Simple high-frequency PIV flow field measurement system

Abstract

The invention relates to the technical field of particle image velocimetry, and discloses a simple high-frequency PIV flow field measurement system, a shading rotor comprises a turntable and a plurality of light holes circumferentially formed in the turntable, the turntable is rotatably arranged at a light source through a driving part, when the turntable rotates, the plurality of light holes sequentially correspond to the light source, and when the turntable rotates, the plurality of light holes correspond to the light source; the light source can periodically pass through the plurality of light holes; and the light conversion device is arranged between the shading rotor and the tracer particle flow field, and the light conversion device is used for converting a point light source emitted by the light source into a sheet light source and irradiating the sheet light source to the tracer particle flow field. The effective exposure time of the camera can be effectively shortened, compared with existing equipment, the particle trailing problem in a shot image is solved, the image quality is improved, and the shot flow field speed is increased to 9 m / s from 2.88 m / s.

Description

Technical Field

[0001] The present invention relates to the technical field of particle image velocimetry, and in particular to a simple high-frequency PIV flow field measurement system. Background Art

[0002] PIV velocimetry principle: Tiny tracer particles are added to a fluid, causing them to move with the fluid. Pulsed laser light is used to illuminate the particles in the flow field, typically using a laser sheet to illuminate a thin region of the flow field. A high-speed camera then captures two or more exposures of the particles. Image processing is then used to analyze the particle displacements in these two consecutive exposures, and the particle velocity distribution is calculated using cross-correlation or autocorrelation methods.

[0003] To reduce the cost of high-frequency PIV equipment, many researchers have adopted continuous lasers instead of pulsed laser systems, and then used computer software to control cameras to periodically capture flow field images. This approach still presents a significant problem: the exposure time of many high-frequency cameras cannot be reduced to a specific value. Longer exposure times result in longer particle trails in the captured images, which limits the flow field speed that can be captured. Existing equipment can only capture velocity fields of 2.88 m / s. If the speed is increased further, the particle trailing phenomenon will lead to inaccurate post-processing results. Summary of the Invention

[0004] The purpose of the present invention is to provide a simple high-frequency PIV flow field measurement system, aiming to solve or improve at least one of the above technical problems.

[0005] To achieve the above objectives, the present invention provides the following solution: The present invention provides a simple high-frequency PIV flow field measurement system, comprising:

[0006] tracer particle flow field;

[0007] a camera, configured to acquire an image of the tracer particle flow field;

[0008] a light source, disposed below the tracer particle flow field;

[0009] a light-shielding rotor comprising a rotating disk and a plurality of light-transmitting holes circumferentially arranged on the rotating disk, wherein the rotating disk is rotatably disposed at the light source via a driving member, and when the rotating disk rotates, the plurality of light-transmitting holes sequentially correspond to the light source;

[0010] A light conversion device is provided between the light-shielding rotor and the tracer particle flow field, and is used for converting the point light source emitted by the light source into a sheet light source and irradiating the sheet light source to the tracer particle flow field.

[0011] Optionally, a controller is further included, connected to the camera and the driving member, and the controller can control the camera to take pictures and control the driving member to drive the turntable to rotate.

[0012] Optionally, a synchronizer is further included, which is arranged above the tracer particle flow field and is connected to the controller. When the synchronizer obtains the light passing through the tracer particle flow field, the camera takes an image of the tracer particle flow field.

[0013] Optionally, the light source includes a continuous laser and a light outlet provided on the continuous laser.

[0014] Optionally, the light conversion device is a conversion lens.

[0015] Optionally, the synchronizer includes a light trigger and a light sensor arranged in the light trigger.

[0016] Optionally, a plurality of light-transmitting holes are arranged at equal intervals with the axis of the turntable as the center.

[0017] Optionally, the driving member includes a motor, and the output shaft of the motor is connected to the axis of the turntable.

[0018] Optionally, the tracer particle flow field includes a car model and a tracer particle inlet and a tracer particle outlet located on both sides of the car model.

[0019] Optionally, the speed adjustment range of the driving member is 100r / s (revolutions per second)-150r / s, and the adjustment accuracy is 0.01r / s.

[0020] The present invention discloses the following technical effects:

[0021] 1. The present invention provides a light-shielding rotor at the light source. Multiple light-transmitting holes on the turntable, in conjunction with the turntable's rotation, allow the light source to periodically pass through the holes. A light conversion device then converts the point light source into a sheet light source. The turntable is then driven by a driver, so that the light-transmitting holes cooperate with the light source. This effectively reduces camera exposure, improves image quality, and increases the captured flow field velocity from 2.88 m / s to 9 m / s.

[0022] 2. The present invention uses a lower-cost continuous laser in conjunction with a light-shielding rotor instead of an expensive pulse laser, which reduces the irradiation time and thus increases the measurable velocity amplitude, and also reduces the overall equipment cost.

[0023] 3. The present invention uses an optical trigger to directly control the camera shooting, and no longer requires a complex synchronization controller to simultaneously control the high-speed camera shooting and the emission of the pulsed laser, further reducing the cost of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0025] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0026] Figure 2 It is a schematic structural diagram of the light source, light-shielding rotor and light conversion device of the present invention;

[0027] Figure 3 This is a schematic diagram of the shading rotor structure of the present invention;

[0028] Figure 4 Schematic diagram of the arrangement of two adjacent light-shielding holes in an embodiment of the present invention;

[0029] Figure 5 A velocity vector diagram obtained by shooting and processing in an embodiment of the present invention;

[0030] Figure 6 Schematic diagram of the control program principle of the present invention.

[0031] In the figure: 1. tracer particle flow field; 2. camera; 3. light source; 4. light-shielding rotor; 41. turntable; 42. light-transmitting hole; 43. driving part; 5. light conversion device; 6. controller; 7. synchronizer. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Reference Figures 1-6 The present invention provides a simple high-frequency PIV flow field measurement system, comprising:

[0035] Tracer particle flow field 1;

[0036] Camera 2, used to obtain images of the tracer particle flow field 1, and camera 2 is a high-speed camera;

[0037] The light source 3 is arranged below the tracer particle flow field 1;

[0038] The light-shielding rotor 4 includes a rotating disk 41 and a plurality of light-transmitting holes 42 circumferentially formed on the rotating disk 41. The rotating disk 41 is rotatably disposed at the light source 3 via a driving member 43. When the rotating disk 41 rotates, the plurality of light-transmitting holes 42 sequentially correspond to the light source 3, and the light source 3 can periodically pass through the plurality of light-transmitting holes 42.

[0039] The light conversion device 5 is disposed between the shading rotor 4 and the tracer particle flow field 1 . The light conversion device 5 is used to convert the point light source emitted by the light source 3 into a sheet light source and irradiate the sheet light source to the tracer particle flow field 1 .

[0040] By setting a shading rotor 4 at the light source 3, the multiple light-transmitting holes 42 on the turntable 41 cooperate with the rotation of the turntable 41 so that the light source 3 periodically passes through the light-transmitting holes 42, and then the light conversion device 5 converts the point light source into a sheet light source, and cooperates with the driving member 43 to drive the turntable 41 to rotate, so that the multiple light-transmitting holes 42 cooperate with the light source 3, thereby effectively reducing the exposure of the camera 2 and improving the image quality.

[0041] The exposure time of camera 2 is 250μs. Due to its inherent limitations, the exposure time of camera 2 cannot be significantly reduced. Using the shading rotor 4, the illumination time of light source 3 can be reduced to 80μs. Experimental calibration shows that the relationship between the actual displacement and pixel displacement corresponding to this example is 180μm / pixel, meaning that the actual distance corresponding to one pixel is 180μm. Particle tailing is generally controlled within four pixels, so the maximum speeds V1 and V2 that can be captured at different exposure times can be calculated:

[0042] Maximum actual distance S:

[0043] S = 4 × 180 μm = 720 μm (1.1)

[0044] V1:

[0045]

[0046] V2:

[0047]

[0048] The shooting frequency of camera 2 is 1500 Hz, that is, 1500 sets of images are taken in 1 second, with 2 images in each set, totaling 3000 images, which are used to calculate the velocity field of the fluid. Under this condition, it is guaranteed that the unsteady flow can be captured with high temporal resolution, thereby obtaining high-quality velocity field information.

[0049] In one embodiment of the present invention, a controller 6 is further included, which is connected to the camera 2 and the driving member 43. The controller 6 can control the camera 2 to shoot and control the driving member 43 to drive the turntable 41 to rotate.

[0050] The controller is a computer and post-processing software set in the computer, preferably the App software PIV-Lab in Matlab.

[0051] In one embodiment of the present invention, a synchronizer 7 is further included, which is arranged above the tracer particle flow field 1. The synchronizer 7 is connected to the controller 6. When the synchronizer 7 obtains the light passing through the tracer particle flow field 1, the camera 2 takes an image of the tracer particle flow field 1.

[0052] In one embodiment of the present invention, the light source 3 includes a continuous laser and a light outlet provided on the continuous laser.

[0053] The continuous laser does not need to add a pulse frequency controller. The emitted laser is a continuous laser without flickering. Its frequency is controlled by the high-speed shading rotor 4. When taking images, you only need to adjust the rotation speed of the shading rotor 4 and the shooting parameters of the camera 2.

[0054] In one embodiment of the present invention, the light conversion device 5 is a conversion lens.

[0055] In one embodiment of the present invention, the synchronizer 7 includes a light trigger and a light sensor disposed in the light trigger.

[0056] The synchronizer 7 is used to monitor light emission and covering. When the light sensor detects that light is transmitted upward, the light trigger signal is transmitted to the controller 6, and the controller 6 will simultaneously control the camera 2 to start shooting. That is, the light trigger is similar to the shooting switch of the camera 2. When light is detected, the shooting is started. This can ensure that the start time of the camera 2 shooting is synchronized with the time when the light source 3 is emitted, and the error does not exceed 1μs.

[0057] In one embodiment of the present invention, a plurality of light-transmitting holes 42 are arranged at equal intervals around the axis of the turntable 41 .

[0058] The distance between the center of the light-transmitting hole 42 and the center of the rotating disk 41 is 40 mm.

[0059] In one embodiment of the present invention, the driving member 43 includes a motor, and the output shaft of the motor is connected to the axis of the turntable 41 .

[0060] In one embodiment of the present invention, the tracer particle flow field 1 includes a car model and a tracer particle inlet and a tracer particle outlet located on both sides of the car model.

[0061] In one embodiment of the present invention, the speed adjustment range of the driving member 43 is 100 r / s-150 r / s, and the adjustment accuracy is 0.01 r / s.

[0062] The number and radius of the light-transmitting holes need to be calculated based on the shooting frequency and rotation speed, and the rotation speed needs to be calculated based on the turntable radius, hole radius and shooting parameters.

[0063] The specific calculation formula is as follows:

[0064] The camera captures 3000 images per second, so each image capture time is 333.3 μs. Since the maximum speed of the motor is 150 r / s, the turntable 41 should have n light-transmitting holes 42:

[0065]

[0066] The angle of rotation of the corresponding image turntable 41 is θ1:

[0067]

[0068] The rotation angle per 1μs is θ2:

[0069]

[0070] The exposure time is 80μs, and the angle corresponding to one light-transmitting hole is θ3:

[0071] θ3=80×0.054°=4.32°(1.7)

[0072] The center of the light hole 42 is 40 mm away from the center of the turntable, so the radius of the light hole 42 can be calculated as:

[0073]

[0074] The radius of the light-transmitting hole 42 is designed to be 1.5 mm.

[0075] The shooting parameters of this embodiment are:

[0076] The shooting frequency is 1500Hz (1500 sets of images per second, 2 images per set).

[0077] Camera exposure time: 250μs. The camera's exposure time cannot be reduced excessively for its own reasons, so a light-shielding rotor is needed to reduce the exposure time to 80μs (the time required for the camera to capture an image. Since the tracer particles generally move at a fast speed, the exposure time cannot be too long, otherwise particle light tails will easily form).

[0078] In a set of images taken by the camera, the time interval between two images is 300 μs (i.e., the movement time of the tracer particles).

[0079] Image resolution: 1024*762 (the number of pixels in the image in the horizontal and vertical coordinate directions. Before the formal experiment, the light source cross-section needs to be calibrated and the image post-processing is based on this. It will determine the correspondence between the number of particle pixel displacements in the image and the actual displacement distance).

[0080] According to the known parameters: exposure time 80 μs, shooting interval 300 μs, shooting frequency 1500 Hz, the center of the light hole 42 is 40 mm away from the center of the turntable.

[0081] At 1500 Hz, the camera should capture 1500 images per second, each containing 2 images, for a total of 3000 images. Therefore, each image capture takes 666.6 μs. Since the motor's maximum speed is 150 rpm, turntable 41 should have 20 light-transmitting apertures 42. The angle of rotation of turntable 41 for each image is 36°, or 0.054° per μs. Therefore, the angle corresponding to each light-transmitting aperture is 4.32°. Therefore, the radius of light-transmitting aperture 42 is calculated to be 1.5086 mm. The designed radius of light-transmitting aperture 42 is 1.5 mm.

[0082] like Figure 4 As shown, the turntable rotates clockwise, and a point light source first passes through one of the light holes 42 for 80 μs (exposure time). It is then blocked for 300 μs (interval between shots). Finally, the point light source passes through the adjacent light hole 42 for another 80 μs (exposure time). The subsequent steps are the same: the turntable 41 continues to rotate, and the point light source periodically passes through the light holes 42.

[0083] PIV velocity measurement principle: Tracer particles are spread in the measured flow field, and under the illumination of light source 3, time series images are continuously obtained using camera 2. Image processing algorithms are applied to obtain the displacement of particles on the image. When the exposure interval Δt = t2-t l =300μs, the pixel displacements Δx and Δy of the particle on the image are obtained. After accounting for the system's optical magnification, the actual displacement distances Δx and Δy are calculated, and the actual particle velocity can be calculated. If Δt is small, this velocity can be used to approximate the instantaneous velocity of the particle at (t1 + t2) / 2. Therefore, PIV measurements use average velocity instead of instantaneous velocity, and tracer particle velocity instead of the flow field velocity at that location.

[0084] This PIV system can capture a specific section of the flow field for measurement, requiring illumination of the flow field using light source 3. The choice of tracer particles depends on the experimental requirements; in this example, DEHS (diethylhexyl sebacate) is selected.

[0085] Specific experimental steps:

[0086] 1. Design turntable 41:

[0087] In this example, the size of the turntable 41 is mainly determined by the shooting parameters. When the light source 3 passes through the light hole 42, the light will be transmitted to the top. That is, the aperture size of the light hole 42 is mainly determined by the exposure time, that is, 80μs above. The shooting interval time is the light source blocking time of 300μs. In this example, the calculation shows that:

[0088] The radius of the turntable 41 is 50 mm, the radius of the light-transmitting holes 42 is 1.5 mm, the rotation speed of the turntable 41 is 150 r / s, and the number of the light-transmitting holes 42 is 20.

[0089] 2. Install the continuous laser, shading rotor 4, and conversion lens so that the light outlet is 50 cm below the model and the minimum thickness of the light source is kept in the middle of the model cross section. After adjusting the continuous laser, slowly rotate the adjustment button until the light source emits light. Adjust the required light intensity according to the experimental requirements to maintain uniform light intensity on the model surface.

[0090] 3. Install high-speed camera 2 perpendicular to the model cross section, turn on the power of high-speed camera 2, connect it to the computer via optical fiber, and use the computer to control high-speed camera 2 and adjust the high-speed camera parameters:

[0091] The shooting frequency is adjusted to 1500 Hz, that is, the time resolution is 666.7 μs, to ensure that the accuracy is small enough, so as to obtain high-quality unsteady flow field velocity vector diagram.

[0092] The exposure time is adjusted to 250 μs and the shooting interval is adjusted to 300 μs, that is, the above Δt=300 μs.

[0093] Adjust the image resolution to 1024*762, that is, the number of pixels in the horizontal and vertical coordinate directions. Before the formal experiment, it is necessary to calibrate the parameters of the light source cross section and use this as a basis for image post-processing. It will determine the correspondence between the number of particle pixel displacements on the image and the actual displacement distance.

[0094] 6. Install the trigger: The light trigger needs to be installed above the experimental space. The light sensor will detect the light source passing through the light hole 42 and being blocked by the turntable 41. It will then directly control the high-speed camera 2 to start capturing a set of images. This cycle will continue until the shooting is completed. The time error between the moment the light trigger detects the light and the moment the camera 2 takes the image is no more than 1 μs.

[0095] 7. Calibration: Place a piece of paper with a scale on the section being imaged, take an image, and use the post-processing software to check the correspondence between actual distance and pixels. Different experimental conditions will have different correspondences, so you need to adjust based on your experimental conditions. In this example, the correspondence is 180μm / pixel, meaning that one pixel corresponds to an actual distance of 180μm.

[0096] 8. Experimental Recording: Start the motor and adjust the motor speed to 150 r / s using the computer. After it stabilizes, open the lens cap of Camera 2 and adjust the exposure to maximum. A light trigger, mounted above the experimental space, directly connects the detection light to High-Speed Camera 2 and controls the camera's recording. The light trigger is also connected to the computer, which activates the detection light. The computer activates the light trigger, which automatically and synchronously controls High-Speed Camera 2 to capture images upon detecting light. High-Speed Camera 2 is connected to the computer via optical fiber, which controls the computer to adjust recording parameters and save experimental data. The light trigger continuously detects light, maintaining continuous recording for Camera 2.

[0097] 9. With an incoming airflow velocity of 9 m / s, use a computer connected to high-speed camera 2 to capture experimental images and save the results. In this example, the capture time is 10 seconds, resulting in 15,000 sets of images. The velocity changes within this 10-second period can be analyzed. The algorithm then calculates the pressure changes or other desired experimental results over the entire period.

[0098] Velocity vector field display: Open the post-processing software, import the experimental images, perform a series of pre-processing, input the calibration data, and perform post-processing. Finally, the experimental velocity vector diagram is obtained. After removing mismatched vectors (manually or automatically), the final data is obtained and displayed. If necessary, an interpolation algorithm can be used to obtain a denser velocity vector distribution.

[0099] like Figure 5 As shown: The average velocity of all tracer particles in each small area is obtained by post-processing. Figure 5 Velocity vector. From the figure, we can see that turbulence similar to the Karman vortex street is formed behind the car model, which is consistent with the actual physical phenomenon.

[0100] A brief introduction to the post-processing principle:

[0101] The software identifies the horizontal and vertical displacements ΔX and ΔY of the tracer particles and brings them into the velocity solution formula:

[0102] U=ΔX / Δt

[0103] V=ΔY / Δt

[0104] The velocity components in the horizontal and vertical coordinate directions on the two-dimensional plane are obtained by solving the problem, and the average velocity vector in a small range is obtained after software calculation and processing.

[0105] In the description of the present invention, it should be understood that the terms "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0106] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A simple high-frequency PIV flow field measurement system, characterized in that: include: Tracer particle flow field (1); A camera (2) for acquiring an image of the tracer particle flow field (1); A light source (3) is arranged below the tracer particle flow field (1); A light-shielding rotor (4) comprises a rotating disk (41) and a plurality of light-transmitting holes (42) circumferentially arranged on the rotating disk (41); the rotating disk (41) is rotatably arranged at the light source (3) via a driving member (43); when the rotating disk (41) rotates, the plurality of light-transmitting holes (42) sequentially correspond to the light source (3); A light conversion device (5) is provided between the light-shielding rotor (4) and the tracer particle flow field (1), and is used to convert the point light source emitted by the light source (3) into a sheet light source and irradiate the sheet light source onto the tracer particle flow field (1).

2. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: It also includes a controller (6) connected to the camera (2) and the driving member (43), and the controller (6) can control the camera (2) to take pictures and control the driving member (43) to drive the turntable (41) to rotate.

3. A simple high-frequency PIV flow field measurement system according to claim 2, characterized in that: The invention also includes a synchronizer (7) which is arranged above the tracer particle flow field (1). The synchronizer (7) is connected to the controller (6). When the synchronizer (7) obtains light transmitted through the tracer particle flow field (1), the camera (2) takes an image of the tracer particle flow field (1).

4. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: The light source (3) comprises a continuous laser and a light outlet arranged on the continuous laser.

5. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: The light conversion device (5) is a conversion lens.

6. A simple high-frequency PIV flow field measurement system according to claim 3, characterized in that: The synchronizer (7) comprises a light trigger and a light sensor arranged in the light trigger.

7. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: A plurality of light-transmitting holes (42) are arranged at equal intervals with the axis of the turntable (41) as the center.

8. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: The driving member (43) includes a motor, and the output shaft of the motor is connected to the axis of the turntable (41).

9. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: The tracer particle flow field (1) comprises a car model and a tracer particle inlet and a tracer particle outlet located on both sides of the car model.

10. A simple high-frequency PIV flow field measurement system according to claim 1, characterized in that: The speed adjustment range of the driving member (43) is 100 r / s (revolutions per second)-150 r / s, and the adjustment accuracy is 0.01 r / s.

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