Bionic flexible pectoral fin wake flow vortex structure observation test system and analysis method

Abstract

The invention discloses a bionic flexible pectoral fin wake vortex structure observation test system and an analysis method. The system comprises a manta ray-simulated flexible pectoral fin test prototype, a circulating water tank, a camera, a pulse laser, tracer particles and an observation domain. The circulating water tank is started, so that the tracer particles are uniformly distributed in the observation domain, and the test site is in a dark environment; and enabling the test prototype, turning on the pulse laser and the camera, after the test prototype is stable, shooting a tracer particle flow image in the to-be-observed plane to obtain two sets of image sets based on time-marching arrangement, and analyzing the bionic flexible pectoral fin wake vortex structure based on the two sets of image sets. According to the method, the speed field information of the bionic flexible pectoral fin wake flow and the quantified vortex nuclear physical quantity distribution characteristics of the to-be-observed domain can be obtained, so that the quantitative relation between the motion parameters and the dynamic characteristics of the flexible pectoral fin is established; rapid prediction of pectoral fin propulsion performance is realized through a mapping function between bionic flexible pectoral fin motion parameters and hydrodynamic characteristics.

Description

Technical Field

[0001] The invention relates to the field of underwater bionic structure testing, and in particular to a bionic flexible pectoral fin wake vortex structure observation test system and an analysis method. Background Art

[0002] As an important equipment for autonomously performing underwater missions, unmanned underwater vehicles have shown great development potential in both military and civilian fields. In particular, bionic underwater vehicles that imitate the propulsion methods of marine organisms have developed rapidly in recent years.

[0003] Bionic underwater vehicles rely on the periodic flapping of bionic fins to generate propulsion. In common underwater bionic application products, the internal motor drives the bionic pectoral fins to move through periodic rotation. Part of the output momentum is converted into the internal energy of the flexible pectoral fins, and the other part is released into the surrounding water, generating a reaction force to drive the vehicle to maneuver. Compared with traditional propeller propulsion devices, the flexible pectoral fin structure of the bionic underwater vehicle can show more significant bending deformation during the propulsion process. Related studies have shown that flexible pectoral fins have better thrust performance and higher propulsion efficiency than rigid pectoral fins. Therefore, it is of great significance to observe the hydrodynamic force and wake flow field vortex structure of the bionic flexible pectoral fin.

[0004] Most of the existing technologies focus on obtaining the hydrodynamic characteristics of bionic flexible pectoral fins. There are mature schemes for mechanical property tests under various motion modes. However, there is still no relatively complete observation system setting and test result post-processing analysis method for the flow field evolution of bionic flexible pectoral fins. It is difficult to establish a mapping relationship between flow field physical quantities and motion parameters, and thus to analyze the changes in mechanical properties caused by the changes in flow field physical quantities under the influence of motion parameters from a numerical perspective. It is impossible to provide a reference from the perspective of fluid mechanics mechanism for the design of underwater bionic structures. Summary of the invention

[0005] The purpose of the present invention is to provide a bionic flexible pectoral fin wake vortex structure observation test system and analysis method to solve the problems existing in the prior art.

[0006] In order to achieve the above tasks, the present invention adopts the following technical solutions:

[0007] A bionic flexible pectoral fin wake vortex structure observation test system includes a manta ray flexible pectoral fin test prototype, a circulating water tank, a camera, a pulsed laser, tracer particles and an observation domain, wherein:

[0008] The manta ray flexible pectoral fin test prototype is installed in the observation area in the circulating water tank and assembled on the position adjustment mechanism arranged on the top of the circulating water tank through a fixing rod; the test prototype is used to move under the driving of the circulating water flow provided by the circulating water tank, so as to observe the wake vortex structure of the pectoral fin of the test prototype;

[0009] The circulating water tank is used to provide a flow field environment for the test prototype, and the section is observed in the circulating water tank according to the size of the test prototype;

[0010] The observation domain is set in the observation section, and the size of the observation domain is determined through observation experiments. The prototype is located in the middle of the edge line of the observation domain;

[0011] The sheet laser provided by the pulsed laser is emitted upward from the glass below the circulating water tank, and the sheet laser plane coincides with the plane to be observed by the camera in the observation domain; the sheet laser should irradiate the evolution area of ​​the wake vortex structure in the observation domain and completely include the test prototype;

[0012] The camera is set on the side of the circulating water tank, and its optical axis is perpendicular to the side glass of the circulating water tank; a calibration plate is installed on the plane to be observed, and the size of the calibration plate should be sufficient to cover the observation area, and the observation area is calibrated using the calibration plate;

[0013] Tracer particles are spread in the circulating water tank, and the diameter and number of the tracer particles are set according to the size of the observation domain and the resolution of the camera;

[0014] Start the circulating water tank to make the tracer particles evenly distributed in the observation domain. The test site should be in a dark environment. Enable the test prototype, turn on the pulse laser and camera, and take the tracer particle flow image in the plane to be observed after the test prototype is stable, and obtain two sets of atlases F0 and F1 based on time-marching arrangement; F0 is the image set taken for the first exposure, and F1 is the image set taken for the second exposure. The analysis of the wake vortex structure of the bionic flexible pectoral fin is carried out based on the two sets of atlases.

[0015] Furthermore, the design process of the manta ray-like flexible pectoral fin test prototype is as follows:

[0016] A coordinate system is established with the chord direction as the x direction, the span direction as the y direction, and the thickness direction as the z direction; firstly, the top view contour of the manta ray is scanned to obtain the point set of the top view outer contour curve of the manta ray; the mouthparts, eyes, and tail curve areas corresponding to the curve at the head of the manta ray in the top view outer contour curve are optimized using static deviation, and the optimization principle is to use a spline curve to make the curve transition as smooth as possible, and obtain the point set of the optimized top view outer contour curve;

[0017] After the point set of the top-view outer contour curve of the test prototype is determined, the span-wise section of the test prototype is designed using the NACA series airfoils. According to different internal load requirements, airfoils with different thickness ratios are used to envelop the surface, and the three-dimensional shape of the test prototype is constructed based on the top-view outer contour curve.

[0018] Furthermore, the flexibility and thickness of the pectoral fin of the test prototype should follow the trend of gradually increasing flexibility from head to tail and from center to sides; design bionic bones with different flexibility distributions according to the number of driving sources of the test prototype; design groove structures with non-uniform distance distribution on the span and chord paths of the bionic bones in the pectoral fin; on each bionic bone, starting from the first groove close to the center of the test prototype, the distance between adjacent grooves is proportionally reduced by 1.5; four motors are set inside the test prototype, of which motor 1 and motor 3 are located on both sides of the front of the test prototype, respectively connected to the front fins on both sides; motor 2 and motor 4 are located on both sides of the rear of the test prototype, respectively connected to the rear fins on both sides; the driving source of the pectoral fin of the test prototype adopts the CPG motion form inspired by biological motion, which is converted into PWM signals to drive motors 1 to 4;

[0019] The central columnar main body and pectoral fins of the test prototype are printed with nylon material, and the bionic muscles on the pectoral fins are made of super-elastic flexible material. A demoulding mold is made according to the three-dimensional shape point set, and the prepared silicone liquid is poured into the mold. The front and rear fins are installed, and the flexible pectoral fins are obtained after vacuum degassing and high-temperature shaping.

[0020] Furthermore, a dynamometer is used to lift the front fin ray upward at the wingtip of the pectoral fin with a fixed force, and the deformation curve of the experimental prototype is projected onto the yOz plane of the coordinate system. The maximum deformation angle θ must be greater than 60°; the x direction of the coordinate system is the chord direction, the y direction is the span direction, and O is the origin of the coordinate system.

[0021] Furthermore, chemical pigments were used to mark several characteristic points on the pectoral fins, and the test prototype was installed in an underwater environment. An underwater camera was used to capture the time-varying motion trajectory of the characteristic points during the motion process; the characteristic points were taken as the tip of the front fin ray, the tip of the rear fin ray, and the chord-wise tip of the rear fin ray; the dynamic deviation of the corresponding positions on the pectoral fins of the real manta ray and the test prototype was Should be less than the preset value:

[0022]

[0023] Where, d 0i (t), d i (t) are the motion trajectories of the i-th feature point on the pectoral fin of the manta ray and the test prototype at time t, respectively.

[0024] Furthermore, the static deviation at a certain point of the curve to be optimized in the top view outer contour curve is defined as:

[0025]

[0026] Where (x0, y0) is the coordinate of the point on the curve before optimization, (x, y) is the coordinate of the point after optimization; the static deviation is no more than 0.1.

[0027] Furthermore, the flow rate in the circulating water tank was set to 0.5BL / s, the size of the observation section was 40BL*15BL*8BL, and BL was the length of the test specimen.

[0028] Furthermore, the calibration error needs to be less than 0.3 pixel. When the camera is turned on, if the resolution of the camera is Rx*Ry, the area of ​​the area corresponding to one pixel unit in the picture taken by the camera is (Lx*Ly) / (Rx*Ry). Lx*Ly is the size of the observation domain.

[0029] Furthermore, the interval dt between the camera's consecutive frames should be synchronized with the pulsed laser and selected according to the expected movement distance of the tracer particles in the flow field:

[0030]

[0031] Where, d exp is the expected moving distance of the tracer particles within the shooting time interval, and U is the flow velocity of the circulating water tank.

[0032] Furthermore, the diameter and number of tracer particles are set according to the size of the observation domain and the camera resolution, as shown below:

[0033]

[0034] N is the number of complete tracer particles in any 64*64 pixel area in the flow field captured by the camera, ranging from 8 to 25, and BL is the length of the test specimen.

[0035] A bionic flexible pectoral fin wake vortex structure observation test analysis method comprises the following steps:

[0036] Step 1: Use the above-mentioned test system to obtain two sets of atlases F0 and F1, use the analysis program PIVLab to remove the surrounding environment and the fixed rod connected to the test prototype captured by each image in the atlas, and then use the high-pass filtering method to filter out the bright spots in each image and enhance the contrast of the images in the atlas, so as to obtain the updated atlases F0 and F1;

[0037] Step 2: Convert the updated atlas F0 and F1 from RGB space to Lab space, and then use the PIVLab open source program to match the corresponding tracer particles in the two images corresponding to the same time in the atlas F0 and F1, and solve the velocity of the corresponding tracer particles;

[0038] Set the active significance threshold of the tracer particles, filter the tracer particles with a value greater than the active significance threshold, and integrate the velocity vectors of all tracer particles to obtain the velocity field of the entire observation domain at this moment; set the acceptable velocity range according to the Gaussian distribution, and filter out the tracer particles outside the mean value of the velocity field by N standard deviations;

[0039] Step 3, a velocity field can be calculated for each pair of images corresponding to all moments in the atlas F0 and F1; the observation domain is divided into grid points according to the spatial position, and the corresponding velocity values ​​are assigned to the grid points corresponding to the tracer particles, so as to obtain the velocity field distribution data V of the observation domain arranged according to time;

[0040] Step 4, using the velocity field distribution data V in the observation domain, adopting the POD modal decomposition method to analyze the main modes in the vortex structure of the pectoral fin wake flow field of the test prototype, and reconstructing the flow field according to the main modes with a large energy share, to obtain the physical quantity matrix set Physics (u, v, Ω) after POD modal decomposition; wherein v represents the superposition of the average velocity and the pulsating velocity at the grid point, u represents the average velocity vector composed of the average values ​​of v at different grid points, and Ω represents the vorticity field;

[0041] Step 5, analyzing the spatial distribution characteristics of each physical quantity in the physical quantity matrix set Physics (u, v, Ω), as well as the mapping relationship between the physical quantity and the motion parameters and mechanical characteristics, and establishing a functional fitting relationship between the pectoral fin wake velocity field and vortex field based on the motion parameters.

[0042] Furthermore, when performing vortex field analysis:

[0043] Taking the Strouhal number as the analysis variable, the vortex core based on the Q criterion is calculated according to the obtained physical quantity matrix set Physics(u, v, Ω) and the Q criterion; the development stage of the corresponding vortex core is judged according to the area of ​​the vortex core, such as shedding, evolution, retention, divergence, etc., and the lateral spacing, longitudinal spacing, jet exit angle, maximum vortex core area and evolution distance, vorticity, etc. between vortex cores are recorded; the geometric distribution and numerical differences of the vortex field under different motion parameters are compared to form a vortex field data set for different motion parameters.

[0044] Furthermore, according to the multi-factor regression analysis method, the motion parameters including the maximum amplitude angle of the motor, the motion frequency, the phase difference and the physical quantities are mapped to function, and the response surface of the physical quantities of the wake field vortex structure based on the motion parameters is obtained, thereby realizing the rapid prediction of the pectoral fin propulsion performance under arbitrary motion parameters.

[0045] Compared with the prior art, the present invention has the following technical features:

[0046] The present invention can realize the acquisition of velocity field information of the wake of the bionic flexible pectoral fin and the quantitative distribution characteristics of the vortex core physical quantity in the observed domain, thereby establishing a quantitative relationship between the motion parameters of the flexible pectoral fin and the dynamic characteristics, and realizing the rapid prediction of the pectoral fin propulsion performance through the mapping function between the motion parameters of the bionic flexible pectoral fin and the hydrodynamic characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a schematic diagram of the manta ray's flexible pectoral fin model and its movement form;

[0048] Figure 2 This is a schematic diagram of the layout of the bionic flexible pectoral fin wake vortex structure observation test system;

[0049] Figure 3 Schematic diagram of velocity field calculation of tracer particles in a certain area in the embodiment;

[0050] Figure 4 The construction and analysis process of the experimental system for observing the vortex structure of the flexible pectoral fin wake of the manta ray. DETAILED DESCRIPTION

[0051] See attached Figure 1 The present invention first provides a bionic flexible pectoral fin wake vortex structure observation test system, the system includes a manta ray flexible pectoral fin test prototype, a circulating water tank, a camera, a pulsed laser, tracer particles and an observation domain, wherein:

[0052] 1. Manta ray flexible pectoral fin test prototype

[0053] The manta ray-like flexible pectoral fin test prototype is installed in the observation area in the circulating water tank and is assembled on a position adjustment mechanism arranged on the top of the circulating water tank through a fixing rod. The position of the test prototype in the circulating water tank can be adjusted by the position adjustment mechanism; the test prototype is used to move under the drive of the circulating water flow provided by the circulating water tank, so that the test system is used to observe the wake vortex structure of the pectoral fin of the test prototype.

[0054] The three-dimensional outline of the manta ray flexible pectoral fin test prototype should be highly similar to the outline of manta rays in nature; its construction process is as follows:

[0055] 1.1 Overall three-dimensional design of the test prototype.

[0056] A coordinate system is established with the chord direction as the x direction, the span direction as the y direction, and the thickness direction as the z direction. First, the top view contour of the manta ray is scanned to obtain the point set S0(x, y) of the top view outer contour curve of the manta ray, where x and y represent the horizontal and vertical coordinates in the top view plane, respectively, and the number of points in the point set is recorded as M. Considering the design and manufacture of conventional manta ray-like submersibles, the top view outer contour curve corresponding to the mouthparts, eyes, and tail curves of the manta ray head curve are optimized. The optimization principle is to use a spline curve to make the curve transition as smooth as possible, and the optimized top view outer contour curve point set S(x, y) is obtained. The specific optimization process is:

[0057] Define the static deviation at a point of the curve to be optimized in the top view outer contour curve:

[0058]

[0059] In the formula, (x0, y0) is the coordinate of the point on the curve before optimization, and (x, y) is the coordinate of the point after optimization. In order to make the contour of the curve after optimization have a high similarity with the contour before optimization, the static deviation at any point should not be too large. In this scheme, 0.1 is taken as the maximum deviation, and χ i ≤0.1,i=1,..., M .

[0060] After the point set of the top view outer contour curve of the test prototype is determined, the span-wise section of the test prototype is designed using the NACA series airfoil. According to different internal load requirements, airfoils with different thickness ratios are respectively used to envelop the surface, and the three-dimensional shape of the test prototype is constructed on the basis of the top view outer contour curve. The point set of the three-dimensional shape is recorded as V (x, y, z). In this embodiment, the NACA0012 airfoil is used to design the test prototype.

[0061] 1.2 Design of pectoral fin of test prototype.

[0062] The flexibility of the pectoral fins (including front and rear fins) of the test prototype should follow the trend of gradually increasing flexibility from head to tail and from center to sides; design bionic skeletons with different flexibility distributions according to the number of driving sources of the test prototype; this scheme is further explained by the design of double-fin bionic skeletons. Design groove structures with non-uniform distance distribution on the span and chord paths of the bionic skeleton in the pectoral fin; on each bionic skeleton, starting from the first groove close to the center of the test prototype, the distance between adjacent grooves is proportionally reduced by 1.5, and the structural strength of the fins should be considered to avoid stress concentration that makes the fins easy to break; four motors are set inside the test prototype, of which motor 1 and motor 3 are located on both sides of the front of the test prototype, respectively connected to the front fins on both sides; motor 2 and motor 4 are located on both sides of the rear of the test prototype, respectively connected to the rear fins on both sides.

[0063] The thickness design of the pectoral fin should also conform to the above-mentioned flexibility trend, that is, the thickness h(x, y) of the fin should satisfy:

[0064]

[0065] In the formula, is the corresponding first-order derivative operator, x and y are the coordinates in the chord direction and span direction respectively.

[0066] The central columnar main body and pectoral fins of the test prototype are printed with high-performance nylon material, and the bionic muscles on the pectoral fins are made of super-elastic flexible material. This scheme uses silicone as the bionic muscle material, and makes a demoulding mold according to the three-dimensional shape point set V (x, y, z). Pour the prepared silicone liquid into the mold, install the front and rear fins, and obtain flexible pectoral fins after vacuum degassing and high-temperature shaping.

[0067] In order to ensure the maximum flexibility of the bionic pectoral fin, the maximum deformation angle of the pectoral fin needs to be measured. Using a tensile force gauge, the front fin ray is lifted upward at the wing tip of the pectoral fin with a fixed value of tension, and the deformation curve of the experimental prototype is projected onto the yOz plane, as shown in Figure 1 As shown, the maximum deformation angle θ is larger than expected, and θ>60° in this solution.

[0068] 1.3 Drive structure design.

[0069] The driving source of the pectoral fin of the test prototype adopts the CPG motion form inspired by biological motion, which is converted into a PWM signal to drive motors 1 to 4; the input is the maximum amplitude angle A of the motor, the motion frequency, and the output phase difference between motors 1 and 2, and the output phase difference between motors 3 and 4.

[0070] Chemical pigments are used to mark several characteristic points on the pectoral fins, and the test prototype is installed in an underwater environment. An underwater camera is used to capture the time-varying motion trajectory of the characteristic points during the motion process; the characteristic points are taken as the tip of the front fin ray, the tip of the rear fin ray, and the chord-wise tip of the rear fin ray; the dynamic deviation of the corresponding positions on the pectoral fins of the real manta ray and the test prototype should be less than the preset value. In order to make the pectoral fin motion of the test prototype similar to that of the real manta ray, the dynamic deviation should not be too large. This scheme takes 0.2, that is:

[0071]

[0072] Where, d 0i (t), d i (t) are the motion trajectories of the i-th feature point on the pectoral fin of the manta ray and the test prototype at time t, respectively.

[0073] 2. Circulating water tank

[0074] The circulating water tank is used to provide a flow field environment for the test prototype, and the flow rate is set to 0.5BL / s. The size of the circulating water tank should be large enough to prevent the test process from being affected by the backflow of the wall and water surface. In this scheme, the size of the observation section of the circulating water tank is 40BL*15BL*8BL. Figure 1 As shown in the figure, BL is the length of the test sample, and SL is the half-side extension of the test sample; the test sample is located in the observation domain in the center of the observation section to ensure that the surrounding flow field is a uniform flat straight flow; there should be fewer large particle impurities in the flow field to ensure the quality of the flow field.

[0075] 3. Observation domain

[0076] The observation domain is set in the observation section. The size (length*width) Lx*Ly of the observation domain is determined through observation experiments. The prototype is located in the middle of the edge line of the observation domain.

[0077] Before observing the wake vortex structure of the pectoral fin of the test prototype, we should have a qualitative understanding of the approximate size and evolution law of its vortex field, and set up the observation domain on this basis. According to the vortex formation number theory, the vortex core continues to draw in the fluid and reaches its maximum size within 3.3 times the characteristic length, and then begins to dissipate. Therefore, for the pectoral fin of the test prototype, if we want to observe the evolution and dissipation process of the wake vortex structure, we need to roughly determine the vortex core size and evolution area through preliminary observation experiments. Furthermore, according to relevant research, when manta rays are propulsive, the wingtip area is the area that provides the greatest contribution to thrust, so it is necessary to observe the structure and evolution of the vortex shed at the wingtip of the pectoral fin; after the observation experiment, it is determined that the evolution and dissipation area of ​​the wake vortex structure that is long enough can be observed is used as the observation domain.

[0078] 4. Pulsed laser

[0079] The sheet laser provided by the pulsed laser is shot upward from the glass below the circulating water tank. The sheet laser plane coincides with the plane to be observed by the camera in the observation domain. The sheet laser plane described in this scheme is at the plane at 0.95 times the span of the test prototype. The sheet laser should mainly irradiate the evolution area of ​​the wake vortex structure in the observation domain and completely include the test prototype, such as Figure 2 shown.

[0080] 5. Camera

[0081] The camera is set on the side of the circulating water tank, and its optical axis is perpendicular to the side glass of the circulating water tank; a calibration plate is installed on the plane to be observed, and the size of the calibration plate should be sufficient to cover the observation domain; the observation domain is calibrated using the calibration plate. In this scheme, the calibration error must be less than 0.3 pixel; turn on the camera, if the camera resolution is Rx*Ry, then the area corresponding to one pixel unit in the picture taken by the camera is (Lx*Ly) / (Rx*Ry).

[0082] Since the observation domain of this scheme is relatively large, the resolution required by the camera is also relatively large. However, the continuous shooting interval of high-resolution cameras is usually long, that is, it is impossible to take into account both resolution and shooting speed. In this scheme, resolution should be given priority, that is, double-frame double-exposure low-frequency and high-resolution shooting. In order to make the flow field shooting clear, a low-frequency and high-energy pulsed laser is selected. Therefore, the shooting interval dt of the camera's continuous frames must be synchronized with the pulsed laser and selected according to the expected movement distance of the tracer particles in the flow field.

[0083]

[0084] Where, d exp is the expected moving distance of the tracer particles within the shooting time interval, and U is the flow velocity of the circulating water tank.

[0085] 6. Tracer particles

[0086] Tracer particles are spread in the circulating water tank. The diameter r of the tracer particles and the number of particles are set according to the following conditions based on the observation area size and camera resolution:

[0087]

[0088] N is the number of complete tracer particles in any 64*64pixel area in the flow field captured by the camera, which should be between 8 and 25. That is, there should be neither too many nor too few tracer particles in the flow field; too many tracer particles will lead to chaotic flow field, while too few tracer particles will lead to incomplete reconstruction of the velocity field and inaccurate calculation results.

[0089] Start the circulating water tank to make the tracer particles evenly distributed in the observation domain. The test site environment should be dark enough and other light interference from the wall should be minimized. Enable the test prototype, turn on the pulsed laser and camera, and take the flow image of the tracer particles in the plane to be observed after the test prototype is stable, and obtain two sets of atlases F0 and F1 based on time-marching arrangement; F0 is the image set taken for the first exposure, and F1 is the image set taken for the second exposure. The analysis of the wake vortex structure of the bionic flexible pectoral fin is carried out based on the two sets of atlases.

[0090] On the basis of the above technical solution, the present invention further provides a bionic flexible pectoral fin wake vortex structure observation test analysis method, comprising the following steps:

[0091] Step 1: Use the above-mentioned test system to obtain two sets of atlases F0 and F1, and use the MATLAB open source PIV analysis program PIVLab to remove the surrounding environment and the fixed rod connected to the test prototype captured in each image in the atlas. Then use the high-pass filtering method to filter out the bright spots in each image and enhance the contrast of the images in the atlas, thereby obtaining the updated atlases F0 and F1.

[0092] Step 2: Convert the updated atlas F0 and F1 from RGB space to Lab space, and then use the PIVLab open source program to match the corresponding tracer particles in the two images corresponding to the same time in the atlas F0 and F1, such as Figure 3 As shown, taking particle No. 8 as an example, solve the velocity of the corresponding jumping particle:

[0093]

[0094] Where (x1, y1) is the pixel position of the 8th active particle in the image of F1, (x0, y0) is the pixel position of the 8th active particle in the image of F0, and the velocity direction of this particle is (x0, y0) pointing to (x1, y1), and dt is the shooting time interval; the active significance of the tracer particle must be greater than 0.8, that is:

[0095] The active saliency calculation area of ​​a tracer particle is 64 pixels, and the pixel p corresponding to the tracer particle in the image of the atlas F0 and F1 is i and p j The Euclidean distance between the colors is d c (p i , p j ), the spatial Euclidean distance is d p (p i , p j ), then the activity significance of the tracer particle is:

[0096]

[0097] The velocity of a tracer particle at the pixel position (x, y) is recorded as the vector V(x, y). The velocity vectors of all tracer particles are integrated to form the velocity field of the entire observation domain at this moment. According to the Gaussian distribution, the acceptable velocity range is set to filter out tracer particles outside the mean of the velocity field by plus or minus 3 standard deviations.

[0098] Step 3, the images corresponding to the remaining moments in the atlas F0 and F1 are processed in the same way as in step 2, and a velocity field can be calculated for each pair of images; the observation domain is divided into grid points according to the spatial position, and the corresponding velocity values ​​are assigned to the grid points corresponding to the tracer particles, so as to obtain the velocity field distribution data V of the observation domain arranged in time, and the data is saved in the .dat standard file format. The moment corresponding to different velocity fields is called a snapshot moment. In order to make the subsequent modal decomposition results of the vortex structure of the flexible pectoral fin of the manta ray have significant conclusions, as many double-frame atlases as possible should be taken in one flapping cycle.

[0099] Step 4, using the observation domain velocity field distribution data V, adopting the POD modal decomposition method to analyze the main modes in the vortex structure of the pectoral fin wake flow field of the test prototype, and reconstructing the flow field according to the main modes with a large energy share, to obtain the physical quantity matrix set Physics(u, v, Ω) after POD modal decomposition; wherein v represents the superposition of the average velocity and the pulsating velocity at the grid point, u represents the average velocity vector composed of the average values ​​of v at different grid points, and Ω represents the vortex field.

[0100] Step 5, analyze the spatial distribution characteristics of each physical quantity in the physical quantity matrix set Physics (u, v, Ω), as well as the mapping relationship between the physical quantity and the motion parameters and mechanical characteristics, and establish a functional fitting relationship between the pectoral fin wake velocity field and the vorticity field based on the motion parameters. The following description is made with the vorticity field as the analysis object.

[0101] (1) For the motion scenario of the test prototype, the Strouhal number is a key dimensionless number that affects the distribution of its vorticity field. In this scheme, the Strouhal number is expressed as:

[0102]

[0103] Where L is the maximum amplitude of the pectoral fin, f is the movement frequency of the pectoral fin, V ∞ is the flow velocity in the observation domain.

[0104] Taking the Strouhal number as the analysis variable, the vortex core based on the Q criterion is calculated according to the obtained physical quantity matrix set Physics(u, v, Ω) and the Q criterion; the development stage of the corresponding vortex core is judged according to the area of ​​the vortex core, such as shedding, evolution, retention, divergence, etc., and the lateral spacing, longitudinal spacing, jet exit angle, maximum vortex core area and evolution distance, vorticity, etc. between vortex cores are recorded; the geometric distribution and numerical differences of the vortex field under different motion parameters are compared to form a vortex field data set for different motion parameters.

[0105] (2) Analyze the remaining physical quantities u and v; according to the multi-factor regression analysis method, map the functional relationship between motion parameters such as the maximum amplitude angle A of the motor, motion frequency, and phase difference φ and flow field physical quantities such as velocity field and vorticity field, and obtain the response surface of the wake field vortex structure physical quantities based on the motion parameters.

[0106] Based on the existing mechanical property data, the fluid dynamics mechanism of mechanical property changes can be analyzed more deeply; by simulating the mapping function between the motion parameters of the manta ray's flexible pectoral fin and the physical quantity change characteristics of the wake field vortex structure, the propulsion performance of the pectoral fin can be quickly predicted under any motion parameters. The logic flow chart of this example is as follows Figure 4 .

[0107] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A bionic flexible pectoral fin wake vortex structure observation test system, characterized in that: It includes a manta ray flexible pectoral fin test prototype, a circulating water tank, a camera, a pulsed laser, tracer particles, and an observation domain, including: The manta ray flexible pectoral fin test prototype is installed in the observation area in the circulating water tank and assembled on the position adjustment mechanism arranged on the top of the circulating water tank through a fixing rod; the test prototype is used to move under the driving of the circulating water flow provided by the circulating water tank, so as to observe the wake vortex structure of the pectoral fin of the test prototype; The circulating water tank is used to provide a flow field environment for the test prototype, and the section is observed in the circulating water tank according to the size of the test prototype; The observation domain is set in the observation section, and the size of the observation domain is determined through observation experiments. The prototype is located in the middle of the edge line of the observation domain; The sheet laser provided by the pulsed laser is emitted upward from the glass below the circulating water tank, and the sheet laser plane coincides with the plane to be observed by the camera in the observation domain; the sheet laser should irradiate the evolution area of ​​the wake vortex structure in the observation domain and completely include the test prototype; The camera is set on the side of the circulating water tank, and its optical axis is perpendicular to the side glass of the circulating water tank; a calibration plate is installed on the plane to be observed, and the size of the calibration plate should be sufficient to cover the observation area, and the observation area is calibrated using the calibration plate; Tracer particles are spread in the circulating water tank, and the diameter and number of the tracer particles are set according to the size of the observation domain and the resolution of the camera; Start the circulating water tank to make the tracer particles evenly distributed in the observation domain. The test site should be in a dark environment. Enable the test prototype, turn on the pulse laser and camera, and take the tracer particle flow image in the plane to be observed after the test prototype is stable, and obtain two sets of atlases F0 and F1 based on time-marching arrangement; F0 is the image set taken for the first exposure, and F1 is the image set taken for the second exposure. The analysis of the wake vortex structure of the bionic flexible pectoral fin is carried out based on the two sets of atlases.

2. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 1 is characterized in that: The design process of the manta ray-like flexible pectoral fin test prototype is as follows: A coordinate system is established with the chord direction as the x direction, the span direction as the y direction, and the thickness direction as the z direction; firstly, the top view contour of the manta ray is scanned to obtain the point set of the top view outer contour curve of the manta ray; the mouthparts, eyes, and tail curve areas corresponding to the curve at the head of the manta ray in the top view outer contour curve are optimized using static deviation, and the optimization principle is to use a spline curve to make the curve transition as smooth as possible, and obtain the point set of the optimized top view outer contour curve; After the point set of the top-view outer contour curve of the test prototype is determined, the span-wise section of the test prototype is designed using the NACA series airfoils. According to different internal load requirements, airfoils with different thickness ratios are used to envelop the surface, and the three-dimensional shape of the test prototype is constructed based on the top-view outer contour curve.

3. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 1 is characterized in that: The flexibility and thickness of the pectoral fin of the test prototype should follow the trend of gradually increasing flexibility from head to tail and from center to sides; design bionic bones with different flexibility distributions according to the number of driving sources of the test prototype; design groove structures with non-uniform distance distribution on the span and chord paths of the bionic bones in the pectoral fin; on each bionic bone, starting from the first groove close to the center of the test prototype, the distance between adjacent grooves is proportionally reduced by 1.5; four motors are set inside the test prototype, of which motor 1 and motor 3 are located on both sides of the front of the test prototype, respectively connected to the front fins on both sides; motor 2 and motor 4 are located on both sides of the rear of the test prototype, respectively connected to the rear fins on both sides; the driving source of the pectoral fin of the test prototype adopts the CPG motion form inspired by biological motion, which is converted into PWM signals to drive motors 1 to 4; The central columnar main body and pectoral fins of the test prototype are printed with nylon material, and the bionic muscles on the pectoral fins are made of super-elastic flexible material. A demoulding mold is made according to the three-dimensional shape point set, and the prepared silicone liquid is poured into the mold. The front and rear fins are installed, and the flexible pectoral fins are obtained after vacuum degassing and high-temperature shaping.

4. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 3 is characterized in that: Use a dynamometer to lift the front fin ray upward at the wing tip of the pectoral fin with a fixed force, and project the deformation curve of the experimental prototype onto the yOz plane of the coordinate system. The maximum deformation angle θ must be greater than 60°; the x direction of the coordinate system is the chord direction, the y direction is the span direction, and O is the origin of the coordinate system.

5. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 1 is characterized in that: Chemical pigments were used to mark several characteristic points on the pectoral fins, and the test prototype was installed in an underwater environment. An underwater camera was used to capture the time-varying motion trajectory of the characteristic points during the motion process. The characteristic points were taken as the tip of the front fin ray, the tip of the rear fin ray, and the chord-wise tip of the rear fin ray. The dynamic deviation of the corresponding positions on the pectoral fins of the real manta ray and the test prototype was Should be less than the preset value: Where, d 0i (t), d i (t) are the motion trajectories of the i-th feature point on the pectoral fin of the manta ray and the test prototype at time t, respectively.

6. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 1, characterized in that: The camera's frame capture interval dt is synchronized with the pulsed laser and is selected based on the expected movement distance of the tracer particles in the flow field: Where, d exp is the expected moving distance of the tracer particles within the shooting time interval, and U is the flow velocity of the circulating water tank.

7. The bionic flexible pectoral fin wake vortex structure observation test system according to claim 1 is characterized in that: The diameter and number of tracer particles are set according to the size of the observation domain and the camera resolution, as shown below: N is the number of complete tracer particles in any 64*64 pixel area in the flow field captured by the camera, ranging from 8 to 25, and BL is the length of the test specimen.

8. A bionic flexible pectoral fin wake vortex structure observation test analysis method, characterized in that: include: Step 1: Use the above-mentioned test system to obtain two sets of atlases F0 and F1, use the analysis program PIVLab to remove the surrounding environment and the fixed rod connected to the test prototype captured by each image in the atlas, and then use the high-pass filtering method to filter out the bright spots in each image and enhance the contrast of the images in the atlas, so as to obtain the updated atlases F0 and F1; Step 2: Convert the updated atlas F0 and F1 from RGB space to Lab space, and then use the PIVLab open source program to match the corresponding tracer particles in the two images corresponding to the same time in the atlas F0 and F1, and solve the velocity of the corresponding tracer particles; Set the active significance threshold of the tracer particles, filter the tracer particles with a value greater than the active significance threshold, and integrate the velocity vectors of all tracer particles to obtain the velocity field of the entire observation domain at this moment; set the acceptable velocity range according to the Gaussian distribution, and filter out the tracer particles outside the mean value of the velocity field by N standard deviations; Step 3, a velocity field can be calculated for each pair of images corresponding to all moments in the atlas F0 and F1; the observation domain is divided into grid points according to the spatial position, and the corresponding velocity values ​​are assigned to the grid points corresponding to the tracer particles, so as to obtain the velocity field distribution data V of the observation domain arranged according to time; Step 4, using the velocity field distribution data V in the observation domain, adopting the POD modal decomposition method to analyze the main modes in the vortex structure of the pectoral fin wake flow field of the test prototype, and reconstructing the flow field according to the main modes with a large energy share, to obtain the physical quantity matrix set Physics (u, v, Ω) after POD modal decomposition; wherein v represents the superposition of the average velocity and the pulsating velocity at the grid point, u represents the average velocity vector composed of the average values ​​of v at different grid points, and Ω represents the vorticity field; Step 5, analyzing the spatial distribution characteristics of each physical quantity in the physical quantity matrix set Physics (u, v, Ω), as well as the mapping relationship between the physical quantity and the motion parameters and mechanical characteristics, and establishing a functional fitting relationship between the pectoral fin wake velocity field and vortex field based on the motion parameters.

9. The bionic flexible pectoral fin wake vortex structure observation test analysis method according to claim 8, characterized in that: When performing vortex field analysis: Taking the Strouhal number as the analysis variable, the vortex core based on the Q criterion is calculated according to the obtained physical quantity matrix set Physics(u, v, Ω) and the Q criterion; the development stage of the corresponding vortex core is judged according to the area of ​​the vortex core, such as shedding, evolution, retention, divergence, etc., and the lateral spacing, longitudinal spacing, jet exit angle, maximum vortex core area and evolution distance, vorticity, etc. between vortex cores are recorded; the geometric distribution and numerical differences of the vortex field under different motion parameters are compared to form a vortex field data set for different motion parameters.

10. The bionic flexible pectoral fin wake vortex structure observation test analysis method according to claim 8, characterized in that: According to the multi-factor regression analysis method, the motion parameters including the maximum amplitude angle of the motor, motion frequency, phase difference and physical quantities are mapped with functional relationships to obtain the response surface of the physical quantities of the wake field vortex structure based on the motion parameters, thereby realizing the rapid prediction of the pectoral fin propulsion performance under arbitrary motion parameters.

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