A holographic grating based fluid vortex motion vortex measurement system and method

Abstract

This invention discloses a fluid vortex motion vortex measurement system and method based on a holographic grating, belonging to the field of fluid mechanics measurement technology. The system includes a laser, a spatial light modulator, a first 4f system, a reflector, a second 4f system, and a photodetector. The laser beam is loaded with a holographic grating pattern having superimposed topological charges by the spatial light modulator, generating a superimposed vortex beam. After being filtered by the 4f system, it illuminates the flow field under test. Scattered light signals are collected, the spectrum is extracted and analyzed, frequency intervals in the spectrum are extracted and averaged to obtain the fluid vortex angular velocity at the beam illumination position, and then the fluid vorticity is calculated according to fluid mechanics formulas (e.g., for a two-dimensional planar vortex, vorticity is twice the angular velocity). This invention does not require strict alignment with the vortex center and has the advantages of being non-contact, having a simplified optical path, strong anti-interference, and wide applicability, making it suitable for measuring vortex parameters in complex flow fields.

Description

Technical Field

[0001] This invention relates to the field of fluid dynamics measurement technology, and in particular to a fluid vortex motion measurement system and method based on a holographic grating. Background Technology

[0002] With the development of fluid dynamics observation technology and the increasing demand for complex flow field research, the precise measurement of fluid vortex motion has become a core issue in fields such as turbulence analysis, aircraft aerodynamic optimization, and ocean circulation monitoring. As a typical structure of fluid motion, the vortex's intensity, topological characteristics, and evolution directly determine the energy transfer and momentum exchange mechanisms of the flow field. However, in actual engineering and natural scenarios, fluid vortices often exhibit unsteady, multi-scale complex characteristics, making them unsteady flow targets that are difficult to accurately capture using conventional contact sensors. To reveal the intrinsic mechanism of vortex motion and ensure the safe and efficient operation of fluid engineering equipment, it is urgent to conduct research on high-precision vortex measurement technology for fluid vortex motion.

[0003] Currently, the mainstream measurement methods include: (1) Particle image-based velocimetry method: First, tracer particles are seeded into the flow field, and the flow field region is irradiated with a dual-pulse laser. Two exposure images of the particles are acquired by a high-speed camera, and the particle displacement is tracked by a cross-correlation algorithm. Then, the velocity field and vortex distribution of the vortex are obtained by inversion. However, this method is limited by the particle following ability and has insufficient ability to capture small-scale vortices in high Reynolds number turbulence. In addition, the image matching calculation is huge and it is difficult to realize real-time online measurement.

[0004] (2) Hot-wire anemometer-based method: A thermal probe is placed in the flow field. The heat exchange effect between the fluid and the probe is used to convert the velocity change into a resistance change signal of the probe. The local velocity of the vortex is calculated through the calibrated voltage-velocity relationship, and then the vortex intensity is derived. However, the probe will interfere with the original flow field, destroy the natural evolution state of the vortex, and can only obtain single-point velocity information. It cannot achieve full-domain measurement of the vortex structure and has poor characterization effect on the spatial distribution of large-scale vortices.

[0005] (3) Laser Doppler velocimetry method: Two intersecting laser beams are emitted to form a measuring body. When the tracer particles pass through the measuring body, they will scatter light signals carrying Doppler frequency shifts. By detecting the correspondence between the frequency shift and the flow velocity, non-contact measurement of vortex flow velocity can be achieved. However, its measurement accuracy is highly dependent on the alignment accuracy between the particle size and the laser beam. For flow fields with low particle concentration, signal loss is likely to occur, and it is difficult to simultaneously obtain vector characteristics such as the topological charge of the vortex.

[0006] (4) Pressure sensor array-based method: Multi-channel pressure sensors are arranged at the flow field boundary to collect the wall pressure pulsation signal caused by vortex motion, and the frequency and intensity information of the vortex is extracted by signal processing methods such as Fourier transform. However, it can only reflect the vortex characteristics of the boundary region and cannot capture the core structure of the vortex inside the flow field. Moreover, it is limited by the sensor response frequency and has a weak ability to identify small-scale vortices with high-frequency pulsation.

[0007] (5) High-speed photography-based method: The dynamic evolution of vortices and tracer media is recorded using a high frame rate camera. The contours and trajectories of the vortices are extracted through image grayscale gradient analysis, which intuitively presents the generation, development and dissipation process of the vortices. However, the measurement results are easily affected by lighting conditions and media transparency. For weak vortices in transparent fluids, the image contrast is insufficient, and quantitative vorticity data of the vortices cannot be obtained directly. It is necessary to rely on complex post-processing algorithms for conversion.

[0008] However, the aforementioned methods for measuring vortex motion in fluids generally suffer from drawbacks such as stringent requirements for the alignment of the measuring device with the vortex core, complex optical paths, high computational demands, or intrusive interference with the flow field. Especially in real-world complex flow fields, the vortex center position may dynamically change, and the stringent alignment requirements significantly limit the convenience and applicability of the measurements. Summary of the Invention

[0009] The present invention aims to overcome the shortcomings of the prior art and provide a fluid vortex motion vortex measurement system and method based on holographic grating, so as to solve the problems of the existing measurement technology, such as strict requirements for optical path alignment, complex calculation, easy interference with the flow field, or inability to directly obtain quantitative vortex parameters.

[0010] In a first aspect, the present invention provides a fluid vortex motion vortex measurement system based on a holographic grating, comprising a laser, a spatial light modulator, a first 4f system, a reflector, a second 4f system, and a photodetector.

[0011] The laser beam emitted by the laser passes sequentially through the spatial light modulator, the first 4f system, the reflector, and the second 4f system before illuminating the flow field under test.

[0012] The scattered light from the measured flow field is converged and received by the photodetector.

[0013] The spatial light modulator is used to load a superimposed topological charge number. The digital holographic grating is used to generate the corresponding superimposed vortex beam.

[0014] The system analyzes the spectral information of the scattered light signal collected by the photodetector, extracts the frequency interval features and calculates the average value to obtain the vortex angular velocity of the measured flow field, and then calculates the fluid vorticity.

[0015] Furthermore, an adjustment system is provided between the laser and the spatial light modulator. The adjustment system includes a waveplate, an aperture stop, an optical isolator, and a lens arranged sequentially along the optical path, which are used to expand, collimate, and filter stray light from the laser beam.

[0016] Furthermore, the first 4f system includes a first lens and a second lens arranged sequentially along the optical path. An aperture stop is provided at the point where the focal planes of the first lens and the second lens overlap, for filtering out elements with superimposed topological charges. A specific diffraction order sub-beam.

[0017] Furthermore, the second 4f system includes a third lens and a fourth lens arranged sequentially along the optical path. An aperture stop is provided at the point where the focal planes of the third lens and the fourth lens overlap, which is used to adjust the size of the beam illuminating the flow field under test.

[0018] Furthermore, it also includes multiple reflectors disposed between the second 4f system and the measured flow field, for adjusting the propagation direction and height of the illumination beam.

[0019] Furthermore, the flow field to be measured is placed on an adjustable-speed magnetic stirrer.

[0020] Furthermore, the spatial light modulator is a pure phase spatial light modulator, and the superimposed topological charge number... The absolute value is greater than or equal to 2. Preferably, it can be increased by... The value of is such that the frequency component of the target rotational Doppler effect is far away from the low-frequency noise interference band, thus improving the signal-to-noise ratio.

[0021] Secondly, the present invention provides a method for measuring the vortex motion of fluid vortices based on a holographic grating, applied to the aforementioned measurement system, comprising the following steps: S1. Generate and adjust the laser beam; S2, Controlling the spatial light modulator to load a superimposed topological charge number The digital holographic grating pattern enables the incident beam to generate a corresponding superimposed vortex beam; S3. The superimposed vortex beam is filtered and modulated using the first 4f system; S4. Guide the modulated beam and irradiate it onto the flow field under test. The center of the beam does not need to be strictly aligned with the vortex center of the flow field under test. Optionally, moderately increasing the lateral offset of the center of the irradiated beam relative to the center of the fluid vortex is beneficial to increasing the proportion of the dominant frequency range. S5. Collect the scattered light signal of the measured flow field; S6. Process the scattered light signal to obtain its spectrum information; S7. Based on the spectrum information, extract the frequency interval of the dominant frequency component and calculate its average value, and use the average value as the vortex angular velocity of the measured flow field. S8. Calculate the fluid vorticity based on the vortex angular velocity.

[0022] Furthermore, in step S7, after normalizing the spectrum information, a frequency region exceeding a preset power threshold is selected, and the interval between adjacent dominant frequency peaks within that region is calculated and the average value is statistically analyzed.

[0023] Furthermore, in step S8, for fluid vortex motion in a two-dimensional plane, the fluid vorticity... ,in The vortex angular velocity is given; for fluid vortex flow in three-dimensional space, the fluid vorticity is the curl of the vortex angular velocity vector.

[0024] Therefore, the fluid vortex motion measurement system and method based on a holographic grating, which adopts the above-described structure, has the following beneficial effects: (1) This invention belongs to non-contact optical measurement, which does not require the seeding of tracer particles into the flow field or the insertion of probes, thus avoiding complex preliminary preparations and interference with the flow field. The core of signal processing lies in performing Fourier transform on the time-domain signal of the collected scattered light and extracting the characteristic frequency intervals in the spectrum for statistical averaging. The algorithm is simple and efficient, without the need for complex image matching or three-dimensional reconstruction calculations, which is conducive to achieving fast and near real-time measurement.

[0025] (2) Compared with some traditional vortex light measurement schemes that require beam combining using Ronchi gratings, the present invention directly generates a holographic grating image with superimposed topological charge through a spatial light modulator, and uses a 4f system combined with an aperture stop to filter out the required superimposed vortex beams, omitting additional beam combining optical paths and precision adjustment steps, making the system structure more compact and stable, and reducing assembly difficulty and cost.

[0026] (3) The core advantage of this invention is that it does not require strict alignment of the illumination beam with the center of rotation and axis of the fluid vortex during measurement. Even if the beam illumination position deviates from the center of the vortex, the system can still effectively extract the frequency characteristics representing the rotational angular velocity from the scattered light signal. This feature greatly reduces the requirements for experimental operation accuracy, making it possible to measure vortices in irregular and dynamically changing flow fields, and enhancing the practicality and environmental adaptability of the method.

[0027] (4) This invention has no special limitations on the flow field being measured and is applicable to various fluid solutions contained in containers of different shapes and sizes. By moving the measuring spot along the radial direction of the vortex or by scanning, the characteristic parameters of the vortex at different locations in the flow field (angular velocity, vorticity) can be obtained, providing an effective means for studying the spatial distribution and gradient changes of the vortex structure. It has broad application prospects in the fields of fluid dynamics mechanism research, performance monitoring of industrial stirring equipment, and environmental fluid dynamics analysis.

[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the measurement system provided in an embodiment of the present invention; Figure 2 A schematic flowchart of the measurement method provided in an embodiment of the present invention; Figure 3 This is an example of a digital holographic grating image generated on a spatial light modulator in an embodiment of the present invention (corresponding to a superimposed topological charge number ±18). Figure 4 This is a statistical result diagram of frequency interval extraction from the analysis of the scattered light signal spectrum in this embodiment of the invention; Figure 5 This is a schematic diagram illustrating the calculation of fluid vorticity at different rotational speeds in an embodiment of the present invention.

[0030] Figure Labels 1. Laser; 2. Adjustment system; 3. Spatial light modulator; 4. First lens; 5. Aperture stop two; 6. Second lens; 7. First mirror; 8. Third lens; 9. Aperture stop three; 10. Fourth lens; 11. Second mirror; 12. Third mirror; 13. Fourth mirror; 14. Flow field under test; 15. Fifth lens; 16. Photodetector. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0033] Example Reference Figure 1 This embodiment provides a specific fluid vortex motion vortex measurement system based on a holographic grating, including a laser 1, an adjustment system 2, a spatial light modulator 3, a first 4f system, a first reflector 7, a second 4f system, a second reflector 11, a third reflector 12, a fourth reflector 13, a flow field to be measured 14, a fifth lens 15, and a photodetector 16.

[0034] The laser 1 is a solid-state laser of model MGL-III-532-2000mW, which outputs a fundamental Gaussian beam with a wavelength of 532nm.

[0035] The adjustment system 2 is located after the laser 1 and includes, in sequence along the optical path, a half-wave plate for adjusting polarization; an aperture stop for initially selecting the center portion of the beam; an optical isolator for preventing backlight damage to the laser; a sixth lens for beam expansion and collimation; and finally outputs a high-quality linearly polarized flat-top beam.

[0036] The spatial light modulator 3 is a pure phase spatial light modulator (e.g., Holoeye PLUTO-2-VIS-016), located at the front focal plane of the first 4f system, used to receive the collimated beam from the adjustment system 2 and load a computer-generated holographic grating.

[0037] The first 4f system consists of a first lens 4 and a second lens 6, both with equal focal lengths, forming a standard 4f imaging system. An aperture stop 5 is provided at the point where the focal planes of the first lens 4 and the second lens 6 overlap.

[0038] The first reflector 7 is located at the rear focal plane of the first 4f system (i.e., the rear focal plane of the second lens 6) and is used to change the direction of the light beam.

[0039] The second 4f system consists of a third lens 8 and a fourth lens 10, thus forming a 4f system. An aperture stop 9 is provided at the point where the focal planes of the third lens 8 and the fourth lens 10 coincide. The light beam reflected by the first reflecting mirror 7 is incident on the third lens 8, which is the front focal plane of the second 4f system.

[0040] The second reflector 11, the third reflector 12 and the fourth reflector 13 are arranged sequentially after the second 4f system and before the measured flow field 14, in order to further elevate the beam modulated by the second 4f system and guide it to the measurement area.

[0041] The measured flow field 14 is contained in a transparent container (such as a glass beaker) and placed on an adjustable-speed magnetic stirrer. By controlling the rotation speed of the stirrer, a stable vortex with a known angular velocity can be generated in the fluid, which facilitates the calibration and verification of the measurement results.

[0042] The fifth lens 15 is located in the scattered light collection optical path of the measured flow field 14 and is used to converge the scattered light at large angles.

[0043] The photodetector 16 (such as a photodiode or photomultiplier tube) is located near the back focal plane of the fifth lens 15 and is used to receive the converged scattered light and convert it into an electrical signal. This electrical signal is then imported into a computer via a data acquisition card for further processing.

[0044] This embodiment also provides a measurement method based on the above system, the process of which is as follows: Figure 2 As shown, the specific steps are as follows: S1. Turn on laser 1 to output a Gaussian beam. After passing through the waveplate, aperture stop 1, optical isolator and sixth lens in adjustment system 2, the beam is expanded, collimated and stray light is filtered out to form a uniform collimated beam with a diameter of about 10 mm.

[0045] S2. Using MATLAB software, generate a graph with superimposed topological loads. (In this embodiment) Phase-type digital holographic gratings, such as Figure 3 As shown. Adjust the program parameters to match the period and size of the generated grating pattern with the pixel size of the spatial light modulator 3 and the actual optical path. Load the grating pattern onto the spatial light modulator 3. After the collimated beam passes through the spatial light modulator 3 loaded with the grating pattern, it undergoes diffraction, producing a beam containing multiple diffraction orders such as 0th order and ±1st order, where the ±1st order beam carries superimposed topological charge information.

[0046] S3. Beams of different diffraction orders gradually separate in space and are incident on the first lens 4. The first lens 4 converges these beams onto its focal plane (i.e., the plane where aperture stop 5 is located). By precisely adjusting the position and size of aperture stop 5, it allows only the passage of single-sided first-order diffraction beams (e.g., +1st order) carrying superimposed topological charges of +18 and -18, while blocking beams of other orders. The beam filtered by aperture stop 5 is then collimated again after passing through the second lens 6, forming a superimposed vortex beam. This beam has a symmetrical petal-shaped intensity distribution, with the number of petals being... .

[0047] S4. The superimposed vortex beam is reflected by the first reflecting mirror 7, changes direction, and enters the second 4f system. It then passes sequentially through the third lens 8, aperture stop 3 9, and fourth lens 10. Aperture stop 3 9 can be used to further control the beam aperture. Subsequently, the beam is guided and elevated by reflections from the second reflecting mirror 11, the third reflecting mirror 12, and the fourth reflecting mirror 13, and finally illuminates the surface of the measured flow field 14 at a certain height and angle, non-centrally. The diameter of the illumination spot is adjusted to approximately 2 mm. Crucially, during this process, the center of the beam spot was not intentionally aligned with the center of the vortex generated by the magnetic stirrer in the fluid; there is a significant lateral offset between the two.

[0048] S5. The superimposed vortex beams illuminating the rotating fluid are scattered. Due to the Doppler effect, the frequency of the scattered light carries information about the fluid's rotation.

[0049] S6. The scattered light is collected by the fifth lens 15 and focused onto the photosensitive surface of the photodetector 16.

[0050] S7 and photodetector 16 convert the optical signal into a time-domain electrical signal, which is then acquired by the data acquisition card at a sufficiently high sampling rate (e.g., 10 kSa / s) and transmitted to the computer.

[0051] S8. In a computer (e.g., using MATLAB), perform a Fourier transform on the acquired time-domain intensity signal to obtain the signal's spectrum. Normalize the spectrum (e.g., normalize to the maximum amplitude).

[0052] S9. On the normalized spectrum, select the frequency region containing obvious peaks whose amplitudes exceed a preset threshold (e.g., 0.5). Calculate the frequency interval between adjacent peaks within this region. Count multiple interval values ​​(e.g.) Figure 4 As shown, the distribution statistics of frequency intervals are illustrated, and the average value of these intervals is calculated. Based on average Calculate the angular velocity corresponding to the fluid vortex. (The unit is Hz, which is revolutions per second). Figure 4In the example, the dominant frequency interval ranges from 0.2000Hz to 0.2518Hz, with an average value of... .

[0053] S10. According to the principles of fluid mechanics, for vortex motion in a two-dimensional plane, its vorticity... (Unit: rad / s) is angular velocity twice as much as The result obtained in step S9 Substitute the values ​​and calculate the fluid vorticity. (like Figure 5 As shown in the figure, the fluid vortex angular velocity at the beam illumination position is obtained based on the average value of the frequency interval, and then the fluid vorticity at different rotational speeds is calculated according to the fluid dynamics formula. The results are in good agreement with the theoretical values ​​verified by camera recording, thus verifying the effectiveness of this method.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A fluid vortex motion measurement system based on a holographic grating, characterized in that, The system includes a laser, a spatial light modulator, a 4f system, a reflector, and a photodetector. The 4f system comprises a first 4f system and a second 4f system. The laser beam emitted from the laser passes sequentially through the spatial light modulator, the first 4f system, the reflector, and the second 4f system before illuminating the flow field under test. The scattered light from the flow field under test is converged and received by the photodetector. The spatial light modulator is used to load a holographic grating to generate a corresponding superimposed vortex beam. The system obtains the measured flow field information by analyzing the spectral information of the scattered light signal collected by the photodetector, extracting frequency interval features, and calculating the vortex angular velocity.

2. The fluid vortex motion measurement system based on holographic grating according to claim 1, characterized in that, An adjustment system is provided between the laser and the spatial light modulator. The adjustment system includes a waveplate, an aperture stop, an optical isolator, and a lens arranged sequentially along the optical path, which are used to expand, collimate, and filter stray light from the laser beam.

3. The fluid vortex motion measurement system based on a holographic grating according to claim 1, characterized in that, The first 4f system includes a first lens and a second lens arranged sequentially along the optical path. An aperture stop is provided at the point where the focal planes of the first lens and the second lens overlap, used to filter out lenses with superimposed topological charges. A specific diffraction order sub-beam.

4. The fluid vortex motion measurement system based on a holographic grating according to claim 1, characterized in that, The second 4f system includes a third lens and a fourth lens arranged sequentially along the optical path. An aperture stop is provided at the point where the focal planes of the third lens and the fourth lens overlap, which is used to adjust the size of the beam illuminating the flow field under test.

5. The fluid vortex motion measurement system based on a holographic grating according to claim 1, characterized in that, It also includes multiple reflectors disposed between the second 4f system and the measured flow field, for adjusting the propagation direction and height of the illumination beam.

6. The fluid vortex motion measurement system based on a holographic grating according to claim 1, characterized in that, The flow field to be measured is placed on an adjustable-speed magnetic stirrer.

7. The fluid vortex motion measurement system based on a holographic grating according to claim 1, characterized in that, The spatial light modulator is a pure phase spatial light modulator, and the superimposed topological charge number The absolute value is greater than or equal to 2.

8. A method for measuring the vortex motion of fluid vortices based on holographic gratings, characterized in that, Applied to the measurement system as described in any one of claims 1-7, comprising the following steps: S1. Generate and adjust the laser beam; S2, Controlling the spatial light modulator to load a superimposed topological charge number The digital holographic grating pattern enables the incident beam to generate a corresponding superimposed vortex beam; S3. The superimposed vortex beam is filtered and modulated using the first 4f system; S4. Guide the modulated beam and illuminate the flow field to be measured. The center of the beam does not need to be strictly aligned with the vortex center of the flow field to be measured. S5. Collect the scattered light signal of the measured flow field; S6. Process the scattered light signal to obtain its spectrum information; S7. Based on the spectrum information, extract the frequency interval of the dominant frequency component and calculate its average value, and use the average value as the vortex angular velocity of the measured flow field. S8. Calculate the fluid vorticity based on the vortex angular velocity.

9. The method for measuring fluid vortex motion based on a holographic grating according to claim 8, characterized in that, In step S7, after normalizing the spectrum information, a frequency region exceeding the preset power threshold is selected, the interval between adjacent dominant frequency peaks in the region is calculated, and the average value is statistically calculated.

10. The method for measuring fluid vortex motion based on a holographic grating according to claim 8, characterized in that, In step S8, for fluid vortex motion in a two-dimensional plane, the fluid vorticity... ,in The vortex angular velocity is given; for fluid vortex flow in three-dimensional space, the fluid vorticity is the curl of the vortex angular velocity vector.

Images