included in the patent protection scope of the present application.​​​

Claims

1. A method of experimental correction of optical distortion at a density interface in a density stratified fluid, characterized by, The method comprises the following steps: S1, obtaining a plurality of continuous original images of the spherical particles in a density interface process, the displacement of the spherical particles between adjacent original images being not more than an average pixel radius of the spherical particles, and a falling direction of the spherical particles being taken as a positive direction of a y-axis; S2, from the continuous original image, taking the image i of the spherical particles before entering the density interface, extracting the spherical particle contour, obtaining the y coordinate of all rows of the spherical particles, and constructing the mapping array of the y axis pixel coordinates y' of the corrected spherical particles and the y axis pixel positions of the original image , specifically: the spherical particles in the image i and the images before the image i do not deform, and the mapping relationship is y' = y ; S3, taking an i+n image, n≥1, representing that n images are processed, extracting a contour of the spherical particles to obtain a y coordinate of all rows of the spherical particles and a center of the spherical particles in the i+n image, specifically, obtaining a pixel width corresponding to all rows of the spherical particles in the i+n image, determining a y-axis pixel coordinate corresponding to a maximum pixel width as the center of the spherical particles, if the maximum pixel width is not unique, taking an average value of the y-axis pixel coordinates as the center of the spherical particles, obtaining a center correction position of the spherical particles in the i+n image according to the mapping array in S2, and obtaining a y-axis pixel coordinate y' of the spherical particles in the i+n image through a correction formula for a row whose y coordinate is greater than the center coordinate of the spherical particles but less than a bottom position coordinate of the spherical particles, and adding the y' obtained in S3 and a corresponding y into the mapping array; S4, repeatedly processing the remaining images in sequence according to S3 to obtain a complete mapping array of the y-axis pixel positions of the original images and the y-axis pixel coordinates y' of the spherical particles after correction; S5, correcting subsequent images in the density layered fluid based on the mapping array obtained in S4; In S3, the y-axis pixel coordinate y' of the spherical particles after correction for a corresponding row is specifically obtained through a correction formula. y'= ； wherein, is the center corrected position of the spherical particle in the image i + n, r is the average pixel radius of the spherical particle, is the pixel width corresponding to the y row in the image i + n.

2. The method of claim 1, wherein, In S1, the average pixel radius of the spherical particles is obtained through an original image of a non-deformed spherical particle.

3. An experimental calibration system for optical distortion at a density interface in a density stratified fluid, comprising: The system is used for implementing the method in any one of claims 1-2, and comprises: an image acquisition module, through which a plurality of continuous images of the spherical particles in a density interface process are obtained; a data processing and storage module, through which a mapping array is processed and stored for correcting subsequent images in the density layered fluid.

4. The system of claim 3, wherein, The image acquisition module is a high-speed camera, and the data processing and storage module is a computer.

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