A method for molecular tagged velocity measurement of steady flow field under high speed wind tunnel

Abstract

The application discloses a kind of molecular marker velocity measurement methods for steady flow field under high-speed wind tunnel, belong to wind tunnel test technical field.To solve the problem of improving the robustness and precision of MTV data processing.The present application includes based on femtosecond laser system in the upper part of test section to the directly below incidence wavelength femtosecond pulse laser generates fluorescent bright line, after flow field stabilizes, trigger image intensifier, using camera to collect test image;Collect the fluorescent bright line reference image under the condition of no wind, as calibration image, place steel plate ruler in the fluorescent bright line plane, calculate space and pixel conversion coefficient;Image correction is carried out to test image, and the test image after rotation correction is obtained using fast fourier transform method, calculate pixel displacement;Using space and pixel conversion coefficient and pixel displacement, conversion is carried out to obtain actual velocity in space.The present application can significantly reduce the quality requirement of test image.

Description

Technical Field

[0001] This invention belongs to the field of wind tunnel testing technology, specifically relating to a molecular marker velocity measurement method for steady flow fields in high-speed wind tunnels. Background Technology

[0002] In wind tunnel testing, velocity measurement is a crucial step in evaluating the aerodynamic performance and flow field characteristics of an aircraft. While existing technologies such as Particle Image Velocimetry (PIV) and hot-wire anemometers are mature, they still have certain limitations, including difficulties in seeding tracer particles, poor tracking performance at hypersonic speeds, and interference with the flow field during the measurement process. Molecular Tagging Velocimetry (MTV), as a non-contact optical measurement method, effectively overcomes these problems by exciting gas molecules with femtosecond lasers and recording their fringe patterns as they evolve over time, demonstrating significant potential, particularly in high Mach number wind tunnel experiments.

[0003] Existing MTV data processing methods largely rely on feature recognition of fluorescent marker lines, which requires high image quality. However, in complex optical environments, factors such as fluorescence intensity distribution, flow field density gradient, and background noise often lead to problems such as excessively close spacing of marker stripes, blurred lines, or overexposure in certain areas, which to some extent restricts the accuracy and applicability of traditional feature recognition algorithms. Summary of the Invention

[0004] The problem this invention aims to solve is to improve the robustness and accuracy of MTV data processing, and proposes a molecular marker velocity measurement method for steady flow fields in high-speed wind tunnels.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels includes the following steps:

[0007] S1. Based on the femtosecond laser system, a femtosecond pulsed laser of a specific wavelength is incident directly downwards from the upper part of the test section to generate a fluorescent bright line. After the flow field stabilizes, an image intensifier is triggered, and the test image is acquired using a camera. ;

[0008] S2. Acquire a baseline image of the fluorescent bright line under windless conditions as the calibration image. Place a steel ruler on the plane of the fluorescent bright line and calculate the spatial and pixel conversion coefficient k;

[0009] S3. Perform image correction on the test image obtained in step S1 to obtain the rotation-corrected test image. ;

[0010] S4. Based on the rotationally corrected test image obtained in step S3, the pixel displacement is calculated using the Fast Fourier Transform method. ;

[0011] S5. Using the spatial-pixel conversion coefficients obtained in step S2 and the pixel displacements obtained in step S4, calculate the actual velocity in space. .

[0012] Furthermore, the specific implementation method of step S1 includes the following steps:

[0013] S1.1. A femtosecond pulsed laser is injected into the upper wall panel of the wind tunnel test section through the observation window. The laser energy is focused and breaks down the air, producing a fluorescent bright line.

[0014] S1.2. Adjust the acquisition timing of the femtosecond laser system. The laser operates at a fixed frequency of L Hz, and the image intensifier performs m frequency divisions within the fixed frequency range of L Hz, with the interval between each peak being [missing value]. The camera performs low-frequency long-exposure acquisition using a frequency of C Hz, which enables the superposition of multiple images to enhance the brightness of the fluorescent lines and reflects the movement of the fluorescent lines in a single acquired image.

[0015] S1.3. Begin the experiment. The laser and camera will operate in trigger mode. Once the flow field stabilizes, the image intensifier will be triggered, and the image data will be saved. .

[0016] Furthermore, the specific implementation method of step S3 includes the following steps:

[0017] S3.1. Process the test image data, and calculate the detected lines using Hough line detection. slope Then calculate the normal slope. ;

[0018] Then calculate the normal slope. The angle between the fluorescent line and the horizontal x-axis is used to obtain the rotation error angle of the fluorescent line caused by equipment installation errors. ;

[0019] S3.2. Test images conduct Rotate the image to obtain the rotation-corrected test image. .

[0020] Furthermore, the specific implementation method of step S4 includes the following steps:

[0021] S4.1. Test images after rotation correction Summing along the direction of the fluorescent bright lines yields the array. ;

[0022] S4.2. For arrays Perform a Fast Fourier Transform to obtain the spectrum P;

[0023] S4.3. Take the frequency of the first peak position in the spectrum P. The reciprocal of the frequency at the first peak position is the pixel displacement between the two fluorescent bright lines, thus obtaining the pixel displacement. .

[0024] The beneficial effects of this invention are:

[0025] This invention discloses a molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels. The method converts fluorescent stripe images from the spatial domain to the frequency domain and accurately extracts the displacement information of the labeled lines through phase analysis, thereby effectively reducing the sensitivity to image morphology quality. The processing mainly includes image correction, frequency domain conversion, phase analysis, and velocity field reconstruction. First, the original fluorescent stripe image is oriented to ensure accuracy. Then, the image is summed along the vertical axis, followed by a Fast Fourier Transform (FFT) to extract sub-pixel-level displacement by finding the maximum peak point. Finally, combined with system calibration parameters, the pixel displacement is converted into actual physical velocity, significantly improving the reliability of data processing. This frequency domain processing method, while ensuring velocity measurement accuracy, significantly reduces the quality requirements of the experimental images, providing a new approach for the application of molecularly labeled velocity measurement (MTV) in wind tunnel-like experimental scenarios. Attached Figure Description

[0026] Figure 1 This is a flowchart of a molecularly labeled velocity measurement method for a steady flow field in a high-speed wind tunnel, as described in this invention.

[0027] Figure 2 This is a schematic diagram of the timing control of the molecular marker velocity measurement system of the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described specific embodiments are merely a part of the embodiments of the invention, and not all of them. The components of the specific embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations, and the invention may also have other embodiments.

[0029] Therefore, the following detailed description of specific embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected specific embodiments of the invention. All other specific embodiments obtained by those skilled in the art based on these specific embodiments without inventive effort are within the scope of protection of this invention.

[0030] To further understand the invention's content, features, and effects, the following specific embodiments are provided, along with accompanying drawings. Figure 1 and attached Figure 2 Detailed explanation is as follows:

[0031] Example 1:

[0032] A molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels includes the following steps:

[0033] S1. Based on the femtosecond laser system, a femtosecond pulsed laser of a specific wavelength is incident directly downwards from the upper part of the test section to generate a fluorescent bright line. After the flow field stabilizes, an image intensifier is triggered, and the test image is acquired using a camera. ;

[0034] Furthermore, the specific implementation method of step S1 includes the following steps:

[0035] S1.1. A femtosecond pulsed laser is injected into the upper wall panel of the wind tunnel test section through the observation window. The laser energy is focused and breaks down the air, producing a fluorescent bright line.

[0036] S1.2. Adjust the acquisition timing of the femtosecond laser system. The laser operates at a fixed frequency of L Hz, and the image intensifier performs m frequency divisions within the fixed frequency range of L Hz, with the interval between each peak being [missing value]. The camera performs low-frequency long-exposure acquisition using a frequency of C Hz, which enables the superposition of multiple images to enhance the brightness of the fluorescent lines and reflects the movement of the fluorescent lines in a single acquired image.

[0037] S1.3. Begin the experiment. The laser and camera will operate in trigger mode. Once the flow field stabilizes, the image intensifier will be triggered, and the image data will be saved. .

[0038] S2. Acquire a baseline image of the fluorescent bright line under windless conditions as the calibration image. Place a steel ruler on the plane of the fluorescent bright line and calculate the spatial and pixel conversion coefficient k;

[0039] S3. Perform image correction on the test image obtained in step S1 to obtain the rotation-corrected test image. ;

[0040] Furthermore, the specific implementation method of step S3 includes the following steps:

[0041] S3.1. Process the test image data, and calculate the detected lines using Hough line detection. slope Then calculate the normal slope. ;

[0042] Then calculate the normal slope. The angle between the fluorescent line and the horizontal x-axis is used to obtain the rotation error angle of the fluorescent line caused by equipment installation errors. ;

[0043] S3.2. Test images conduct Rotate the image to obtain the rotation-corrected test image. .

[0044] S4. Based on the rotationally corrected test image obtained in step S3, the pixel displacement is calculated using the Fast Fourier Transform method. ;

[0045] Furthermore, the specific implementation method of step S4 includes the following steps:

[0046] S4.1. Test images after rotation correction Summing along the direction of the fluorescent bright lines yields the array. ;

[0047] S4.2. For arrays Perform a Fast Fourier Transform to obtain the spectrum P;

[0048] S4.3. Take the frequency of the first peak position in the spectrum P. The reciprocal of the frequency at the first peak position is the pixel displacement between the two fluorescent bright lines, thus obtaining the pixel displacement. .

[0049] S5. Using the spatial-pixel conversion coefficients obtained in step S2 and the pixel displacements obtained in step S4, calculate the actual velocity in space. .

[0050] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0051] Although this application has been described above with reference to specific embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of this application. In particular, as long as there is no structural conflict, the features in the specific embodiments disclosed in this application can be combined with each other in any way. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, this application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for measuring molecularly labeled velocities in a steady flow field under high-speed wind tunnel conditions, characterized in that, Includes the following steps: S1. Based on the femtosecond laser system, a femtosecond pulsed laser of a specific wavelength is incident directly downwards from the upper part of the test section to generate a fluorescent bright line. After the flow field stabilizes, an image intensifier is triggered, and the test image is acquired using a camera. ; S2. Acquire a baseline image of the fluorescent bright line under windless conditions as the calibration image. Place a steel ruler on the plane of the fluorescent bright line and calculate the spatial and pixel conversion coefficient k; S3. Perform image correction on the test image obtained in step S1 to obtain the rotation-corrected test image. ; S4. Based on the rotationally corrected test image obtained in step S3, the pixel displacement is calculated using the Fast Fourier Transform method. ; S5. Using the spatial-pixel conversion coefficients obtained in step S2 and the pixel displacements obtained in step S4, calculate the actual velocity in space. .

2. The molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels according to claim 1, characterized in that, The specific implementation method of step S1 includes the following steps: S1.

1. A femtosecond pulsed laser is injected into the upper wall panel of the wind tunnel test section through the observation window. The laser energy is focused and breaks down the air, producing a fluorescent bright line. S1.

2. Adjust the acquisition timing of the femtosecond laser system. The laser operates at a fixed frequency of L Hz, and the image intensifier performs m frequency divisions within the fixed frequency range of L Hz, with the interval between each peak being [missing value]. The camera performs low-frequency long-exposure acquisition using a frequency of C Hz, which enables the superposition of multiple images to enhance the brightness of the fluorescent lines and reflects the movement of the fluorescent lines in a single acquired image. S1.

3. Begin the experiment. The laser and camera will operate in trigger mode. Once the flow field stabilizes, the image intensifier will be triggered, and the image data will be saved. .

3. The molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels according to claim 2, characterized in that, The specific implementation method of step S3 includes the following steps: S3.

1. Process the test image data, and calculate the detected lines using Hough line detection. slope Then calculate the normal slope. ; Then calculate the normal slope. The angle between the fluorescent line and the horizontal x-axis is used to obtain the rotation error angle of the fluorescent line caused by equipment installation errors. ; S3.

2. Test images conduct Rotate the image to obtain the rotation-corrected test image. .

4. The molecularly labeled velocity measurement method for steady flow fields in high-speed wind tunnels according to claim 3, characterized in that, The specific implementation method of step S4 includes the following steps: S4.

1. Test images after rotation correction Summing along the direction of the fluorescent bright lines yields the array. ; S4.

2. For arrays Perform a Fast Fourier Transform to obtain the spectrum P; S4.

3. Take the frequency of the first peak position in the spectrum P. The reciprocal of the frequency at the first peak position is the pixel displacement between the two fluorescent bright lines, thus obtaining the pixel displacement. .

Images