Apparatus and method for measuring spatial three-dimensional velocity field of constant flow

Abstract

The application discloses a device and method for measuring constant flow space three-dimensional velocity field, and belongs to the technical field of fluid motion measurement. The device comprises a camera (1), two or more light sources (2) with different colors, a test water tank (3), tracer particles (4), frosted glass (5) and a coordinate grid (6). The application uses two or more light sources (2) with different colors to irradiate the test water tank (3), so that the tracer particles (4) moving with the water flow in the test water tank (3) generate point-like shadows with different colors on the frosted glass (5); according to the positions of the corresponding color point-like shadows, the three-dimensional coordinate values of the tracer particles (4) are calculated by using a specific algorithm; the three-dimensional coordinate value changes of the same tracer particles (4) at different time points are compared, and the three-dimensional flow velocity of the space position can be obtained; and the three-dimensional velocity field of the constant flow of the observation space can be constructed by continuous observation. The application can obtain the three-dimensional velocity field of the constant flow space by using simple equipment, and is low in cost, simple in method and convenient to use.

Description

Technical Field

[0001] This invention relates to the field of fluid motion measurement technology, specifically to a device and method for observing the three-dimensional velocity field in a steady flow spatial region. Background Technology

[0002] Fluid motion measurement technology is an important tool in experimental fluid mechanics, playing a significant role in studying and solving a range of fluid mechanics problems in hydraulic and marine engineering. Flow field visualization methods are commonly used to measure fluid motion. Early measurements involved injecting dyes, floating objects, or tracer particles into the fluid as tracers, followed by intermittent photography to obtain the tracer's position at different times, thus revealing its trajectory and velocity. With the development of image processing technology, PIV (Particle Image Velocimetry) has rapidly advanced. It features non-contact, full-field measurement capabilities and can provide rich flow field information. However, due to the use of laser sheet light sources, PIV technology can only measure two-dimensional or three-dimensional flow fields in a plane. Measuring three-dimensional flow fields in space requires setting up multiple sheet light sources or continuously moving a single sheet light source, resulting in very complex and expensive experimental equipment, hindering widespread adoption. Summary of the Invention

[0003] A simple and easy-to-use device and method are provided for measuring the three-dimensional velocity field of a constant flow in a fluid spatial region. After continuous measurement and analysis, the information of the three-dimensional velocity field of the constant flow in the measurement spatial region is obtained.

[0004] This invention provides a novel three-dimensional velocity field observation device for a constant flow space, comprising a camera (1), two or more light sources (2) of different colors, a test water tank (3), tracer particles (4), frosted glass (5), and a coordinate grid (6). The light sources (2) include two or more light sources of different colors, and the light emitted by the light sources is a parallel beam or a nearly parallel beam; the test water tank (3) is made of transparent material and is filled with a fluid with a constant flow state; the tracer particles (4) are opaque particles with a density close to that of the test fluid, and their movement can be regarded as consistent with the movement of the test fluid at their location. The number of tracer particles (4) in the test fluid should not be too large to ensure that the average spacing of the tracer particles (4) is greater than the longitudinal coordinate deviation value Δx; the coordinate grid (6) is a transparent thin sheet with black coordinate grid lines on it. When the measurement is carried out, the axis of the camera (1) and the axes of each light source (2) are arranged in the same plane. The axis of the camera (1) and the axis of the light source (2) are at an angle of 0° to 70°; the axes of each light source are at an angle of 30° to 140°; the test water tank (3) is located between the camera (1) and the light source (2).

[0005] Another aspect of the present invention provides a method for observing a three-dimensional velocity field in a constant flow space based on the above-mentioned device, comprising the following steps:

[0006] Step 1: Inject test fluid into test water tank (3), and after debugging, ensure that the flow state of test fluid remains constant. Place coordinate grid (6) on the outside of one side wall of light source (2) in test water tank (3). Under the condition of using only ambient scattered light, use camera (1) to capture the original position of coordinate grid (6). Determine the x and z coordinate values ​​of each pixel in the captured image based on the original position of coordinate grid (6).

[0007] Step 2: Place a camera (1) on one side wall of the test water tank (3) and attach frosted glass (5) to block the ambient light. Turn on each color light source (2) separately, use the camera (1) to capture the projection of the coordinate grid (6) on the frosted glass (5), and calculate the longitudinal coordinate deviation value Δx of the original position of different x coordinate values ​​on the coordinate grid (6) when different color light sources (2) are irradiated. After completion, remove the coordinate grid (6).

[0008] Step 3: Block the ambient light and turn on all light sources (2). Spread tracer particles (4) in the test fluid with constant flow. Stop spreading tracer particles (4) when the average spacing of tracer particles (4) in the test fluid is less than the longitudinal coordinate deviation value Δx.

[0009] Step 4: Use camera (1) to capture test video, analyze test images frame by frame or at equal intervals, and search for the projection of tracer particles (4) on frosted glass (5) layer by layer from bottom to top in the vertical (Z-axis direction) of each frame image. Find a set of projections produced by the particles with the same z coordinate value, the same number as the number of light sources, but different colors, record the z coordinate value of the particles, and then determine the direction of the light source according to the color of the particle projection and record the vertical coordinate position x' of the projection produced by different light sources. At the same time, calculate the corresponding vertical coordinate deviation value Δx. The actual y coordinate value and x coordinate value of the particles can be calculated from the vertical coordinate position of the projection and the vertical coordinate deviation value. According to the time interval Δt of the test images and the spatial position of the particles at different times, calculate the three-dimensional flow velocity data of the corresponding spatial position in the test tank (3).

[0010] Step 5: Divide the observation space into units, each unit being a cube of the same size. The side length of the cube is 5.0 to 8.0 times the average flow velocity of the test fluid multiplied by the interval time Δt of the test images.

[0011] Step 6: Continue measuring until the velocity data of each unit in the observation space is greater than 30. Take the median of the velocity data of each unit in the observation space to construct the three-dimensional velocity field of steady flow in the observation area. Attached Figure Description

[0012] AppendixFigure 1 This is a schematic diagram of the measurement process device arrangement of the present invention. The X-axis is longitudinal, the Y-axis is transverse, the Z-axis is vertical, 1 is a camera, 2 is a light source, 3 is a test water tank, 4 is a tracer particle, and 5 is frosted glass.

[0013] Appendix Figure 2 This is a schematic diagram of the calibration process device layout and a schematic diagram of the tracer particle position calculation of the present invention. The X-axis is vertical, the Y-axis is horizontal, the Z-axis is vertical, 5 is frosted glass, and 6 is the coordinate grid. Detailed Implementation

[0014] 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.

[0015] Taking a device equipped with two approximately parallel light sources, red and green, as an example, the specific implementation of the present invention will be described.

[0016] The axes of the red and green light sources (2) are arranged on the same plane as the axis of the camera (1). The angle between the axis of the camera (1) and the axes of the red and green light sources (2) is 45°. The red light source is S1 and the green light source is S2. The angle between the axes of the two light sources is 90°. A transparent test water tank (3) made of plexiglass is placed between the light source (2) and the camera (1). A transparent fluid is injected into the test water tank (3). After debugging, a constant flow to be tested is formed in the test water tank (3). The red and green light sources (2) are not turned on. A coordinate grid (6) is attached to the outside of the side wall of the test water tank (3) where the light source (2) is placed. The original position of the coordinate grid (6) is captured by the camera (1) under the condition of using only ambient scattered light. The x and z coordinate values ​​of each pixel in the captured image are calculated by linear interpolation based on the position of the coordinate grid (6) on the captured image. The elevation coordinates of the test image (i.e., the x and z coordinate values ​​of each pixel in the test image corresponding to the actual space) are established. Place a camera (1) on one side wall of the test tank (3) and attach frosted glass (5) to block ambient light. Turn on the red light source S1 and use the camera (1) to capture the projection of the coordinate grid (6) onto the frosted glass (5). Compare the coordinate projection generated by the red light source S1 with the original coordinate position to calculate the longitudinal coordinate deviation value Δx1. Since the light source emits an approximately parallel beam, the longitudinal coordinate deviation value Δx1 has a linear relationship with the original coordinate value x. Therefore, the longitudinal coordinate deviation value Δx1 generated by the red light source S1 is Δx1 = C1 * x, where C1 is a constant coefficient. Turn off the red light source S1 and turn on the green light source S2. Repeat the above steps to obtain the longitudinal coordinate deviation value Δx2 generated by the green light source S2 is Δx2 = C2 * x, where C2 is a constant coefficient. Remove the coordinate grid (6) to complete the calibration process.

[0017] Block out ambient light, turn on the red and green light sources (2), turn on the camera (1) to continuously capture images, and slowly release the tracer particles (4). If the average spacing of the tracer particles (4) is found to be less than the average value of the longitudinal coordinate deviation during the experiment, the release should be paused. Continuously analyze two adjacent frames or two frames with equal intervals. When analyzing each test image, search along the Z-axis from bottom to top. When two adjacent red and green tracer particle projections with the same z-coordinate value are found, it is considered that the two projections are generated by the same tracer particle, and the z-coordinate value is recorded. The red projection is generated by the green light source S2, and its longitudinal coordinate value is x'2; the green projection is generated by the red light source S1, and its longitudinal coordinate value is x'1; based on the longitudinal coordinate values ​​of these two projections, the longitudinal coordinate deviation values ​​Δx2=C2*x'2 and Δx1=C1*x'1 can be calculated; based on the formulas x=(Δx1*x'2+Δx2*x'1) / (Δx1+Δx2) and y=b(x'1+x'2) / (Δx1+Δx2), the x and y coordinate values ​​of the tracer particle can be calculated, where b is the width of the test tank (3). By comparing the spatial coordinate changes of the tracer particle in the two test images and the image interval time Δt, the instantaneous three-dimensional velocity (u) of the actual spatial position of the tracer particle at the previous frame time can be obtained. x , u y , u z At this point, it is important to note that the positional change of the same tracer particle in different experimental images should not exceed the minimum longitudinal coordinate deviation value; otherwise, the interval Δt between two frames needs to be shortened.

[0018] The observation space was divided into units of equal-sized cubes, with the cube side length being 6.67 times the average flow velocity of the test fluid multiplied by the interval Δt between test images. Continuous observation was performed to acquire three-dimensional flow velocity data (u) for each unit of the observation space. x , u y , u z When the velocity data of all units is greater than 30, the observation can be stopped. The three-dimensional velocity measurement value of each unit is obtained by taking the median of the velocity data of each unit in the X, Y and Z axes. Based on this, the three-dimensional velocity field of the steady flow in the observation area is constructed.

Claims

1. A method for measuring a three-dimensional velocity field in a steady-flow space, using an apparatus for measuring a three-dimensional velocity field in a steady-flow space, said apparatus comprising a camera (1), a light source (2), a test tank (3), tracer particles (4), frosted glass (5), and a coordinate grid (6), characterized in that, The axis of the camera (1) and the axis of the light source (2) are arranged in the same plane. The axis of the camera (1) and the axis of the light source (2) are at an angle of 0° to 70°. The light source (2) contains two or more light sources of different colors. The axes of each light source are at an angle of 30° to 140°. The light emitted by the light source is a parallel beam or a nearly parallel beam. The test water tank (3) is made of transparent material and is arranged between the camera (1) and the light source (2). The water tank is filled with a fluid with a constant flow state. The tracer particles (4) are opaque particles with a density close to that of the test fluid. The coordinate grid (6) is a transparent sheet with black coordinate grid lines on it. The coordinate grid lines are in the direction of the longitudinal x-axis along the direction of fluid movement. The coordinate grid lines are perpendicular to the x-axis and perpendicular to the z-axis. The direction perpendicular to the plane where the coordinate grid (6) is located is the transverse y-axis. The method for measuring the three-dimensional velocity field of a constant flow space includes the following steps: Step 1: Inject a fluid with a constant flow state into the test tank (3). Place a coordinate grid (6) on the outside of one side wall of the test tank (3) where a light source (2) is placed. Use a camera (1) to capture the original position of the coordinate grid (6) under the condition of using only ambient scattered light. Determine the x and z coordinate values ​​of each pixel in the captured image based on the original position of the coordinate grid (6). Step 2: Place a camera (1) on one side wall of the test water tank (3) and attach frosted glass (5) to block the ambient light. Turn on each color light source (2) separately, use the camera (1) to capture the projection of the coordinate grid (6) on the frosted glass (5), and calculate the longitudinal coordinate deviation value Δx of the original position of different longitudinal coordinate values ​​on the coordinate grid (6) when illuminated by different color light sources. After completion, remove the coordinate grid (6) to complete the calibration process. Step 3: Block the ambient light and turn on all light sources (2). Spread tracer particles (4) in the test fluid with constant flow. When the average spacing of the tracer particles (4) in the test fluid is less than the average value of the longitudinal coordinate deviation, temporarily stop spreading the tracer particles (4). Step 4: Use camera (1) to capture test video, analyze test images frame by frame or at equal intervals, and search for the projection of tracer particles (4) on frosted glass (5) layer by layer from bottom to top in each frame. After identifying a set of projections produced by the particles with the same z coordinate value, the same number as the number of light sources, but different colors, record the z coordinate value of the identified particles. Then, determine the direction of the light source based on the color of the particle projection and record the vertical coordinate position x' of the projection produced by different light sources. Calculate the corresponding vertical coordinate deviation value Δx. Calculate the actual y coordinate value and x coordinate value of the identified particles from the projection vertical coordinate position x' and the vertical coordinate deviation value Δx. Calculate the three-dimensional flow velocity data of the spatial position of the identified particles at the first frame time based on the interval time Δt between two test images and the spatial position change of the same identified particles in the two test images. Step 5: Divide the observation space into units, each unit being a cube of the same size. The side length of the cube is 5.0 to 8.0 times the average flow velocity of the test fluid multiplied by the interval time Δt of the test images. Step 6: Continue measuring until the velocity data of each unit in the observation space is greater than 30. Take the median of the velocity data of each unit in the observation space to construct the three-dimensional velocity field of steady flow in the observation area.

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