Apparatus and method for measuring fluid convective-diffusive motion velocity field

Abstract

This invention discloses an apparatus and method for measuring the velocity field of fluid convection and diffusion. The apparatus includes a camera (1), a light source (2), a test water tank (3), a frosted glass (4), and a computer hardware and software system (5) for analyzing and processing image data. This invention uses a parallel or nearly parallel light beam generated by the light source (2) to pass through the test water tank (3). When the light beam passes through the test fluid undergoing convection and diffusion inside the test water tank (3), it is deflected and projected onto the frosted glass (4) to form shadow images of varying brightness. The camera (1) captures the shadow images, which are then transmitted to the computer hardware and software system (5) for analyzing and processing the image data. The computer uses optical flow algorithms to analyze the positional changes of fluid clusters in the shadow images over time, thus obtaining the velocity field of the test fluid convection and diffusion. This invention uses a simple apparatus to obtain the velocity field of convection and diffusion within a fluid, is inexpensive, simple, and convenient to use.

Description

Technical Field

[0001] This invention relates to the field of fluid motion measurement technology, and specifically to a device and method for measuring the velocity field of fluid convection and diffusion. Background Technology

[0002] Convection-diffusion is an important form of fluid motion in the natural environment, encompassing numerous physical phenomena such as pollution diffusion in water bodies, atmospheric convection, and mantle convection. The study of convection-diffusion primarily investigates the motion and variation of a physical quantity (such as temperature or the concentration of substances dissolved in the fluid) carried by fluid particles, with the velocity field of the fluid motion being a key focus. Currently, PIV (Particle Image Velocimetry) is commonly used, indirectly measuring the transient velocity distribution of the flow field by measuring the displacement of tracer particles over a known time interval. If the tracer particles have sufficiently high flow tracking ability, their motion can accurately reflect the motion state of the flow field. PIV systems consist of a laser generator, optical arm, high-speed camera, synchronizer, and other equipment; they are typically expensive, require sophisticated installation and layout, and are not convenient to use.

[0003] Convective diffusion within a fluid is driven by density changes due to variations in temperature or concentration, resulting in buoyancy-driven motion. These temperature or concentration differences also alter the fluid's refractive index. The shadowgraph method uses a light beam passing through fluids with different refractive indices. The light is deflected as it passes through the fluid, creating shadows of varying brightness on the projection plane. The shadowgraph method can capture dynamic shadow images of convection diffusion within a fluid, directly reflecting the convection diffusion process. Optical flow technology analyzes and identifies objects and their motion states in an image by varying the brightness of individual pixels over time. By analyzing and identifying the contours of fluid clumps in shadow images using optical flow technology and comparing the positional changes of these clumps at different times, the velocity field of the convection diffusion motion can be obtained. Summary of the Invention

[0004] To provide a simple and easy method for measuring the velocity field of convective diffusion motion inside a fluid, the velocity field of convective diffusion motion is obtained by combining the shadow map formed by the difference in refractive index of the fluid mass during convective diffusion with optical flow technology.

[0005] This invention provides an apparatus and method for measuring the velocity field of convective diffusion motion inside a fluid. The apparatus includes a camera (1), a light source (2), a test water tank (3), a frosted glass (4), and a computer hardware and software system (5) for analyzing and processing image data. Specifically: the axis of the camera (1) and the axis of the light source (2) are on the same straight line; the test water tank (3) is made of transparent material and is placed between the camera (1) and the light source (2), with both side walls perpendicular to the axis between the camera (1) and the light source (2); the frosted glass (4) is attached to the side wall of the test water tank (3) near the camera (1), and has coordinate scales to determine the actual position of each pixel in the test image; the light source (2) generates a parallel beam of light, which passes through the fluid undergoing convective diffusion motion inside the test water tank (3). Due to the different refractive indices of the fluid clusters undergoing convective diffusion motion, the light beam will deflect differently and will be reflected in the frosted glass. A shadow image with different brightness is formed on the glass (4); the dynamic shadow image formed by the fluid undergoing convective diffusion is captured by the camera (1) and transmitted to the computer hardware and software system (5) for analyzing and processing graphic data. The dynamic shadow image is analyzed and processed by the optical flow technology algorithm on the computer. The shadow image is extracted frame by frame or at equal intervals for analysis. Since the time interval between the two shadow images is very short, the shape of the small fluid clusters undergoing convective diffusion does not change much, and most of the fluid clusters are still within the observation area. The light and dark shadows generated by the movement of each fluid cluster will also move, but the total brightness of the two shadow images is almost unchanged. The pixel brightness of the same small fluid clusters in the two images is the same. P The velocity of the fluid mass is related to ∂ P / ∂ t + u (∂ P / ∂ x ) + v (∂ P / ∂ y ) = 0 (where u for x Directional velocity, v for y Directional velocity, t To determine the relationship between time and volume, the image is divided into multiple units of equal-sized squares. Assuming the fluid motion within each unit is uniform, the velocity of the fluid within that unit can be obtained. u , v The relationship between the brightness of cells at adjacent positions and at adjacent times (the next frame) is as follows:

[0006]

[0007] The above equation is an overdetermined system of equations, from which we can obtain ( u , vThe least squares solution of the experiment is obtained by calculating the velocity field of the previous frame of the image in the observation area unit by unit. By continuously analyzing all images in the experiment, the velocity field of fluid convection and diffusion motion in the observation area at each time period is finally constructed. Attached Figure Description

[0008] Appendix Figure 1 This is a schematic diagram of the device for measuring the velocity field of fluid convection and diffusion according to the present invention. 1 is a camera, 2 is a light source, 3 is a test water tank, 4 is frosted glass, and 5 is a computer hardware and software system. Detailed Implementation

[0009] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0010] Arrange the axis of the camera (1) and the axis of the light source (2) on the same straight line; place the test water tank (3) between the camera (1) and the light source (2), ensuring that the two side walls of the test water tank (3) are perpendicular to the axes of the camera (1) and the light source (2), and attach the frosted glass (4) to the side wall of the test water tank (3) near the camera (1); turn on the light source (2) to form a parallel beam, and the beam passes through the test water tank (3) and is projected onto the frosted glass (4); turn on the camera (1) and connect it to the computer hardware and software system (5), and adjust the camera (1) until it can obtain a clear view of the test area on the frosted glass (4). Based on the coordinate scale position on the frosted glass (4) in the captured image, the actual coordinate value of each pixel in the captured image is calculated using linear interpolation. The test fluid is injected into the test water tank (3) to form convective diffusion motion. The parallel beam generated by the light source (2) passes through the test fluid and projects a dynamic shadow image on the frosted glass (4). The dynamic shadow image is recorded using a camera (1) and input into the computer hardware and software system (5) for analyzing and processing graphic data. The position change of the fluid cluster in the shadow image over time is analyzed by the computer using optical flow technology algorithm to obtain the velocity field of the convective diffusion motion of the test fluid.

Claims

1. A method for measuring the velocity field of fluid convection and diffusion, using an apparatus for measuring the velocity field of fluid convection and diffusion, comprising a camera (1), a light source (2), a test water tank (3), frosted glass (4), and a computer hardware and software system (5), characterized in that, The axis of the camera (1) coincides with the axis of the light source (2). The test water tank (3) is made of transparent material and is arranged between the camera (1) and the light source (2). The side wall of the test water tank (3) near the camera (1) is perpendicular to the axis of the camera (1), and the side wall near the light source (2) is perpendicular to the axis of the light source (2). The method includes the following steps: Step 1: Turn on the light source (2) to form a parallel beam. The beam passes through the test water tank (3) and is projected onto the frosted glass (4). Turn on the camera (1) and connect it to the computer hardware and software system (5). Adjust the camera (1) until it can acquire a clear image of the test area on the frosted glass (4). Calculate the actual coordinate values ​​of each pixel in the captured image based on the coordinate scale on the frosted glass (4). Step 2: Inject the test fluid into the test water tank (3) to form convective diffusion motion. The light beam generated by the light source (2) passes through the test fluid and forms a dynamic shadow image on the frosted glass (4). Use the camera (1) to record the dynamic shadow image and input it into the computer hardware and software system (5) for analysis and processing of graphic data. Step 3: Analyze the shadow image frame by frame or at equal frame intervals using a computer. Divide two adjacent frames or two frames at equal frame intervals into units. Each unit is a square of the same size with a side length of 8 to 32 pixels. Establish a set of equations relating the fluid velocity and pixel brightness changes within each unit based on optical flow algorithms. Calculate the fluid velocity within each unit to obtain the velocity field of the experimental fluid convection and diffusion motion at the previous frame. Step 4: Analyze the dynamic shadow image frame by frame or at equal frame intervals to obtain the velocity field of fluid convection and diffusion motion at each moment during the experiment.

2. The method for measuring the velocity field of fluid convection and diffusion as described in claim 1, characterized in that, The light source (2) consists of a point light source and a lens, and the light beam emitted by the light source is a parallel light beam.

3. The method for measuring the velocity field of fluid convection and diffusion as described in claim 1, characterized in that, The frosted glass (4) is attached to the outside of the side wall of the test water tank (3) near the camera (1), and there are coordinate scales on the frosted glass.

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