Flow field velocity measurement and visualization apparatus and method

Abstract

The application discloses a flow field velocity measurement and visualization device and method, relates to the technical field of flow field display measurement, and comprises a dual-color laser emission system, a surface dielectric barrier discharge system, a Cassegrain optical shaping system, a fluorescent signal detection system and a data processing system; wherein the dual-color laser emission system is used for generating femtosecond pulse laser and first laser, and the Cassegrain optical shaping system is used for converging and collimating the femtosecond pulse laser and the first laser and then focusing the same to a detection area to cooperatively excite nitrogen molecules; the surface dielectric barrier discharge system forms a plasma environment in the detection area; the fluorescent signal detection system is used for collecting nitrogen molecule fluorescent signals in the detection area; and the data processing system is used for controlling timing and realizing flow field structure visualization characterization of the detection area and velocity measurement of the flow field in the detection area, so that the application can synchronously obtain a velocity vector near a wall surface and flow field structure parameters under millimeter-level spatial resolution and microsecond-level time scale.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flow field display measurement, in particular to a flow field velocity measurement and visualization device and method. BACKGROUND

[0002] The flow field of a hypersonic low-density wind tunnel has significant non-equilibrium characteristics and complex physical mechanisms, which mainly manifest in the following aspects: 1) Dominance of rarefied gas effect: In a low-density environment, the mean free path of gas molecules can reach millimeter level, leading to the invalidation of the continuous medium assumption, and the flow presents significant rarefied effect; 2) Coupling of high-temperature gas effect: When the speed of the aircraft exceeds 10 Mach, the temperature of the shock layer generated by the compression of high-speed airflow can reach tens of thousands of degrees Celsius, causing the dissociation and even ionization of oxygen and nitrogen molecules, forming a multi-component non-equilibrium plasma flow field containing atoms, ions and electrons, which will lead to significant differences in aerodynamic thermodynamic parameters (thermal conductivity, viscosity coefficient) from conventional conditions, and the spatiotemporal scale of chemical reactions is highly coupled with the scale of fluid motion, further increasing the complexity of measurement and modeling; 3) Special characteristics of in-wall flow: Under low-density conditions, the ratio of boundary layer thickness to aircraft characteristic size increases significantly, and the velocity slip and temperature jump phenomena in the boundary layer are prominent. In addition, the flow separation zone caused by the interaction of shock wave / boundary layer expands, leading to strong non-stationarity of the wall pressure and heat flow distribution, which puts extreme requirements on the spatial resolution and dynamic response of the measurement technology.

[0003] To address the above problems, the existing flow field display technology faces the following key bottlenecks: 1) Failure of traditional particle image velocimetry (PIV) technology: In a low-density environment, the density of gas molecules is extremely low, and it is impossible to effectively mark by scattering tracer particles. Even if particles are forcibly injected, the following property of the particles will be severely reduced due to the rarefied effect, and the momentum exchange between the particles and the airflow may change the intrinsic characteristics of the flow field, leading to distorted results; 2) Significant limitations of NO-PLIF technology (NO-PLIF is a non-contact optical diagnostic method based on laser spectroscopy): Although the planar laser-induced fluorescence technology based on nitric oxide tracer does not require external particles, the gas injection it relies on causes two problems: first, the high concentration of the tracer gas (such as NO) causes changes in the chemical composition of the local flow field, interfering with the true flow field structure; second, some tracer gases are highly toxic, posing safety and environmental risks in a closed wind tunnel system; 3) Coverage blind area exists in electron beam fluorescence technology: Although the electron beam fluorescence technology can achieve large-scale flow field velocity and density measurement, its fluorescence lifetime (usually in the order of microseconds) is insufficient for low-density resolution. More importantly, the electron beam is easily affected by electromagnetic interference and material shielding effect in the in-wall area, resulting in missing data in the key area.

[0004] Current technical problems are mainly reflected in the insufficient ability to measure in situ near the wall: existing methods are unable to simultaneously obtain velocity vectors and flow field structure parameters near the wall at millimeter-level spatial resolution and microsecond-level time scale. Summary of the Invention

[0005] To address the limitations of current technologies in in-situ measurement near the wall and to achieve simultaneous acquisition of velocity vectors and flow field structure parameters near the wall at millimeter-level spatial resolution and microsecond-level time scale, this invention provides a flow field velocity measurement and visualization device, comprising:

[0006] Dual-color laser emission system, surface dielectric barrier discharge system, Cassegrain optical path shaping system, fluorescence signal detection system, and data processing system;

[0007] The dual-color laser emission system generates a femtosecond pulsed laser and a first laser. The Cassegrain optical path shaping system converges and collimates the femtosecond pulsed laser and the first laser, focusing them onto the detection area. The first laser pre-excites nitrogen molecules in the detection area, while the femtosecond pulsed laser induces stimulated emission of nitrogen in the detection area. The first laser and the femtosecond pulsed laser synergistically excite nitrogen molecules. The surface dielectric barrier discharge system creates a plasma environment within the detection area. The fluorescence signal detection system collects fluorescence signals from nitrogen molecules within the detection area, obtaining a fluorescence image for acquiring flow field structure information and a fluorescence signal flow video for measuring flow field velocity. The data processing system controls the timing of each system in the device, visualizes the flow field structure of the detection area based on the fluorescence image, and measures the velocity of the flow field in the detection area based on the fluorescence signal flow video.

[0008] This invention utilizes a dual-color laser emission system combined with ultrafast diagnostic technology of femtosecond lasers. Taking advantage of its extremely short pulse width (<100 fs) and high peak power characteristics, selective excitation and fluorescence capture of molecular vibrational energy levels are achieved near the wall surface through beam shaping and multi-wavelength synergistic excitation. This invention can avoid the interference problem of traditional tracers and enhance the local fluorescence signal through planar dielectric barrier discharge, thereby breaking through the spatial and temporal resolution limits of near-wall flow field measurement and providing key data support for the aerodynamic design and thermal protection optimization of hypersonic vehicles.

[0009] Preferably, the dual-color laser emission system includes a femtosecond pulse laser for generating femtosecond pulse lasers and a seed laser for generating a first laser. In the dual-color laser emission system, the seed laser outputs a first laser of a specific wavelength to pre-excite a specific energy level of nitrogen molecules, and the high-energy laser pulse generated by the femtosecond pulse laser induces stimulated emission in air (nitrogen), and the two work together to excite nitrogen molecules.

[0010] Preferably, the Cassegrain optical path shaping system includes: a first parabolic mirror, a second parabolic mirror, and a hyperboloid mirror; wherein, the first parabolic mirror is used to converge and collimate the femtosecond pulse laser and reflect it to the hyperboloid mirror; the second parabolic mirror is used to converge and collimate the first laser and reflect it to the hyperboloid mirror; the hyperboloid mirror is used to reflect and focus the femtosecond pulse laser and the first laser onto the detection area. Through the first parabolic mirror, the second parabolic mirror, and the hyperboloid mirror, in conjunction with the optical path adjustment device, the dual-color laser composed of the femtosecond pulse laser and the first laser is accurately focused onto the detection area, correcting optical path distortion.

[0011] Preferably, the surface dielectric barrier discharge system includes: a discharge electrode assembly, a dielectric layer, and a power supply; it includes an outer electrode and an inner electrode located above and below the dielectric layer, respectively. The positive terminal of the power supply is connected to the high-voltage input terminal of the inner electrode, and the negative terminal of the power supply is grounded. The grounding terminal of the inner electrode is connected to the high-voltage input terminal of the outer electrode, and the grounding terminal of the outer electrode is grounded. The inner electrode is embedded in the dielectric surface of the inner wall of the wind tunnel, and the outer electrode is arranged on the outer wall of the wind tunnel to reduce interference with the wind tunnel flow field. The inner and outer electrodes are arranged parallel to each other on the wall of the wind tunnel to be tested. The area of ​​the dielectric layer is larger than the area of ​​the inner electrode to ensure that the electric field uniformly covers the measurement area, thereby generating a large amount of plasma in the area to be tested to improve the fluorescence excitation efficiency. Driven by the power supply, the surface dielectric barrier discharge system forms a plasma environment in the detection area, further enhancing the excitation efficiency of nitrogen molecules.

[0012] Preferably, the fluorescence signal detection system includes: a lens, an ICCD camera, a high-speed camera, and a filter;

[0013] The system includes lenses mounted on both the ICCD camera and the high-speed camera. A filter is installed at the front end of each lens. The lens collects and collimates the fluorescence signal of nitrogen molecules within the detection area; the filter suppresses background light in the nitrogen molecule fluorescence signal; the ICCD camera acquires a fluorescence image; and the high-speed camera captures a video of the fluorescence signal flow. In the fluorescence signal detection system, the fluorescence collecting lens group collects the fluorescence signal. After background interference is filtered out by the filter, the ICCD camera captures a fluorescence image to obtain flow field structure information, and the high-speed camera captures a video of the fluorescence signal flow for velocity measurement.

[0014] Preferably, the data processing system includes a timing controller and a data processing unit; the timing controller controls the timing of each system in the device; the data processing unit visualizes the flow field structure of the detection area based on the fluorescence image, and measures the velocity of the flow field in the detection area based on the fluorescence signal flow video. The timing controller ensures timing synchronization of all components, and the data processing unit, based on the MATLAB platform, performs image conversion, information extraction, and computational analysis on the acquired data to achieve visualized flow field structure and velocity measurement.

[0015] Preferably, the visualization of the flow field structure in the detection region based on the fluorescence image specifically includes:

[0016] Based on the threshold segmentation algorithm, the computer selects the effective area collected by the fluorescence signal detection system to obtain the effective area video, reducing background interference and computational data volume. The effective area video is then converted into frame-by-frame images according to the time series to obtain the sequence image.

[0017] The edge information of the fluorescent dot matrix in the sequence image is extracted based on the edge detection algorithm. The fluorescent dot matrix region is retained based on the edge information, and the background part outside the fluorescent dot matrix is ​​removed to obtain the first fluorescent dot matrix sequence image.

[0018] Image enhancement processing is performed on the first fluorescent dot matrix sequence image to obtain the second fluorescent dot matrix sequence image;

[0019] The spatial resolution of the fluorescence signal detection system is obtained based on the scale calibration calculation of the fluorescence signal detection system.

[0020] Based on the initial image and the second fluorescence array sequence image of the flow field in the detection area, the target surface imaging offset is calculated using a cross-correlation algorithm.

[0021] The actual fluorescence spatial offset is calculated by multiplying the spatial resolution of the fluorescence signal detection system and the target surface imaging offset.

[0022] Preferably, the target surface imaging offset is calculated as follows:

[0023]

[0024]

[0025] in, This represents the horizontal offset of the fluorescence image of the next frame relative to the previous frame. This represents the vertical offset of the subsequent frame relative to the previous frame of the fluorescence image. Here, M is the number of pixels in the vertical direction of the image sequence, and N is the number of pixels in the horizontal direction of the image sequence. The pixel index of the fluorescence image in the vertical direction. This represents the pixel index of the fluorescence image in the horizontal direction. This is the grayscale value of the previous frame of the fluorescence image. This represents the grayscale value of the next frame of the fluorescence image. The pixel offset of the acquired image;

[0026] Based on the actual fluorescence spatial offset, the velocity distribution of the fluorescence dot matrix in the second fluorescence dot matrix sequence image is calculated.

[0027] The flow field structure is obtained based on the velocity distribution of the fluorescence dot matrix in the second fluorescence dot matrix sequence image and the fluorescence image.

[0028] Preferably, the velocity distribution of the fluorescent dot matrix is ​​calculated as follows:

[0029]

[0030] in, The velocity distribution of the fluorescent dots in the second fluorescent dot matrix sequence image is the flow field velocity field distribution. This represents the actual fluorescence spatial offset. The sampling time interval, The number of pixel offsets in the second fluorescent dot matrix sequence image. For the spatial resolution of the fluorescence signal detection system, The time of the previous frame. This refers to the time of the next frame.

[0031] Preferably, the Cassegrain optical path shaping system further includes: an optical path adjustment device, which is used to adjust the relative position and angle between the first parabolic mirror and the hyperboloid mirror, and between the second parabolic mirror and the hyperboloid mirror.

[0032] The present invention also provides a method for measuring and visualizing flow field velocity, the method being based on the aforementioned flow field velocity measurement and visualization device, the method comprising:

[0033] Step 1: Set the output parameters of the dual-color laser emission system; set the power frequency and voltage of the surface dielectric barrier discharge system; adjust the position and angle of each optical path component in the Cassegrain optical path shaping system; adjust the gating and gain of the fluorescence signal detection system;

[0034] Step 2: Turn on the dual-color laser emission system, surface dielectric barrier discharge system, and fluorescence signal detection system, and adjust the Cassegrain optical path to focus the first laser and the femtosecond pulse laser on the detection area;

[0035] Step 3: Use a fluorescence signal detection system to acquire fluorescence images emitted by nitrogen molecules under stimulated emission, and obtain fluorescence images for acquiring flow field structure information and fluorescence signal flow videos for measuring flow field velocity;

[0036] Step 4: The data processing system visualizes the flow field structure of the detection area based on the fluorescence image, and measures the velocity of the flow field in the detection area based on the fluorescence signal flow video.

[0037] One or more technical solutions provided by this invention have at least the following technical effects or advantages:

[0038] This invention enables the simultaneous acquisition of velocity vectors and flow field structure parameters near the wall at millimeter-level spatial resolution and microsecond-level time scale.

[0039] This invention utilizes a tunable semiconductor laser to excite nitrogen molecules to produce fluorescence, achieving precise matching of emission wavelengths. Simultaneously, the plasma channel generated by the excitation, under the action of a high-power femtosecond laser, accelerates particle collisions with nitrogen molecules, enhancing the fluorescence signal to a certain extent. The combined use of two-color laser beams effectively improves excitation efficiency. By applying an electric field to the flow field under test using surface dielectric barrier discharge, the nitrogen gas molecules between the electrode and the dielectric surface are broken down, forming plasma that promotes collisions between free electrons and nitrogen molecules, achieving fluorescence enhancement. This avoids the shortcomings of low fluorescence intensity and short fluorescence lifetime caused by energy and wavelength issues in two-color beams. This invention excites common substances in the flow field to generate tracer particles, avoiding changes in the flow field structure caused by the dispersion of tracer particles and avoiding external disturbances. This invention utilizes a Cassegrain optical path shaping system to solve the spherical aberration caused by long-distance transmission of air lasers, while accurately focusing the two-color laser onto the test area, avoiding laser beam distortion and divergence problems caused by high temperature, high pressure, and high dynamic environments, ensuring that the laser can accurately act on the detection area and improving measurement accuracy. Attached Figure Description

[0040] The accompanying drawings, which are provided to further illustrate embodiments of the invention and constitute a part of this invention, are not intended to limit the scope of the invention.

[0041] Fig. 1 Schematic diagram of the flow field velocity measurement and visualization device

[0042] Fig. 2 This is a schematic diagram of the Cassegrain optical path shaping system;

[0043] Fig. 3 This is a flowchart of the post-processing of the present invention;

[0044] Among them, 1-femtosecond pulsed laser, 2-seed laser, 3-first parabolic mirror, 4-hyperboloid mirror, 5-detection area, 6-inner electrode, 7-dielectric layer, 8-outer electrode, 9-power supply, 10-high-speed camera, 11-ICCD camera, 12-lens, 13-filter, 14-timing controller, 15-data processing unit, and 16-second parabolic mirror. Detailed Implementation

[0045] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, where there is no conflict, the embodiments of the present invention and the features thereof can be combined with each other.

[0046] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0047] Those skilled in the art should understand that, in the disclosure of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting this invention.

[0048] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0049] Example 1;

[0050] Please refer to Figs. 1-3 The present invention provides a flow field velocity measurement and visualization device, the device comprising:

[0051] The system comprises a dual-color laser emission system, a surface dielectric barrier discharge system, a Cassegrain optical path shaping system, a fluorescence signal detection system, and a data processing system. The dual-color laser emission system includes a femtosecond pulsed laser 1 and a seed laser 2. The femtosecond pulsed laser is a Ti:sapphire femtosecond laser with a center wavelength of 800 nm and a repetition rate of 1000 Hz, serving as the pump source for the air laser. The seed laser is a wavelength-tunable semiconductor laser with an output wavelength selected in the 337 nm band, used to generate the first laser beam. Nitrogen molecules undergo stimulated emission under the synergistic effect of the femtosecond laser pulse and the seed laser pulse, producing partial fluorescence.

[0052] The surface dielectric barrier discharge system includes: a discharge electrode assembly, a dielectric layer 7, and a power supply 9; it includes an outer electrode 8 and an inner electrode 6 located above and below the dielectric layer, respectively. The positive terminal of the power supply is connected to the high-voltage input terminal of the inner electrode, and the negative terminal of the power supply is grounded. The grounding terminal of the electrode is connected to the high-voltage input terminal of the outer electrode, and the grounding terminal of the outer electrode is grounded. The inner electrode is implanted into the dielectric surface of the inner wall of the wind tunnel. The inner electrode adopts a stainless steel mesh structure to reduce interference with the flow field and enhance the uniformity of the discharge. The outer electrode is a copper electrode plate. The dielectric layer is selected with a high dielectric constant. A 2mm thick quartz glass with a diameter of 3.8 μm is used to cover the surface of the inner electrode. The power supply can output a sinusoidal AC high voltage with a frequency of 5-30kHz and a peak voltage of 10-20kV to drive the electrodes to generate a stable surface dielectric barrier discharge, forming a plasma environment within the detection area 5. An external power supply can drive a stable spatial electric field to be formed between the copper electrode plates, which helps to enhance the fluorescence signal intensity.

[0053] The Cassegrain optical path shaping system includes: a first parabolic mirror 3, a second parabolic mirror 16, a hyperboloid mirror 4, and an optical path adjustment device. The first parabolic mirror focuses and collimates the femtosecond pulse laser and reflects it to the hyperboloid mirror; the second parabolic mirror focuses and collimates the first laser and reflects it to the hyperboloid mirror; the hyperboloid mirror reflects and focuses the femtosecond pulse laser and the first laser onto the detection area. The focal length of the first and second parabolic mirrors is 1500mm. The hyperboloid mirror is fixed to the optical axis of the first and second parabolic mirrors by a bracket. The optical path adjustment device includes an angle adjustment knob and a displacement adjustment rail, used to precisely adjust the relative position and angle of the first and second parabolic mirrors and the hyperboloid mirror to adapt to the complex optical path changes required in hypersonic flow fields.

[0054] The fluorescence signal detection system includes a lens 12, a filter 13, an ICCD camera 11, and a high-speed camera 10. The lens is a fluorescence collecting lens group using a large numerical aperture lens in the ultraviolet to visible light band, used to collect and collimate the fluorescence signal of nitrogen molecules in the detection area. The filter is a bandpass filter with a center wavelength corresponding to the characteristic fluorescence wavelengths of nitrogen molecules (391nm, 427nm), used to suppress background light interference. The bandpass wavelength of the filter is within the wavelength range of the nitrogen fluorescence signal (391nm, 427nm). This design aims to effectively remove stray light and other interference, extracting only the fluorescence signal image. The ICCD camera is an enhanced ICCD camera with a temporal resolution of 1ns and a spatial resolution of 1920×1080 pixels, used to capture highly sensitive fluorescence images. The high-speed camera is used to capture video images of the fluorescence signal flowing with the flow field.

[0055] The data processing system includes a timing controller 14 and a data processing unit 15. The timing controller is used to realize the timing synchronization of the power supply, ICCD camera and high-speed camera in the femtosecond pulse laser, seed laser, surface dielectric barrier discharge system, with a triggering accuracy of 10 ns. The data processing unit is developed based on the MATLAB platform and has image processing algorithms such as fluorescence signal enhancement, noise reduction and three-dimensional reconstruction, which are used to realize the characterization of the flow field structure by the fluorescence signal.

[0056] The energy fluctuation range of the laser pulse generated by the femtosecond laser unit is within ±5% of the set value to ensure the stability of nitrogen molecules excited by induced air laser stimulated radiation.

[0057] Within the detection area, nitrogen molecules (N2) are excited by the combined effects of dual-color laser and SDBD plasma. * The discrete set of spatial fluorescent markers formed by stimulated emission is called a fluorescent dot array.

[0058] The wavelength adjustment precision of the seed laser reaches 0.01 nm, enabling precise excitation of specific energy levels of nitrogen molecules, enhancing the fluorescence signal, and thus more accurately characterizing the flow field structure through the fluorescence signal.

[0059] The power supply of the surface dielectric barrier discharge system has a frequency adjustment accuracy of 0.1kHz and a peak voltage adjustment accuracy of 0.1kV to precisely control the plasma environment generated by the surface dielectric barrier discharge. The inner electrode, as a high-voltage electrode, is connected to the power supply and implanted on the dielectric surface of the wind tunnel inner wall to achieve uniform discharge. The outer electrode is exposed and grounded to avoid discharge quenching caused by charge accumulation on the dielectric surface.

[0060] Among them, the lens in the fluorescence signal detection system, namely the fluorescence collecting lens, has a transmittance of ≥90% in the wavelength range of 337-427nm, so as to efficiently collect the fluorescence signal of nitrogen molecules, improve the signal intensity, and enhance the sensitivity of flow field structure characterization.

[0061] The ICCD camera has a dynamic range of at least 120 dB to capture fluorescence signals of nitrogen molecules of varying intensities, providing support for fine characterization of the flow field structure. The high-speed camera has a frame rate of up to 10 kHz per second, a temporal resolution of up to 10 ns, and a resolution of 2048 × 2048, enabling the capture of subtle dynamic changes in the hypersonic flow field. The filters are mounted in front of the ICCD and high-speed camera lenses to reduce background interference from laser light and the environment, thereby enhancing the signal-to-noise ratio.

[0062] The seed laser is a tunable semiconductor laser with an output wavelength controlled at 337nm, corresponding to the emission wavelength of nitrogen molecules, which enhances the excitation efficiency. The output power can be adjusted according to the needs, with an adjustment range of 0-100mW, and is used to excite specific energy levels of nitrogen molecules.

[0063] Furthermore, the 337nm seed laser possesses high photon energy. When it irradiates nitrogen molecules, the photon energy is absorbed by the nitrogen molecules. The ionization energy provided by this wavelength is approximately 3.5-3.7 eV according to the following photon energy formula. Nitrogen molecules absorb photons, generating plasma through multiphoton ionization.

[0064] ;

[0065] in, It is ionization energy. Let be Planck's constant. At the speed of light, The incident laser wavelength;

[0066] Furthermore, under the action of an 800nm ​​femtosecond laser pulse, free electrons in the plasma absorb laser energy and gain kinetic energy. The kinetic energy relationship of the stimulated electrons is as follows:

[0067] ;

[0068] in, For the kinetic energy of electrons, The wavelength for absorbing laser light is;

[0069] Furthermore, the 800nm ​​infrared femtosecond laser has high power density and wavelength, which can effectively drive free electrons to accelerate and collide with nitrogen molecules, transferring some energy to the nitrogen molecules and exciting them to higher energy levels. When these nitrogen molecules excited to higher energy levels transition back to lower energy levels, they emit fluorescence, thus achieving fluorescence enhancement.

[0070] The first and second parabolic mirrors converge parallel light to the vicinity of their focal point. The hyperboloid mirror, located near this focal point, reflects the light converged by the first and second parabolic mirrors again, focusing the light onto the target area and generating a certain fluorescence signal. This also achieves long-distance focusing and eliminates spherical aberration.

[0071] Furthermore, the gas is broken down to form plasma, which contains a large number of active particles, such as high-energy electrons colliding with nitrogen molecules. According to the law of conservation of energy, the kinetic energy of the electrons is converted into the internal energy of the nitrogen molecules, thereby exciting the nitrogen to achieve energy level transitions and generate fluorescence signals. The enhancement of fluorescence signals can be described as follows:

[0072] ;

[0073] ;

[0074] in, It is Planck's constant. It is the frequency of fluorescent photons. For electrons, It is a nitrogen molecule. For excited-state nitrogen molecules;

[0075] In the cross-correlation analysis based on tracer particle images, the size of the cross-correlation window is determined by factors such as the sampling time interval. The maximum possible displacement is estimated based on the fluid velocity and the camera shooting time interval, and generally three times the maximum displacement is selected as the window side length.

[0076] Furthermore, the data processing unit can be a computer, which is mainly used for video acquisition, video cropping, video frame extraction, background subtraction, image enhancement, cross-correlation particle offset calculation, flow field velocity field calculation, velocity field inversion, and flow field structure visualization. Video acquisition is mainly used to capture video files of the flow field's evolution over time using a high-speed camera; video cropping is used to extract the effective regions from the acquired images; video frame extraction converts the video files into frame-by-frame image files; background subtraction is used to remove the bright and dark background caused by shadows outside the boundaries, as well as the radiation and scattered light from the laser source; image enhancement uses grayscale binarization and other methods to improve the signal-to-noise ratio of the bitmap; cross-correlation particle offset calculation uses cross-correlation algorithms to calculate the relative displacement of the images before and after the flow field passes through; velocity field inversion uses the hypersonic flow field velocity field calculation formula to calculate the transient velocity; flow field visualization is mainly displayed by acquiring fluorescence images.

[0077] Furthermore, the laser intensity of a femtosecond laser can be adjusted according to the signal-to-noise ratio of the image acquired by the imaging system, and incoherent lasers can also be used to reduce diffraction effects. For transient measurement requirements, high-power pulsed lasers can be used for Ti:sapphire femtosecond lasers.

[0078] The first and second parabolic mirrors must possess high reflectivity (above 90%) and high-precision optical surfaces to reduce scattering and distortion during light reflection, ensuring sufficient light energy collection and transmission. The hyperboloid mirror requires high reflectivity (above 90%), and its position and angle must be strictly controlled during installation. Pitch fine-tuning ensures the dual-color laser is focused on the target area, generating stable plasma. Under the synergistic effect of the dual-color laser and the electric field, nitrogen molecules absorb the laser light to generate a plasma channel and gain kinetic energy, promoting stimulated emission to produce a fluorescence signal.

[0079] The dielectric layer can be a quartz glass insulator, which is placed in the test area near the wall of the wind tunnel. The upper and lower surfaces of the quartz glass insulator are seamlessly attached to the near wall of the wind tunnel and the stainless steel mesh copper electrode, respectively. The three are parallel to each other. The quartz glass insulator is connected to the inner wall of the wind tunnel and the mesh copper electrode, and the copper electrode plate is connected to the outer wall of the wind tunnel by an adhesive (epoxy resin).

[0080] The copper electrode plate consists of multiple parallel plate electrodes connected in parallel with equal spacing between adjacent plates, eliminating the risk of concentrated discharge from the parallel plate electrodes, improving discharge stability, thereby ensuring plasma stability and enhancing fluorescence signal; the high-voltage electrode is a stainless steel mesh copper electrode, which increases the contact area between the dielectric and the discharge gas, replenishes the gas consumed during the discharge process in a timely manner, and makes the electric field distribution more uniform.

[0081] The quartz glass between the stainless steel mesh copper electrode and the top wall of the quartz glass insulator has a certain thickness, acting as the dielectric in the dielectric barrier discharge, and this thickness ensures the shock resistance of the quartz material. However, the thickness of the quartz glass material cannot be too thick, otherwise successful dielectric barrier discharge cannot be guaranteed. In this invention, the thickness of the quartz glass material is set to 2mm. This ensures that the quartz glass insulator has a certain thickness while guaranteeing successful surface dielectric barrier discharge, thus improving the shock resistance of the ceramic material.

[0082] The following section introduces methods for measuring and visualizing flow field velocity:

[0083] Select a suitable position on the side optical window of the flow field and place the image acquisition system. The order of placement is as follows: high-speed camera, lens and filter, and ICCD camera, lens and filter on the opposite side. The three are connected by a snap-fit ​​method. Set the appropriate gate width and timing through the timing controller, adjust the lens focal length, and clearly image the dot matrix pattern onto the high-speed camera and ICCD camera.

[0084] The timing controller controls the timing and sampling interval of the femtosecond laser, semiconductor laser, and camera image acquisition.

[0085] When the high-speed camera is working, it captures the dot matrix pattern on the screen, forms a video file, and transmits it to the post-processing computer via a transmission line.

[0086] The computer performs the following steps on the acquired video files in sequence: comparison bitmap correction, image cropping, video frame export, bitmap extraction, background subtraction, image enhancement, and cross-correlation particle offset calculation to finally characterize the flow field velocity field.

[0087] The ICCD camera can characterize the flow field using fluorescence signal images, such as observing the distribution and intensity changes of fluorescence images to determine whether there are characteristic structures such as vortices, jets, and boundary layers.

[0088] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0089] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A flow field velocity measurement and visualization device, characterized in that, The device includes: Dual-color laser emission system, surface dielectric barrier discharge system, Cassegrain optical path shaping system, fluorescence signal detection system, and data processing system; The dual-color laser emission system generates a femtosecond pulsed laser and a first laser. The Cassegrain optical path shaping system converges and collimates the femtosecond pulsed laser and the first laser, focusing them onto the detection area. The first laser pre-excites nitrogen molecules in the detection area, while the femtosecond pulsed laser induces stimulated emission of nitrogen in the detection area. The first laser and the femtosecond pulsed laser synergistically excite nitrogen molecules. The surface dielectric barrier discharge system creates a plasma environment within the detection area. The fluorescence signal detection system collects fluorescence signals from nitrogen molecules within the detection area, obtaining a fluorescence image for acquiring flow field structure information and a fluorescence signal flow video for measuring flow field velocity. The data processing system controls the timing of each system in the device, visualizes the flow field structure of the detection area based on the fluorescence image, and measures the velocity of the flow field in the detection area based on the fluorescence signal flow video. The surface dielectric barrier discharge system includes: a discharge electrode assembly, a dielectric layer, and a power supply; the discharge electrode assembly includes an outer electrode and an inner electrode located above and below the dielectric layer, respectively; the positive terminal of the power supply is connected to the high-voltage input terminal of the inner electrode, the negative terminal of the power supply is grounded, and the grounding terminal of the outer electrode is grounded; the inner electrode is implanted on the surface of the dielectric layer located on the inner wall of the wind tunnel, the outer electrode is arranged on the outer wall of the wind tunnel, and the inner and outer electrodes are arranged parallel to each other on the wall of the wind tunnel to be tested, with the area of ​​the dielectric layer being larger than the area of ​​the inner electrode.

2. The flow field velocity measurement and visualization device according to claim 1, characterized in that, The dual-color laser emitting system includes a femtosecond pulse laser for generating femtosecond pulse lasers and a seed laser for generating a first laser.

3. The flow field velocity measurement and visualization device according to claim 1, characterized in that, The Cassegrain optical path shaping system includes: a first parabolic mirror, a second parabolic mirror, and a hyperboloid mirror; wherein, the first parabolic mirror is used to converge and collimate the femtosecond pulse laser and reflect it to the hyperboloid mirror; the second parabolic mirror is used to converge and collimate the first laser and reflect it to the hyperboloid mirror; the hyperboloid mirror is used to reflect and focus the femtosecond pulse laser and the first laser onto the detection area.

4. The flow field velocity measurement and visualization device according to claim 1, characterized in that, The fluorescence signal detection system includes: a lens, an ICCD camera, a high-speed camera, and a filter; The ICCD camera and the high-speed camera are both equipped with the aforementioned lens, and the filter is installed at the front end of the lens. The lens is used to collect and collimate the fluorescence signal of nitrogen molecules in the detection area; the filter is used to suppress the background light in the fluorescence signal of nitrogen molecules; the ICCD camera is used to obtain fluorescence images; and the high-speed camera is used to obtain video of the fluorescence signal flow.

5. The flow field velocity measurement and visualization device according to claim 1, characterized in that, The data processing system includes a timing controller and a data processing unit; the timing controller is used to control the timing of each system in the device; the data processing unit is used to visualize the flow field structure of the detection area based on the fluorescence image, and to measure the velocity of the flow field in the detection area based on the fluorescence signal flow video.

6. The flow field velocity measurement and visualization device according to claim 1, characterized in that, The visualization and characterization of the flow field structure in the detection region based on the fluorescence image specifically includes: The effective area acquired by the fluorescence signal detection system is selected to obtain the effective area video. The effective area video is then converted into frame-by-frame images according to the time sequence to obtain the sequence image. The edge information of the fluorescent dot matrix in the sequence image is extracted based on the edge detection algorithm, and the background part outside the fluorescent dot matrix is ​​removed based on the edge information to obtain the first fluorescent dot matrix sequence image. The spatial resolution of the fluorescence signal detection system is obtained based on the scale calibration calculation of the fluorescence signal detection system. Image enhancement processing is performed on the first fluorescent dot matrix sequence image to obtain the second fluorescent dot matrix sequence image; Based on the initial image and the second fluorescence array sequence image of the flow field in the detection area, the target surface imaging offset is calculated using a cross-correlation algorithm. The actual fluorescence spatial offset is calculated by multiplying the spatial resolution of the fluorescence signal detection system and the target imaging offset. Based on the actual fluorescence spatial offset, the velocity distribution of the fluorescence dot matrix in the second fluorescence dot matrix sequence image is calculated. The flow field structure is obtained based on the velocity distribution of the fluorescent dots in the second fluorescent dot matrix sequence image and the fluorescence image.

7. The flow field velocity measurement and visualization device according to claim 6, characterized in that, The velocity distribution of the fluorescence lattice in the second fluorescence lattice sequence image is calculated as follows: in, The velocity distribution of the fluorescent dots in the second fluorescent dot matrix sequence image. This represents the actual fluorescence spatial offset. The sampling time interval, The number of pixel offsets in the second fluorescent dot matrix sequence image. For the spatial resolution of the fluorescence signal detection system, The time of the previous frame. This refers to the time of the next frame.

8. The flow field velocity measurement and visualization device according to claim 3, characterized in that, The Cassegrain optical path shaping system further includes an optical path adjustment device, which is used to adjust the relative position and angle between the first parabolic mirror and the hyperboloid mirror, as well as between the second parabolic mirror and the hyperboloid mirror.

9. A method for measuring and visualizing flow field velocity, characterized in that, The method is based on any one of the flow field velocity measurement and visualization devices described in claims 1-8, and the method includes: Step 1: Set the output parameters of the dual-color laser emission system; set the power frequency and voltage of the surface dielectric barrier discharge system; adjust the position and angle of each optical path component in the Cassegrain optical path shaping system; adjust the gating and gain of the fluorescence signal detection system; Step 2: Turn on the dual-color laser emission system, surface dielectric barrier discharge system, and fluorescence signal detection system, and adjust the Cassegrain optical path to focus the first laser and the femtosecond pulse laser on the detection area; Step 3: Use a fluorescence signal detection system to acquire fluorescence images emitted by nitrogen molecules under stimulated emission, and obtain fluorescence images for acquiring flow field structure information and fluorescence signal flow videos for measuring flow field velocity; Step 4: The data processing system visualizes the flow field structure of the detection area based on the fluorescence image, and measures the velocity of the flow field in the detection area based on the fluorescence signal flow video.

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