Flow field velocity measurement and visualization device and method

Abstract

The invention discloses a flow field velocity measurement and visualization device and method, and relates to the technical field of flow field display measurement, and the device comprises a two-color laser emission system, a surface dielectric barrier discharge system, a Cassegrain light path shaping system, a fluorescence signal detection system and a data processing system. Wherein the two-color laser emission system is used for generating femtosecond pulse laser and first laser, and the femtosecond pulse laser and the first laser are converged and collimated by the Cassegrain light path shaping system and then are focused to a detection area to cooperatively excite nitrogen molecules; the surface dielectric barrier discharge system forms a plasma environment in the detection area; the fluorescence signal detection system is used for collecting nitrogen molecule fluorescence signals in the detection area; and the data processing system is used for controlling a time sequence, realizing visual representation of a flow field structure in a detection area and realizing speed measurement of a flow field in the detection area, and the device and the method can realize synchronous acquisition of a speed vector and flow field structure parameters near a wall surface under a millimeter-level spatial resolution and a microsecond-level time scale.

Description

Technical Field

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

[0002] The flow field of ultra-low density wind tunnels has significant non-equilibrium characteristics and complex physical mechanisms, and its characteristics are mainly reflected in the following aspects: 1) Dominance of rarefied gas effect: In a low-density environment, the mean free path of gas molecules can reach the millimeter level, resulting in the failure of the continuous medium assumption and the flow showing a significant rarefied effect; 2) Coupling of high-temperature gas effects: When the aircraft speed exceeds Mach 10, the shock layer temperature generated by the compression of the high-speed airflow can reach tens of thousands of degrees Celsius, causing the dissociation or even ionization of oxygen and nitrogen molecules, forming a multi-component non-equilibrium plasma flow field containing atoms, ions and electrons, which will cause the aerodynamic thermodynamic parameters (heat conduction, viscosity coefficient) to be significantly different from those under conventional conditions. The time and space scales of chemical reactions in the flow field are highly coupled with the scale of fluid motion, further exacerbating the complexity of measurement and modeling; 3) Special characteristics of the flow at the wall: Under low-density conditions, the ratio of the boundary layer thickness to the characteristic size of the aircraft increases significantly, and the velocity slip and temperature jump phenomena in the boundary layer are prominent. In addition, the expansion of the flow separation zone caused by the shock wave / boundary layer interaction leads to strong unsteadiness in the wall pressure and heat flux distribution, which places extreme demands on the spatial resolution and dynamic response of the measurement technology.

[0003] To address these issues, existing flow field visualization technologies face the following key bottlenecks: 1) Traditional particle image velocimetry (PIV) fails: Gas molecules in low-density environments are extremely dense, making effective labeling impossible by seeding tracer particles. Even with forced particle injection, the particle's tracking ability is severely reduced due to the rarefaction effect, and the momentum exchange between the particles and the airflow can alter the intrinsic flow characteristics, distorting the results. 2) NO-PLIF (nitric oxide planar laser-induced fluorescence) technology has significant limitations. (NO-PLIF, a non-contact optical diagnostic method based on laser spectroscopy, is a non-contact optical diagnostic method based on laser spectroscopy.) Although NO-PLIF, a nitric oxide tracer-based PIV technique, does not require external particles, its reliance on gas injection leads to two issues: First, excessive concentrations of tracer gases (such as NO) can alter the local flow field chemical composition, interfering with the true flow field structure. Second, some tracer gases are highly toxic, posing safety and environmental risks in closed wind tunnel systems. 3) Electron beam fluorescence (EBF) suffers from coverage blind spots: While EBF can measure flow velocity and density over a wide range, its fluorescence lifetime (typically on the order of microseconds) and low-density resolution are insufficient. More importantly, the electron beam is susceptible to electromagnetic interference and material shielding effects in the wall area, resulting in data loss in key areas.

[0004] The current technical problems are mainly reflected in the insufficient in-situ measurement capability of the wall: the existing methods make it difficult to synchronously obtain the velocity vector 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 current technology's insufficient in-situ measurement capabilities on the wall surface, and to achieve simultaneous acquisition of velocity vectors and flow field structural parameters near the wall surface at millimeter-level spatial resolution and microsecond-level time scale, the present invention provides a flow field velocity measurement and visualization device, comprising:

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

[0007] Among them, the dual-color laser emission system is used to generate femtosecond pulse laser and a first laser; the Cassegrain optical path shaping system is used to converge and collimate the femtosecond pulse laser and the first laser and focus them to the detection area; the first laser is used to pre-excite the nitrogen molecules in the detection area, and the femtosecond pulse laser is used to induce the nitrogen in the detection area to produce stimulated radiation, and the first laser and the femtosecond pulse laser cooperate to excite the nitrogen molecules; the surface dielectric barrier discharge system is used to form a plasma environment in the detection area; the fluorescence signal detection system is used to collect the fluorescence signals of nitrogen molecules in the detection area, and obtain a fluorescence image for obtaining flow field structure information and a fluorescence signal flow video for measuring the flow field velocity; the data processing system is used to control the timing of each system in the device, and to realize the visualization representation of the flow field structure in the detection area based on the fluorescence image, and to realize the velocity measurement of the flow field in the detection area based on the fluorescence signal flow video.

[0008] Among them, the present invention utilizes a dual-color laser emission system combined with ultrafast diagnostic technology of femtosecond laser, and utilizes its extremely short pulse width (<100fs) and high peak power characteristics, through beam shaping and multi-wavelength collaborative excitation, to achieve selective excitation and fluorescence capture of molecular vibration energy levels near the wall. The present invention can circumvent the interference problem of traditional tracers and enhance the local fluorescence signal in the form of planar dielectric barrier discharge, thereby breaking through the spatial and temporal resolution limits of near-wall flow field measurements, and providing key data support for the aerodynamic design and thermal protection optimization of hypersonic aircraft.

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

[0010] Preferably, the Cassegrain optical path shaping system includes: a first parabolic reflector, a second parabolic reflector, and a hyperbolic reflector; wherein the first parabolic reflector is used to converge and collimate the femtosecond pulse laser and reflect it to the hyperbolic reflector; the second parabolic reflector is used to converge and collimate the first laser and reflect it to the hyperbolic reflector; and the hyperbolic reflector is used to reflect and focus the femtosecond pulse laser and the first laser onto the detection area. The first parabolic reflector, the second parabolic reflector, and the hyperbolic reflector, in conjunction with the optical path adjustment device, accurately focus the two-color laser light consisting of the femtosecond pulse laser and the first laser 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; the outer electrode and inner electrode are located above and below the dielectric layer, respectively; the positive electrode of the power supply is connected to the high-voltage input terminal of the inner electrode, the negative electrode of the power supply is grounded, the ground terminal of the inner electrode is connected to the high-voltage input terminal of the outer electrode, and the ground terminal of the outer electrode is grounded; the inner electrode is implanted on 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 the wind tunnel wall to be measured, and the area of ​​the dielectric layer is larger than that of the inner electrode, ensuring that the electric field uniformly covers the measurement area, thereby generating a large amount of plasma in the measurement area and improving the fluorescence excitation efficiency. In particular, when 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 comprises: a lens, an ICCD camera, a high-speed camera and a filter;

[0013] The ICCD camera and the high-speed camera are both equipped with the lens, with the filter mounted on the front end of the lens. The lens is used to collect and collimate the fluorescence signal of nitrogen molecules within the detection area; the filter is used to suppress background light in the nitrogen molecule fluorescence signal; the ICCD camera is used to obtain a fluorescence image; and the high-speed camera is used to obtain a flow video of the fluorescence signal. The fluorescence collection lens assembly in the fluorescence signal detection system, i.e., the lens, collects the fluorescence signal. After background interference is filtered out by the filter, the ICCD camera captures the fluorescence image to obtain flow field structure information, and the high-speed camera captures the flow video of the fluorescence signal for velocity measurement.

[0014] Preferably, 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 in 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. The timing controller of the data processing system ensures the timing synchronization of each component, and the data processing unit, based on the MATLAB platform, performs image conversion, information extraction, and computational analysis on the collected data to achieve flow field structure visualization and velocity measurement.

[0015] Preferably, the visualization of the flow field structure of the detection area 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 the amount of calculation data, and converts the effective area video into frame-by-frame pictures according to the time series to obtain a sequence image;

[0017] extracting edge information of the fluorescent dot matrix in the sequence image based on an edge detection algorithm, retaining the fluorescent dot matrix area based on the edge information, and removing the background portion outside the fluorescent dot matrix to obtain a first fluorescent dot matrix sequence image;

[0018] performing image enhancement processing on the first fluorescent dot matrix sequence image to obtain a second fluorescent dot matrix sequence image;

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

[0020] Based on the initial image of the flow field in the detection area and the second fluorescent dot matrix sequence image, the target surface imaging offset is calculated using the cross-correlation algorithm;

[0021] The actual fluorescence spatial offset is calculated based on the product of 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, is the horizontal offset of the next frame relative to the previous frame of fluorescence image, is the vertical offset of the next frame relative to the previous frame of fluorescence image, is the cross-correlation function, M is the number of pixels in the vertical direction of the sequence image, N is the number of pixels in the horizontal direction of the sequence image, is the pixel index of the fluorescence image in the vertical direction, is the pixel index of the fluorescence image in the horizontal direction, is the grayscale value of the previous frame of fluorescence image, is the grayscale value of the next frame of fluorescence image, is the pixel offset of the acquired image;

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

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

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

[0029]

[0030] in, is the velocity distribution of the fluorescent dot matrix in the second fluorescent dot matrix sequence image, i.e., the velocity field distribution of the flow field, is the actual fluorescence spatial offset, is the sampling time interval, is the number of pixel offsets of the second fluorescent dot matrix sequence image, is the spatial resolution of the fluorescence signal detection system, is the moment of the previous frame image, is the moment of the next frame image.

[0031] Preferably, the Cassegrain optical path shaping system further comprises: an optical path adjustment device, wherein the optical path adjustment device is used to adjust the relative positions and angles between the first parabolic reflector and the hyperbolic reflector, and between the second parabolic reflector and the hyperbolic reflector.

[0032] The present invention also provides a flow field velocity measurement and visualization method, which is based on the flow field velocity measurement and visualization device, and includes:

[0033] Step 1: Set the output parameters of the dual-color laser emission system; set the power supply 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; and adjust the gate and gain of the fluorescence signal detection system.

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

[0035] Step 3: Using a fluorescence signal detection system to collect fluorescence images emitted by stimulated radiation from nitrogen molecules, obtaining fluorescence images for obtaining flow field structure information and fluorescence signal flow videos for measuring flow field velocity;

[0036] Step 4: The data processing system realizes visualization representation of the flow field structure in the detection area based on the fluorescent image, and the data processing system realizes velocity measurement of the flow field in the detection area based on the fluorescent signal flow video.

[0037] The one or more technical solutions provided by the present invention have at least the following technical effects or advantages:

[0038] The present invention can achieve synchronous acquisition of velocity vector and flow field structure parameters near the wall surface at millimeter-level spatial resolution and microsecond-level time scale.

[0039] The present invention utilizes a tunable semiconductor laser to excite nitrogen molecules to produce fluorescence, achieving strict emission wavelength matching. Simultaneously, the plasma channel generated by the excitation is acted upon by a high-power femtosecond laser, achieving particle acceleration and collision with nitrogen molecules, which can enhance the fluorescence signal to a certain extent. The combined use of two-color laser beams effectively improves the excitation efficiency. A surface dielectric barrier discharge is used to apply an electric field to the flow field to be measured, causing nitrogen molecules between the electrode and the dielectric surface to be broken down, forming a plasma that promotes collisions between free electrons and nitrogen molecules, achieving fluorescence enhancement and avoiding the shortcomings of the two-color beam, which suffer from low fluorescence intensity and short fluorescence lifetime due to energy and wavelength issues. The present invention excites common substances in the flow field to produce tracer particles, avoiding changes in the flow field structure caused by the diffusion of tracer particles and external disturbances. The present invention utilizes a Cassegrain optical path shaping system to resolve the spherical aberration generated by long-distance transmission of air lasers, while accurately focusing the two-color laser onto the area to be measured, avoiding laser beam distortion and divergence 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of the present invention, and do not constitute a limitation of the embodiments of the present invention;

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

[0042] Figure 2 Schematic diagram of the Cassegrain optical path shaping system;

[0043] Figure 3 This is a post-processing flow chart of the present invention;

[0044] Among them, 1-femtosecond pulse laser, 2-seed laser, 3-first parabolic mirror, 4-hyperbolic 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, 16-second parabolic mirror. DETAILED DESCRIPTION

[0045] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features therein can be combined with each other without conflict.

[0046] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0047] Those skilled in the art should understand that, in the disclosure of the present invention, the terms "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the above terms should not be understood as limiting the present invention.

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

[0049] Embodiment 1;

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

[0051] The system includes 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 pulse laser 1 and a seed laser 2. The femtosecond pulse laser uses a titanium sapphire femtosecond laser with a central wavelength of 800nm ​​and a repetition rate of 1000Hz, serving as the pump source for the air laser. The seed laser uses a wavelength-tunable semiconductor laser with an output wavelength selected in the 337nm band, which is used to generate the first laser. The nitrogen molecules are stimulated to emit partial fluorescence under the synergistic action of the femtosecond laser pulses and the seed laser pulses.

[0052] The surface dielectric barrier discharge system includes: a discharge electrode assembly, a dielectric layer 7 and a power supply 9; the outer electrode 8 and the inner electrode 6 are located above and below the dielectric layer respectively, the positive electrode of the power supply is connected to the high voltage input terminal of the inner electrode, the negative electrode of the power supply is grounded, the ground terminal of the electrode is connected to the high voltage input terminal of the outer electrode, the ground terminal of the outer electrode is grounded, and the inner electrode is implanted on 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 adopts a high dielectric constant =3.8 quartz glass, 2mm thick, covers the inner electrode surface. The power supply is capable of outputting a sinusoidal AC high voltage with a frequency of 5-30kHz and a peak voltage of 10-20kV, which is used 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 the formation of a stable spatial electric field between the copper electrode plates, which helps enhance the intensity of the fluorescence signal.

[0053] The Cassegrain optical path shaping system includes: a first parabolic reflector 3, a second parabolic reflector 16, a hyperbolic reflector 4, and an optical path adjustment device. The first parabolic reflector is used to converge and collimate the femtosecond pulse laser and reflect it to the hyperbolic reflector; the second parabolic reflector is used to converge and collimate the first laser and reflect it to the hyperbolic reflector; and the hyperbolic reflector is used to reflect and focus the femtosecond pulse laser and the first laser onto the detection area. The focal length of the first and second parabolic reflectors is 1500mm, and the hyperbolic reflector is fixed to the optical axis of the first and second parabolic reflectors via a bracket. The optical path adjustment device includes an angle adjustment knob and a displacement adjustment guide rail for precisely adjusting the relative position and angle of the first and second parabolic reflectors and the hyperbolic reflector to accommodate 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 collection lens group that uses a lens with a large numerical aperture in the ultraviolet to visible light band, which is 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 wavelength of nitrogen molecules (391nm, 427nm), which is used to suppress background light interference. The bandpass wavelength of the filter is within the wavelength range of the nitrogen fluorescence signal (391nm, 427nm). The purpose of this design is to enable the filter to effectively remove interference such as stray light and only extract the fluorescence signal image. The ICCD camera is an enhanced ICCD camera with a time resolution of 1ns and a spatial resolution of 1920×1080 pixels, which is used to capture high-sensitivity fluorescence images. The high-speed camera is used to capture video images of the fluorescence signal flowing along 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 achieve timing synchronization of femtosecond pulse laser, seed laser, power supply, ICCD camera and high-speed camera in the surface dielectric barrier discharge system, with a triggering accuracy of 10ns; 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 is used to realize the characterization of flow field structure by fluorescence signal.

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

[0057] Among them, in the detection area, nitrogen molecules (N2 * ) The set of discrete spatial fluorescent marker points formed by stimulated radiation is called a fluorescent dot matrix.

[0058] Among them, the wavelength adjustment accuracy of the seed laser reaches 0.01nm, so as to achieve precise excitation of specific energy levels of nitrogen molecules, enhance the fluorescence signal, and thus more accurately characterize 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, so as to precisely control the plasma environment generated by the surface dielectric barrier discharge. The inner electrode is connected to the power supply as a high-voltage electrode and implanted on the dielectric surface of the inner wall of the wind tunnel to achieve uniform discharge. The outer electrode is grounded as an exposed ground level to avoid discharge quenching caused by charge accumulation on the dielectric surface.

[0060] Among them, the lens in the fluorescence signal detection system is the fluorescence collection lens, which 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 120dB, enabling it to capture nitrogen molecule fluorescence signals of varying intensities, supporting detailed characterization of flow field structures. The high-speed camera can achieve a frame rate of 10k frames per second, a temporal resolution of 10ns, and a resolution of 2048x2048 pixels, enabling the detection of subtle dynamic changes in hypersonic flow fields. Filters installed in front of the ICCD and high-speed camera lenses reduce background interference from the laser and the environment, enhancing the signal-to-noise ratio.

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

[0063] Furthermore, the 337nm seed laser has 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 3.5-3.7eV, according to the following photon energy formula. Nitrogen molecules absorb the photons, generating plasma through multiphoton ionization.

[0064] ;

[0065] in, is the ionization energy, is Planck's constant, is the speed of light, is the incident laser wavelength;

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

[0067] ;

[0068] in, is the kinetic energy of the electron, The wavelength of the absorbed laser is;

[0069] Furthermore, the 800nm ​​infrared femtosecond laser has a high power density and wavelength, which can effectively drive free electrons to accelerate and collide with nitrogen molecules, transferring some of the energy to the nitrogen molecules, causing the nitrogen molecules to be excited to a higher energy level. These nitrogen molecules excited to a high energy level will emit fluorescence when they transition back to a low energy level, thereby achieving fluorescence enhancement;

[0070] Among them, the first parabolic reflector and the second parabolic reflector converge parallel light to their focal point. The hyperbolic reflector is located near the focal point and reflects the light converged by the first parabolic reflector and the second parabolic reflector again, so that the light is focused on the target area, generating a certain fluorescence signal, and at the same time achieving long-distance focusing and eliminating spherical aberration.

[0071] Furthermore, the gas is broken down to form plasma, in which a large number of active particles, such as high-energy electrons, collide 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 transition and generate fluorescence signals. The fluorescence signal enhancement can be described as follows:

[0072] ;

[0073] ;

[0074] in, is Planck's constant, is the frequency of the fluorescence photon, For electronics, is a nitrogen molecule, is an excited nitrogen molecule;

[0075] Among them, when performing 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. 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 for high-speed cameras to capture video files of the flow field's evolution over time; video cropping is used to extract the effective area in the captured image; video frame extraction converts the video file into a frame-by-frame image file; background subtraction is used to subtract the bright and dark background generated by shadows outside the boundary, as well as laser light source radiation and scattered light; image enhancement uses grayscale binarization and other methods to improve the signal-to-noise ratio of the dot matrix; cross-correlation particle offset calculation uses a cross-correlation algorithm to calculate the images before and after the flow field passes through to obtain relative displacement; velocity field inversion uses the hypersonic flow field velocity field calculation formula to calculate the transient velocity; flow field visualization is mainly displayed by collecting fluorescence images.

[0077] Furthermore, the laser intensity of the femtosecond laser can be adjusted according to the signal-to-noise ratio of the image captured by the imaging system, and the laser can also use incoherent lasers to reduce diffraction effects. For transient measurement needs, titanium sapphire femtosecond lasers can be used as high-power pulsed lasers.

[0078] The first and second parabolic reflectors must possess excellent reflectivity (above 90%) and high-precision optical surfaces to reduce scattering and distortion during light reflection, ensuring sufficient light energy collection and transmission. The hyperbolic reflector also requires high reflectivity (above 90%), and its position and angle must be strictly controlled during installation. Fine-tuning the pitch ensures that the two-color laser is focused on the target area and generates a stable plasma in the target area. Under the synergistic effect of the two-color laser and the electric field, nitrogen molecules absorb the two-color laser light to generate a plasma channel and gain a certain amount of kinetic energy, promoting stimulated emission to produce a fluorescence signal.

[0079] The dielectric layer can be a quartz glass insulator, which is arranged in the test area near the wall of the wind tunnel. The upper and lower surfaces of the quartz glass insulator are seamlessly bonded 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, the mesh copper electrode, and the copper electrode plate is connected to the outer wall of the wind tunnel through an adhesive (epoxy resin).

[0080] The copper electrode plate is composed of multiple parallel flat electrodes connected in parallel, with equal spacing between adjacent flat electrodes. This eliminates the risk of concentrated discharge on the parallel flat electrodes and improves discharge stability, thereby ensuring plasma stability and enhancing the fluorescence signal. The high-voltage electrode is a stainless steel mesh copper electrode, which can increase the contact area between the dielectric and the discharge gas, replenish the gas consumed during the discharge process in a timely manner, and make 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. This thickness ensures the quartz material's shock resistance. At the same time, the quartz glass material should not be too thick, otherwise successful dielectric barrier discharge cannot be guaranteed. In the present invention, the quartz glass material thickness is set to 2 mm. This ensures that the quartz glass insulator has a certain thickness while ensuring successful surface dielectric barrier discharge, thereby improving the ceramic material's shock resistance.

[0082] The following is an introduction to the flow field velocity measurement and visualization method:

[0083] Select a suitable location on the optical window on the side of the flow field and place the image acquisition system. The order of placement is the high-speed camera, lens, and filter, and the ICCD camera, lens, and filter on the opposite side. The three are connected by snap-fitting. Use the timing controller to set the appropriate gate width and timing, adjust the lens focal length, and clearly image the dot pattern onto the high-speed camera and ICCD camera.

[0084] The timing controller controls the timing and sampling interval of the femtosecond laser, the semiconductor laser and the camera to collect images;

[0085] When the high-speed camera is working, it collects the dot pattern on the screen to form a video file, which is then transmitted to a post-processing computer via a transmission line;

[0086] The computer sequentially performs comparison dot matrix correction, image cropping, video frame export, dot matrix extraction, background subtraction, image enhancement, and cross-correlation particle offset calculation on the collected video file to finally achieve flow field velocity field characterization;

[0087] The ICCD camera can use the fluorescence signal image to characterize the flow field, such as observing the distribution and intensity changes of the fluorescence image to determine whether there are characteristic structures such as vortex, jet, boundary layer, etc.

[0088] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0089] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A flow field velocity measurement and visualization device, characterized in that: The device comprises: Two-color laser emission system, surface dielectric barrier discharge system, Cassegrain optical path shaping system, fluorescence signal detection system and data processing system; Among them, the dual-color laser emission system is used to generate femtosecond pulse laser and a first laser; the Cassegrain optical path shaping system is used to converge and collimate the femtosecond pulse laser and the first laser and focus them to the detection area; the first laser is used to pre-excite the nitrogen molecules in the detection area, and the femtosecond pulse laser is used to induce the nitrogen in the detection area to produce stimulated radiation, and the first laser and the femtosecond pulse laser cooperate to excite the nitrogen molecules; the surface dielectric barrier discharge system is used to form a plasma environment in the detection area; the fluorescence signal detection system is used to collect the fluorescence signals of nitrogen molecules in the detection area, and obtain a fluorescence image for obtaining flow field structure information and a fluorescence signal flow video for measuring the flow field velocity; the data processing system is used to control the timing of each system in the device, and to realize the visualization representation of the flow field structure in the detection area based on the fluorescence image, and to realize the velocity measurement of the flow field in the detection area based on the fluorescence signal flow video.

2. The flow field velocity measurement and visualization device according to claim 1, characterized in that: The dual-color laser emission system includes a femtosecond pulse laser for generating femtosecond pulse laser 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 reflector, a second parabolic reflector and a hyperbolic reflector; wherein the first parabolic reflector is used to converge and collimate the femtosecond pulse laser and reflect it to the hyperbolic reflector; the second parabolic reflector is used to converge and collimate the first laser and reflect it to the hyperbolic reflector; the hyperbolic reflector is used to reflect and focus the femtosecond pulse laser and the first laser to the detection area.

4. The flow field velocity measurement and visualization device according to claim 1, characterized in that: 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 respectively located above and below the dielectric layer, the positive electrode of the power supply is connected to the high-voltage input end of the inner electrode, the negative electrode of the power supply is grounded, and the ground end of the outer electrode is grounded; the inner electrode is implanted on 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. The inner and outer electrodes are arranged parallel to the wind tunnel wall to be measured, and the area of ​​the dielectric layer is larger than the area of ​​the inner electrode.

5. 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 lens is installed on both the ICCD camera and the high-speed camera, and the filter is installed on 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 fluorescence signal flow video.

6. 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 realize the visualization representation of the flow field structure in the detection area based on the fluorescence image, and to realize the velocity measurement of the flow field in the detection area based on the fluorescence signal flow video.

7. The flow field velocity measurement and visualization device according to claim 1, characterized in that: The method of realizing a visual representation of the flow field structure of the detection area based on the fluorescence image specifically includes: Selecting the effective area collected by the fluorescence signal detection system to obtain an effective area video, converting the effective area video into frame-by-frame pictures according to the time sequence to obtain a sequence image; extracting edge information of the fluorescent dot matrix in the sequence image based on an edge detection algorithm, and removing background portions other than the fluorescent dot matrix based on the edge information to obtain a first fluorescent dot matrix sequence image; The spatial resolution of the fluorescence signal detection system is obtained based on the calibration calculation of the fluorescence signal detection system; performing image enhancement processing on the first fluorescent dot matrix sequence image to obtain a second fluorescent dot matrix sequence image; Based on the initial image of the flow field in the detection area and the second fluorescent dot matrix sequence image, the target surface imaging offset is calculated using the cross-correlation algorithm; The actual fluorescence spatial offset is calculated based on the product of the spatial resolution of the fluorescence signal detection system and the target surface imaging offset; Based on the actual fluorescence spatial offset, the velocity distribution of the fluorescence dot array in the second fluorescence dot array sequence image is calculated; The flow field structure is acquired based on the velocity distribution of the fluorescent dot array in the second fluorescent dot array sequence image and the fluorescent image.

8. The flow field velocity measurement and visualization device according to claim 7, characterized in that: The calculation method of the velocity distribution of the fluorescent dot matrix in the second fluorescent dot matrix sequence image is: in, is the velocity distribution of the fluorescent dot matrix in the second fluorescent dot matrix sequence image, is the actual fluorescence spatial offset, is the sampling time interval, is the number of pixel offsets of the second fluorescent dot matrix sequence image, is the spatial resolution of the fluorescence signal detection system, is the moment of the previous frame image, is the moment of the next frame image.

9. 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 positions and angles between the first parabolic reflector and the hyperbolic reflector, and between the second parabolic reflector and the hyperbolic reflector.

10. A flow field velocity measurement and visualization method, characterized in that: The method is based on the flow field velocity measurement and visualization device according to any one of claims 1 to 9, and the method comprises: Step 1: Set the output parameters of the dual-color laser emission system; set the power supply 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; and adjust the gate and gain of the fluorescence signal detection system. Step 2: Turn on the dual-color laser emission system, the surface dielectric barrier discharge system, and the fluorescence signal detection system, and adjust the Cassegrain optical path so that the first laser and the femtosecond pulse laser are focused on the detection area; Step 3: Using a fluorescence signal detection system to collect fluorescence images emitted by stimulated radiation from nitrogen molecules, obtaining fluorescence images for obtaining flow field structure information and fluorescence signal flow videos for measuring flow field velocity; Step 4: The data processing system realizes visualization representation of the flow field structure in the detection area based on the fluorescent image, and the data processing system realizes velocity measurement of the flow field in the detection area based on the fluorescent signal flow video.

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