Three-dimensional reconstruction method of hydrofoil complex cavitation form based on binocular orthogonal view angle

Abstract

The invention discloses a three-dimensional reconstruction method of a hydrofoil complex cavitation bubble form based on a binocular orthogonal visual angle, and belongs to the field of cavitation bubble interface capture in the field of fluid measurement. According to the invention, two high-speed cameras are utilized to obtain time sequence images of two visual angles of the cavitation bubble. An initial boundary is extracted based on a boundary detection method, image noisy points are removed through a connected region labeling method, and a continuous and clear cavitation interface is obtained through a morphological closed operation method. Then, according to the actual morphological characteristics of the vacuoles, the sections of the vacuoles in all the flow directions are reconstructed through assumption of a positive ellipse, a rotating ellipse and a semi-ellipse, and therefore a three-dimensional space structure is reconstructed. According to the method, the three-dimensional and dynamic cavitation structure can be accurately reconstructed by using relatively simple binocular experiment steps.

Description

Technical Field

[0001] The present invention relates to the capture of cavitation interfaces in the field of fluid measurement, and particularly to a three-dimensional reconstruction method for complex cavitation morphologies of hydrofoils based on binocular orthogonal perspectives. This method is based on cavitation images from two perspectives with time resolution. Through the ellipse fitting method, combined with the assumptions of cylindrical, double-helical, and semi-circular cavitation interfaces, the cavitation under different working conditions near the tip of the hydrofoil is reconstructed. Background Art

[0002] The cavitation phenomenon is a complex hydrodynamic process in the field of multiphase flows and widely exists in hydraulic machinery such as ship propellers. During the evolution process, cavitation is affected by its own instability and the non-uniform pressure field of the flow field, and has high unsteadiness and three-dimensional characteristics. The volume pulsation of cavitation will cause local radiation noise and vibration, and its spatial distribution will affect the three-dimensional vortex structure in the flow field, inducing the generation of turbulence. Therefore, the accurate estimation of the three-dimensional morphology of cavitation is of great significance.

[0003] Traditional three-dimensional reconstruction methods for bubbles mainly target bubble flows in free fields. These bubbles are mostly circular and elliptical, which are very different from the bubble morphologies and scales generated by propellers or hydrofoils. Most traditional bubble reconstruction methods use basic ellipse fitting algorithms. However, this algorithm is not accurate enough for fitting slender ribbon-like double-helical tip vortex cavitation and sheet cavitation. The long and short axis information of the cavitation cannot be accurately obtained from the cavitation images of two perspectives. At the same time, some binocular reconstruction methods based on computer vision generally require clear texture information on the gas-liquid interface, which will greatly increase the requirements for the light source for relatively stable cavitation conditions.

[0004] For example, patent application number CN117058335A discloses a method for three-dimensional morphology reconstruction of bubbles, a device, an electronic device, and a storage medium. Although it mentions the method of bubble slicing and reconstruction based on ellipse fitting, it does not consider the problem of ellipse rotation. In actual situations, the side lengths of the circumscribed rectangles of the ellipses extracted from two perspectives are not necessarily parallel to the long and short axes of the actual ellipses, which will underestimate the actual cavitation volume. Summary of the Invention

[0005] To overcome the deficiencies in the prior art, the present invention provides a three-dimensional reconstruction method for complex cavitation morphologies of hydrofoils based on binocular orthogonal perspectives. The present invention can capture the cavitation images evolving in time series from two perspectives, and combine the assumptions of cylindrical, double-helical, and semi-circular cavitation interfaces to obtain the pulsation conditions of the actual cavitation diameter, area, and volume.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives, which includes the following steps:

[0008] S1: Arrange a CMOS high-speed camera on each of two orthogonal perspectives of the flow field condition to be measured, and control the two CMOS high-speed cameras to synchronously collect the cavitation images of the flow field condition to be measured to achieve synchronous acquisition of sequential images; the flow field condition to be measured is the vortex cavitation and surface sheet cavitation formed based on the hydrofoil.

[0009] S2: Divide the surface sheet cavitation and the downstream vortex cavitation according to the position where the cavitation is located, and classify the vortex cavitation into columnar vortex cavitation and double-helical vortex cavitation according to the difference between the maximum and minimum diameters of the vortex cavitation in a single view.

[0010] S3: Physically calibrate the CMOS high-speed camera, and establish two-dimensional coordinate systems of the xy plane and the xz plane with the tip of the hydrofoil as the origin in the cavitation image.

[0011] S4: Use the boundary detection method based on the Canny algorithm to locate the gas-liquid interface of the collected cavitation image, set the filter parameters and algorithm thresholds according to the partition, and obtain the binary boundary image.

[0012] S5: Calculate and label the connected regions of the binary boundary image, delete the connected region objects in the image that are smaller than the set number of pixels, and perform morphological closing operation on the images in each partition respectively to obtain the cavitation contour.

[0013] S6: According to the classification result of S2, select the corresponding reconstruction model for the cavitation slice in the yz plane and perform cavitation contour slice reconstruction.

[0014] S7: Stack the cavitation contour slices obtained in S6 layer by layer along the flow direction, and finally reconstruct the three-dimensional shape of the cavitation.

[0015] According to the preferred embodiment of the present invention, in S1, a signal synchronizer is used to control the two CMOS high-speed cameras to achieve synchronous acquisition of sequential images. After the acquisition is completed, all the images are transmitted to the processing end for storage and processing.

[0016] According to the preferred embodiment of the present invention, S2 includes classifying the part on the surface of the hydrofoil as sheet cavitation, and its flow cross-section can be regarded as a semi-elliptical sheet structure; the part separated from the hydrofoil is vortex cavitation; according to the difference between the maximum diameter D max and the minimum diameter D min of the cavitation in a single view, classify the vortex cavitation: classify the vortex cavitation with (D max - D min ) / D min < 30% as columnar vortex cavitation, otherwise classify it as double-helical vortex cavitation.

[0017] According to a preferred embodiment of the present invention, S3 includes: placing calibration reference objects near the cavitation generation position respectively and making the CMOS high-speed camera focus, taking calibration images from two perspectives respectively, and calculating the image magnification ratios of the two perspectives; subsequently, establish two-dimensional coordinate systems of the xy plane and the xz plane in the image with the tip of the hydrofoil as the origin, wherein the x direction is the free-stream direction, the y direction is the vertical direction, and the z direction is the spanwise direction of the hydrofoil.

[0018] According to a preferred embodiment of the present invention, S4 includes: first, performing a convolution operation between the Gaussian smoothing operator and the image to remove noise, setting different filter standard deviations for the sheet cavitation and vortex cavitation regions respectively to extract boundary information; calculating the amplitude and direction of the image gradient using the derivative operator; performing non-maximum suppression on the global image gradient amplitude, comparing the gradient amplitudes in the gradient direction, and retaining the local maximum of the gradient amplitude as the boundary, eliminating non-boundary pixels; detecting and connecting edges using the double-threshold algorithm, setting two thresholds of high and low according to the contrast of the image, retaining the strong edges with gradient amplitudes greater than the high threshold and the weak edges with gradient amplitudes between the high and low thresholds and connected to the strong edges, and eliminating other edges.

[0019] According to a preferred embodiment of the present invention, when performing morphological closing operation in S5, for the vortex cavitation region, a square structuring element is used for erosion and closing; for the sheet cavitation region, due to the large curvature of the boundary, a disk-shaped structuring element is used for processing.

[0020] According to a preferred embodiment of the present invention, in S6, for the columnar vortex cavitation region, a positive ellipse assumption is used for fitting. For the double-helical cavitation, it is still assumed that the flow direction slice shape of the cavitation is an ellipse, and these ellipse slices rotate at a constant angular velocity along the flow direction, corresponding to two helical lines. For the attached sheet cavitation region, a semi-ellipse geometric model with a constant deflection angle α is used to fit its cross-sectional slice.

[0021] According to a preferred embodiment of the present invention, when performing layer-by-layer stacking in S7, in the transition region between the sheet cavitation and the vortex cavitation, the Gaussian function is used to smooth the cavitation diameter and the center position.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) The vortex cavitation reconstruction model in the present invention fully considers the three-dimensional spatial evolution law of the cavitation itself, overcomes the limitations of the prior art that cannot reconstruct the distorted structure and the error introduced by underestimating the actual cavitation diameter, and has stronger adaptability and accuracy especially for typical vortex cavitation structures such as the double-helical shape.

[0024] (2) The multiple cavitation physical models with sub-regions proposed in the present invention fully consider the structural characteristics of the attached sheet cavitation on the hydrofoil surface and its transition relationship with the downstream vortex cavitation, effectively overcoming the information loss problem existing in the reconstruction of complex cavitation morphologies in the prior art. At the same time, the sub-region setting can effectively remove the interference of water impurity particles and free bubbles, restoring some occluded regions in the original image. Compared with the existing bubble orthogonal reconstruction technology, this method can achieve higher-precision three-dimensional reconstruction when facing complex working conditions with multiple cavitation morphologies, and ensure the coherence between cavitation structures.

[0025] (3) The cavitation reconstruction method in the present invention has low requirements for illumination, only requiring the information of the two outer boundaries. Therefore, it is applicable to some scenes with uneven illumination distribution, effectively overcoming the problem of high dependence on surface texture information in the existing parallel binocular reconstruction, and having stronger adaptability in complex or harsh environments. At the same time, the low illumination requirement also enables this method to be used for the dynamic three-dimensional structure reconstruction of cavitation with high time resolution, capable of obtaining real-time bubble volume information, greatly increasing the spatio-temporal resolution of information such as cavitation diameter, area, and volume. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic flow chart of the three-dimensional morphology reconstruction of the cavitation interface based on binocular orthogonal perspectives provided by an embodiment of the present invention;

[0027] Figure 2 It is an experimental layout diagram provided by an embodiment of the present invention based on binocular orthogonal perspectives;

[0028] Figure 3 It is the original cavitation images from two perspectives provided by an embodiment of the present invention; among them, top view (left), side view (right);

[0029] Figure 4 It is the gas-liquid boundary obtained after pre-processing the cavitation shadow image provided by an embodiment of the present invention; among them, top view (left), side view (right);

[0030] Figure 5 It is the gas-liquid boundary after boundary coherence processing provided by an embodiment of the present invention; among them, top view (left), side view (right);

[0031] Figure 6 It is the reconstruction principle of columnar vortex cavitation provided by an embodiment of the present invention;

[0032] Figure 7 It is the calculation principle of the major and minor axes of the double-helical vortex cavitation slice provided by an embodiment of the present invention;

[0033] Figure 8 It is the calculation principle of the attached sheet cavitation slice provided by an embodiment of the present invention;

[0034] Figure 9 The three-dimensional cavitation reconstruction result provided by the embodiment of the present invention; among them, columnar vortex cavitation (upper), double helical cavitation (lower);

[0035] Figure 10 The comparison between the actual image and the reconstruction result of the columnar vortex cavitation provided by the embodiment of the present invention; among them, (a) side view, (b) top view;

[0036] Figure 11 The comparison between the actual image and the reconstruction result of the double helical vortex cavitation provided by the embodiment of the present invention; among them, (a) side view, (b) top view;

[0037] Figure 12 The comparison between the actual image and the reconstruction result of the attached sheet cavitation under the double helical condition provided by the embodiment of the present invention; among them, (a) side view, (b) top view. Detailed implementation manners

[0038] The method of the present invention will be further described below through specific embodiments. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention and do not impose any limitation on it.

[0039] The method proposed by the present invention is mainly applied to the three-dimensional shape reconstruction of cavitation, and is particularly suitable for reconstructing cavitation under different working conditions near the tip of a hydrofoil. Among them, a hydrofoil is a wing moving in water. For example, the blade of a ship's propeller is a kind of hydrofoil.

[0040] In order to facilitate the description of the technical process and technical effects of the present invention, the following embodiments simulate the actual working conditions of a hydrofoil by building a simulation experiment platform, so as to introduce in detail the implementation process of the three-dimensional shape reconstruction of cavitation of the present invention and illustrate the processing results of each step. However, it should be noted that the present invention is applicable to the three-dimensional shape reconstruction of cavitation generated during the working process of an actual hydrofoil (ship propeller blade), and is also applicable to the three-dimensional shape reconstruction of vortex cavitation and sheet cavitation in a hydrofoil hydrodynamics experiment platform or an industrial simulation platform of any simulation scale.

[0041] As Figure 1 shown, the embodiment of the present invention discloses a method for three-dimensional cavitation shape reconstruction with binocular orthogonal perspectives. The method steps include:

[0042] (1) Obtain the time-sequence images of cavitation from two perspectives

[0043] Build a corresponding hydrofoil simulation experiment platform according to the working conditions of the flow field to be measured during the actual working process of the ship propeller blade. The layout schematic diagram is as Figure 2As shown in the figure, it includes a hydrofoil model 2-1, a water tunnel test section 2-2, a CMOS high-speed camera 2-3, an LED lamp 2-4, a signal synchronizer 2-5, and a PC 2-6. The hydrofoil model 2-1 is installed in the central visible area of the water tunnel test section 2-2, with an angle of attack of 9°. Two CMOS high-speed cameras 2-3 are respectively arranged at the orthogonal perspectives above and on the side of it, both equipped with 105mm focal length lenses. The continuous light source of the LED lamp 2-4 passes through the acrylic plate to illuminate the area near the hydrofoil where cavitation is about to occur, and calibration targets are placed to physically calibrate the two CMOS high-speed cameras 2-3 respectively. During the experiment, run the water tunnel and adjust the internal pressure to the target cavitation number, and wait for the cavitation to fully develop. After adjusting to the target cavitation, stabilize the pressure and observe the cavitation state. Adjust the camera exposure time and lens focus to ensure the clarity of the gas-liquid interface. After the cavitation stabilizes, the trigger signal is input into the signal synchronizer 2-6 through the PC 2-5 instruction, and the signal synchronizer 2-6 will control the CMOS high-speed camera 2-3 to collect cavitation images. The signal synchronizer 2-6 is used to control the two CMOS high-speed cameras 2-3 to achieve synchronous acquisition of sequential images. After the acquisition is completed, all the images are transmitted to the PC 2-5 for storage.

[0044] (2) Cavitation Zoning and Vortex Cavitation Classification

[0045] In the current experiment, the cavitation is zoned according to its location. The part on the hydrofoil surface is classified as sheet cavitation, and its flow cross-section can be regarded as a semi-elliptical sheet structure. The part detached from the hydrofoil is vortex cavitation, presenting a cylindrical or double-helical shape. According to the difference between the maximum diameter (D max ) and the minimum diameter (D min ) of the cavitation in a single view, the vortex cavitation is classified: the vortex cavitation with (D max - D min ) / D min < 30% is classified as columnar vortex cavitation; the vortex cavitation not less than this threshold is classified as double-helical vortex cavitation, presenting a twisted ribbon shape.

[0046] (3) Physical Calibration of CMOS High-Speed Camera

[0047] Calibration references are respectively placed near the cavitation generation position in the water tunnel and the CMOS high-speed camera is focused, and calibration images from two perspectives are respectively taken to calculate the image magnification of the two perspectives. Subsequently, a two-dimensional coordinate system of the (x, y) plane and the (x, z) plane is respectively established in the image with the tip of the hydrofoil as the origin, where the x direction is the free stream direction, the y direction is the vertical direction, and the z direction is the spanwise direction of the hydrofoil.

[0048] (4) Preprocessing of Cavitation Shadow Images

[0049] After adjusting to obtain the target cavitation bubble, collect and acquire the cavitation bubble image, for example Figure 3 the instantaneous images from two perspectives shown. Use the edge detection method based on the Canny algorithm to locate the gas-liquid interface and obtain the binary image of the cavitation bubble boundary. The principle is as follows:

[0050] First, use the Gaussian smoothing operator to convolve with the image to remove noise. Process the vortex cavitation bubble region and the sheet cavitation bubble part separately, and set different filter standard deviations according to the image contrast and clarity of the gas-liquid interface. For the boundary region of the sheet cavitation bubble on the hydrofoil surface, a smaller standard deviation (0.05 - 0.1) is selected to retain more details. For the boundary region of the vortex cavitation bubble on the hydrofoil surface, a larger standard deviation (0.1 - 0.3) is selected to remove the interference of some free bubbles around the cavitation bubble.

[0051] Use the derivative operator to calculate the magnitude and direction of the image gradient. Among them, , are the gray gradients in the x and y directions of the image respectively. The magnitude reflects the boundary strength of the image, and the gradient direction is perpendicular to the edge direction.

[0052] Perform non-maximum suppression on the global image gradient magnitude in the gradient direction, that is, consider the local maximum of the image gradient magnitude in this direction as the boundary and remove other non-boundary pixels.

[0053] Use the double-threshold algorithm to detect and connect edges, that is, judge the relationship between the image gradient magnitude at each position and the set threshold. Set two thresholds, a high threshold and a low threshold, according to the image contrast. In this experiment, the high threshold is set to 0.25 (25% of the maximum gradient), and the low threshold is set to 0.1 (10% of the maximum gradient). Retain the strong edges with gradient magnitudes greater than the high threshold and the weak edges with gradient magnitudes between the high and low thresholds and connected to the strong edges, and remove other edges. Figure 4 This is an example of a columnar vortex cavitation bubble image processed through the above steps.

[0054] (5) Boundary coherence processing

[0055] Calculate the connected regions of the binary boundary image and label them, and delete the connected region objects with less than M pixels in the image. For the sheet cavitation region, M is taken as 20, and for the vortex cavitation region, M is taken as 40. This step can remove the interference of some small bubbles and the information on the hydrofoil surface in the original image, but there are still some discontinuous or non - continuous regions in the boundary image, and the boundary contours of some regions are not smooth enough. It is necessary to further perform morphological closing operation on the image. In this experiment, for the vortex cavitation region, a square structuring element with a side length of 40 pixels is used for erosion and closing. For the sheet cavitation region, due to the large curvature of the boundary, a circular structuring element with a radius of 30 pixels is used for processing. Taking the columnar cavitation as an example, the boundary images from two perspectives are as Figure 5 shown.

[0056] At this time, there are still some unclosed boundaries at the transition position between the attached sheet cavitation and the vortex cavitation. Use a Gaussian weighted moving average filter to smooth the data vectors of the obtained cavitation center coordinates and the diameter distribution along the flow direction to ensure the continuity of the boundary.

[0057] (6) Reconstruct the shape of the cavitation slice

[0058] According to the classification results in step (2), select the corresponding reconstruction model for the cavitation slice in the yz plane to reconstruct the cavitation contour slice; 6 - 1) Columnar vortex cavitation

[0059] For the columnar vortex cavitation region, the positive ellipse hypothesis is used for fitting. Take the cavitation boundary pixels corresponding to the same flow - direction position in the top - view image and the side - view bubble image as the major axis and minor axis of the ellipse slice respectively. Assume that its major and minor axes are perpendicular to the y and z coordinate axes respectively, and reconstruct the ellipse slice at this flow - direction position, with a flow - direction thickness of 1 pixel. Figure 6 The ellipse slices at each flow - direction position obtained using the positive ellipse hypothesis are given, and stacked to form a columnar cavity structure. The ellipse equation used is:

[0060]

[0061] where z0 and y0 are the span - wise and vertical coordinates of the ellipse center respectively, which can be obtained from the upper and lower boundaries [y1, y2], [z1, z2] of the cavitation in the two views:

[0062]

[0063] a and b are the major and minor axes, corresponding to the diameter information in the two views:

[0064]

[0065] 6 - 2) Double - helical vortex cavitation

[0066] For the double - helical cavitation bubble, it is still assumed that the flow - direction cross - sectional shape of the cavitation bubble is an ellipse, and these elliptical slices rotate at a constant angular velocity along the flow direction, corresponding to two helical lines. The reconstruction method of the major and minor axes of its cross - section is shown in Figure 7 as follows. Assuming that the major and minor axes a and b of the ellipse are unknowns, and the angle between the major axis and the z - axis is β, the equation of the ellipse becomes:

[0067]

[0068] z0 and y0 are the span - wise and vertical coordinates of the ellipse center respectively. From the equations of two horizontal and vertical tangent lines, we can get:

[0069]

[0070] p and q are the differences between the upper and lower boundaries of the cavitation bubble obtained from the side view and the top view respectively:

[0071]

[0072] By combining the above several formulas, we can get:

[0073]

[0074] Then the values of a and b for each slice can be obtained, and the inclined elliptical slices can be reconstructed.

[0075] 6 - 3) Reconstruction of attached sheet - type cavitation bubbles

[0076] For the attached sheet - type cavitation bubble region, a semi - elliptical geometric model with a constant deflection angle α is used to fit its cross - sectional slices, as shown in Figure 8 as follows. The value of α is consistent with the angle of attack of the hydrofoil. In this experiment, α = 9°. For the major and minor axes a and b of the semi - ellipse, they are also calculated by the above formula:

[0077]

[0078] where p and q are the differences between the upper and lower boundaries of the cavitation bubble obtained from the side view and the top view respectively;

[0079] For the values of the span - wise and vertical coordinates z0 and y0 of the ellipse center, in order to simplify the calculation, the right - lower corner point (z2, y1) can be regarded as coinciding with the end - point of the semi - ellipse, and then they can be calculated by the following formula:

[0080]

[0081] The corresponding elliptical slices can be obtained by using the above formula. Assuming that the part of the sheet - type cavitation bubble on the hydrofoil surface is all semi - elliptical structures, an ellipse needs to be cut by a straight line passing through the ellipse center (z0, y0), that is, the region satisfying the following formula is retained:

[0082]

[0083] When the cavity begins to detach from the hydrofoil, its shape gradually transitions from semi-elliptical to elliptical. For this transition region, an inclined straight line that moves linearly in the vertical direction is used to cut the ellipse, that is, the region of the following formula is retained:

[0084]

[0085] where h(x) is a function of the flow direction position x. Denote the flow direction interval of the transition region as [x1, x2] (x2 is downstream of x1), and h(x) is calculated by the following formula:

[0086]

[0087] where p(x2) represents the difference between the upper and lower boundaries of the cavity obtained from the side view of the slice at the position x2 in the flow direction.

[0088] (7) Three-dimensional cavity reconstruction

[0089] Stack the slices of each region obtained by the above method layer by layer in the flow direction, and finally reconstruct the three-dimensional shape of the cavity. In the transition region between the sheet cavity and the vortex cavity, the Gaussian function is used to smooth the cavity diameter and the center position. The finally obtained three-dimensional structure of the cavity is as Figure 9 shown.

[0090] It can be seen from the reconstruction results of the columnar vortex cavity that there is an obvious wavy structure on the surface of the vortex cavity under this working condition. Its trajectory deflects greatly in the spanwise direction when it just detaches from the hydrofoil, and then becomes flat in the downstream. At this time, the area occupied by the sheet cavity attached to the surface of the hydrofoil is relatively small. For the double-spiral cavity, the sheet cavity area increases significantly and the cavity thickness increases. The part of the vortex cavity that detaches from the hydrofoil undergoes obvious torsion, and its structure is similar to a twisted "ribbon". The reconstruction results of the two cavities are in good agreement with the results of the high-speed cavity images as a whole.

[0091] Further, local vortex cavity images of two views are selected for comparison with the two-dimensional field of view intercepted from the three-dimensional reconstruction results.

[0092] Figure 10 For the comparison results of the columnar vortex cavity, it can be seen that the size and trajectory of the reconstructed cavity are in good agreement with the actual photographed results, and the wavy distribution of the gas-liquid interface is also clearly identified, indicating that the cross-section assumption of the regular ellipse conforms to the actual physical characteristics of the columnar cavity and the reconstruction results are good.

[0093] Figure 11For the comparison results of double - helix - shaped vortex cavitation, the distributions of the trajectories and diameters of the cavitation bubbles at each flow - direction position also match well with the results of the original images obtained by high - speed photography. The twisted shapes, wavelengths, and node distributions of the cavitation bubbles are highly consistent. This indicates that the rotational hypothesis of the ellipse is more in line with the physical structure of the double - helix - shaped cavitation bubbles, and the reconstruction results are good.

[0094] Figure 12 The double - helix mode with larger attached - sheet cavitation bubbles was selected for comparison. It can be seen that the cavitation - bubble shapes shown in the reconstruction results are roughly the same as the actual attached - sheet cavitation results. In the top - view, the contrast of the gas - liquid interface at the upper boundary of the cavitation bubble is small, and in the transition region where the attached - sheet cavitation detaches from the hydrofoil and connects to the vortex cavitation, the interface is not clear and the reflection is relatively serious, introducing a small amount of error into the reconstruction. Nevertheless, judging from the contour matching degree, the current method reconstructs the attached - sheet cavitation - bubble structure well.

[0095] Further evaluate the error of the current reconstruction method. Considering that the cavitation itself will be disturbed over time, the repeatability of calculating the volume of the vortex cavitation is extracted in the interval of the flow - direction position [20 mm 70 mm] from the tip of the hydrofoil, and the repeatability of calculating the volume and surface area of the attached - sheet cavitation is extracted in the interval [-13.0 mm 15.5 mm]. The mean value, standard deviation, and repeatability calculation formulas for the volume (surface area) are as follows:

[0096]

[0097]

[0098]

[0099] Where N is the number of sample groups, which is 20, and 25 samples are selected for each group. is the average value of the volume (surface area) calculated for each group of samples, is the average volume (surface area) of all samples, S is the sample standard deviation, is the uncertainty of the volume (surface area), that is, the test repeatability. The respective repeatability results are shown in Table 1. The volume repeat rates of the column - shaped cavitation bubbles and double - helix cavitation bubbles are 4.1% and 1.6% respectively. The volume and surface - area repeat rates of the attached - sheet cavitation bubbles are 1.2% and 0.75% respectively, and the measurement repeatability is better than 5%.

[0100] Table 1 - Statistics of 3D reconstruction repeatability

[0101]

[0102] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. For those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives, characterized in that, It includes the following steps: S1: Arrange a CMOS high-speed camera on each of two orthogonal views of the flow field condition to be measured, and control the two CMOS high-speed cameras to synchronously collect the cavitation images of the flow field condition to be measured to achieve synchronous acquisition of sequential images; the flow field condition to be measured is the vortex cavitation and surface sheet cavitation formed based on a hydrofoil. S2: Divide the surface sheet cavitation and downstream vortex cavitation according to the position where the cavitation is located, and classify the vortex cavitation into columnar vortex cavitation and double-helical vortex cavitation according to the difference between the maximum and minimum diameters of the vortex cavitation in a single view. S3: Physically calibrate the CMOS high-speed camera, and establish two-dimensional coordinate systems in the xy plane and xz plane with the tip of the hydrofoil as the origin in the cavitation image. S4: Use the edge detection method based on the Canny algorithm to locate the gas-liquid interface of the collected cavitation image, set the filter parameters and algorithm thresholds according to the partition, and obtain the binary edge image. S5: Calculate and label the connected regions of the binary edge image, delete the connected region objects in the image that are smaller than the set number of pixels, and perform morphological closing operations on the images in each partition respectively to obtain the cavitation contour. S6: According to the classification result of S2, select the corresponding reconstruction model for the cavitation slice in the yz plane and perform cavitation contour slice reconstruction. S7: Stack the cavitation contour slices obtained in S6 layer by layer along the flow direction, and finally reconstruct the three-dimensional shape of the cavitation.

2. The three-dimensional reconstruction method of the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein In S1, a signal synchronizer is used to control the two CMOS high-speed cameras to achieve synchronous acquisition of sequential images. After the acquisition is completed, all the images are transmitted to the processing end for storage and processing.

3. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein S2 includes classifying a part of the hydrofoil surface as sheet cavitation, whose flow cross-section is regarded as a semi-elliptical sheet structure; the part separated from the hydrofoil is vortex cavitation; classifying the vortex cavitation according to the difference between the maximum diameter D max and the minimum diameter D min : classifying the vortex cavitation with (D max - D min ) / D min < 30% as columnar vortex cavitation, otherwise classifying it as double-helical vortex cavitation.

4. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, characterized in that, The S3 includes: Place calibration reference objects near the cavitation generation position respectively and make the CMOS high-speed camera focus, take calibration images of the two views respectively, and calculate the image magnification ratios of the two views; then establish two-dimensional coordinate systems in the xy plane and xz plane with the tip of the hydrofoil as the origin in the image, where the x direction is the free stream direction, the y direction is the vertical direction, and the z direction is the spanwise direction of the hydrofoil.

5. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, characterized in that, The S4 includes: First, use a Gaussian smoothing operator to convolve with the image to remove noise, and set different filter standard deviations for the sheet cavitation and vortex cavitation regions respectively to extract the boundary information. Use a derivative operator to calculate the amplitude and direction of the image gradient. Perform non-maximum suppression on the global image gradient amplitude, compare the gradient amplitudes in the gradient direction, and retain the local maximum of the gradient amplitude as the boundary, and eliminate non-boundary pixels. Use a double-threshold algorithm to detect and connect edges, set two thresholds, high and low, according to the contrast of the image, retain the strong edges with gradient amplitudes greater than the high threshold and the weak edges with gradient amplitudes between the high and low thresholds and connected to the strong edges, and eliminate other edges.

6. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein In S5, when performing morphological closing operations, for the vortex cavitation region, use a square structuring element for erosion and closing; for the sheet cavitation region, due to the large curvature of the boundary, use a disk-shaped structuring element for processing.

7. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein In S6, for the columnar vortex cavitation region, use a positive ellipse hypothesis for fitting, including: Take the cavitation boundary pixels corresponding to the same flow direction position in the top-down image and the side-view bubble image as the major and minor axes of the elliptical slice respectively; assume that the major and minor axes are perpendicular to the y and z coordinate axes respectively, and reconstruct the elliptical slice at this flow direction position with a flow direction thickness of 1 pixel. Stack the elliptical slices at each flow direction position obtained by the regular ellipse assumption to form a cylindrical cavity structure; the elliptical equation is: ； where z0 and y0 are the spanwise and vertical coordinates of the ellipse center respectively, obtained from the upper and lower boundaries of the cavitation [y1, y2], [z1, z2] in the two views: ； a and b are the major and minor axes, corresponding to the diameter information in the two views: 。 8. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, characterized in that In S6, for the double-helical cavitation, it is still assumed that the cross-sectional shape of the cavitation in the flow direction is an ellipse, and these elliptical slices rotate at a constant angular velocity along the flow direction, corresponding to two helical lines, including: Assume that the major and minor axes a and b of the ellipse are unknowns, and the angle between the major axis and the z-axis is β, then the equation of the ellipse becomes: ； where z0 and y0 are the spanwise and vertical coordinates of the ellipse center respectively, From the two horizontal and vertical tangent equations: ； p and q are the differences between the upper and lower boundaries of the cavitation obtained from the side view and the top view respectively: ； Combine the formulas: ； That is, the values of a and b for each slice are obtained, and the inclined elliptical slice is reconstructed.

9. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein In S6, for the attached sheet cavitation region, a semi-elliptical geometric model with a constant deflection angle α is used to fit its cross-sectional slice, including: The value of α is consistent with the angle of attack of the hydrofoil. For the major and minor axes a and b of the semi-ellipse, there are: ； p and q are the differences between the upper and lower boundaries of the cavitation obtained from the side view and the top view respectively, For the values of the spanwise and vertical coordinates z0 and y0 of the ellipse center, by regarding the lower right point (z2, y1) as coinciding with the end point of the semi-ellipse, they are calculated by the following formula: ； The corresponding elliptical slice can be obtained by using the above formula; Assume that the part of the sheet cavitation on the hydrofoil surface is all semi-elliptical structures, then a straight line passing through the ellipse center (z0, y0) is needed to cut the ellipse, that is, keep the region of the following formula: ； When the cavitation begins to detach from the hydrofoil, its shape gradually transitions from a semi-ellipse to an ellipse; for this transition region, an inclined straight line moving linearly in the vertical direction is used to cut the ellipse, that is, keep the region of the following formula: ； where h(x) is a function of the flow direction position x. Denote the flow direction interval of the transition region as [x1, x2], x2 is downstream of x1, and h(x) is calculated by the following formula: ； where p(x2) represents the difference between the upper and lower boundaries of the cavitation obtained from the side view in the slice at the flow direction position x2.

10. The three-dimensional reconstruction method for the complex cavitation morphology of a hydrofoil based on binocular orthogonal perspectives according to claim 1, wherein In S7, when stacking layer by layer, a Gaussian function is used to smooth the cavitation diameter and the center position in the transition region between the sheet cavitation and the vortex cavitation.

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