Propeller two-phase flow multi-scale cavitation bubble measurement system based on laser digital holography

Abstract

The invention discloses a propeller two-phase flow multi-scale cavitation bubble measurement system based on laser digital holography, and belongs to the field of cavitation flow measurement. The device comprises an experiment section, an experiment model, a supporting rotating shaft, a polarization laser, a concave lens, a first convex lens, a second convex lens, a digital camera, a lens, a dimmer, a synchronous triggering system and a data analysis system. Linear polarization laser is emitted by the polarization laser and is collected by the first convex lens after being expanded by the concave lens, the collected light is collected and collimated by the second convex lens after being collected at the focus of the first convex lens, and parallel light is formed and irradiated to a cavitation area generated by the propeller in the experimental section. Part of the light is diffracted and deviated to form object light when passing through the vacuole, the light which does not pass through the vacuole forms reference light, and alternately dark and bright stripes formed by interference of the object light and parallel light are refracted by the dimmer and the lens and recorded on a digital camera negative film. According to the invention, non-contact measurement of the two-phase flow field of the propeller is realized by using interference recording and diffraction reconstruction.

Description

Technical Field

[0001] The present invention belongs to the field of cavitation flow measurement, and relates to a multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography. Background Art

[0002] Propeller cavitation refers to the process in which, under certain conditions, when the rotation of the propeller causes the local pressure of the liquid-phase medium to be lower than the local saturation vapor pressure, the liquid phase changes into the gas phase. There are various unsteady processes with different time scales during its development process: including the cavity pulsation at the convection scale, the cavity collapse at the microsecond scale, and the turbulent and vortex motions at various scales. In particular, the periodic fracture and shedding of large-scale cavitation clusters will cause drastic changes in the flow field, and then trigger a series of problems such as propeller vibration, noise, and cavitation erosion. To carry out the mechanism analysis of the multi-scale cavitation two-phase flow of the propeller, the currently commonly used cavitation measurement techniques include invasive measurement and non-invasive measurement. Among them, invasive measurement will disturb the two-phase flow field and affect the measurement accuracy; non-invasive measurement is mainly based on optical imaging technology. Although it can avoid interference, traditional methods such as schlieren method can only obtain two-dimensional cavitation distribution information, and there is a limitation of lacking three-dimensional space characteristics. Therefore, there is an urgent need for a three-dimensional imaging method to realize the detailed study of the flow details and spatial field quantities during the multi-scale evolution process of cavitation, so as to deepen the understanding of the propeller cavitation mechanism.

[0003] Laser digital holography technology is an optical testing technology based on the principles of light diffraction and interference, with three-dimensional space characteristics. It uses a laser as the interference light source, and by recording the interference information of the object light and the reference light caused by the movement of the cavitation, including amplitude and phase, a digital holographic image is generated. With the help of the diffraction reconstruction algorithm, the three-dimensional space coordinates and morphological characteristics of the cavitation can be accurately restored. Applying the laser digital holography technology to the measurement of the multi-scale cavitation field of the propeller can dynamically monitor the three-dimensional morphology and spatial information of the cavitation in the two-phase flow field, and quantitatively evaluate the evolution process of the multi-scale cavitation. As an optical measurement method, its non-invasive measurement can measure the three-dimensional characteristics of the target without affecting the measured object, so it has unique advantages in the measurement of the three-dimensional characteristics of micro-scale cavitation. Summary of the Invention

[0004] In order to solve the problems of difficult quantitative characterization and few information dimensions in the multi-scale cavitation evolution process of the propeller by traditional measurement techniques, the purpose of the present invention is to provide a multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography, which uses its interference recording and diffraction reconstruction to realize non-invasive measurement of the propeller two-phase flow field, restore the real-time cavitation morphology and distribution in the flow field, and analyze the unsteady evolution process of the cavitation, and then suppress the cavitation noise of the propeller by controlling the initial cavitation.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] The multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention includes an experimental section, an experimental model, a support rotating shaft, a polarized laser, a concave lens, a first convex lens, a second convex lens, a digital camera, a lens, a neutral density filter, a synchronous trigger system, and a data analysis system.

[0007] The optical path is as follows: The polarized laser emits linearly polarized laser, which is expanded by the concave lens and then converged by the first convex lens. The converged light converges at the focal point of the first convex lens and then crosses and is collimated by the second convex lens to form parallel light, which irradiates the cavitation region generated by the propeller in the experimental section. Part of the light diffracts when passing through the cavitation bubbles and is offset to form the object light, while the light that does not pass through the cavitation bubbles forms the reference light. The bright and dark fringes formed by the interference of the object light and the parallel light are refracted by the neutral density filter and the lens and recorded on the negative of the digital camera.

[0008] The synchronous trigger system is connected to the digital camera and triggers the camera to start collecting interference fringes through a step signal. The digital camera saves the collected images to the data analysis system, and restores the multi-scale cavitation flow condition of the propeller through information enhancement, depth restoration, and three-dimensional reconstruction.

[0009] The polarized laser is used to emit polarized laser with temporal coherence and spatial coherence. To make the light after passing through the lens group be parallel light, the distance between the first convex lens and the second convex lens should satisfy the following condition:

[0010] s = f1 + f2

[0011] Where s is the distance between the first convex lens and the second convex lens, and f1 and f2 are the focal lengths of the first convex lens and the second convex lens.

[0012] Preferably, the distance between the concave lens and the first convex lens is appropriately increased so that the laser is fully expanded by the refraction of the concave lens and then converged, thereby obtaining parallel light with more uniform brightness.

[0013] The propeller is installed on the support rotating shaft in the experimental section, and the propeller plane is perpendicular to the water flow direction in the experimental section. The support rotating shaft fixes the propeller and realizes the rotation of the propeller.

[0014] The lens is installed at the front end of the high-speed camera and is mainly used to adjust the focal length and aperture size. The neutral density filter is installed on the lens to absorb the light intensity of the interference fringes generated by the cavitation bubbles and avoid direct laser irradiation from damaging the digital camera. Preferably, the digital camera is a high-speed camera that records holographic images at a higher frame rate.

[0015] The true size of the pixels recorded by the digital camera should be less than half of the minimum holographic interference fringe spacing to be able to restore the high-frequency signals of the object light. Additionally, the true size of the pixels should not be too small to avoid the distribution of diffracted light being too sparse. To reasonably utilize the pixels to record a larger range under the condition of satisfying the sampling theorem, the distance between the digital camera film and the wall of the experimental section should meet the following conditions:

[0016]

[0017] where z0 is the distance between the digital camera film and the wall of the experimental section, N is the number of pixel points on the hologram, d p is the pixel size, and λ is the laser wavelength.

[0018] The trigger system uses a synchronous data acquisition instrument to control the imaging system to start acquisition by sending a trigger signal.

[0019] The data analysis system uses an integrated holographic image processing method to save the acquired holographic images and perform information enhancement, depth restoration, and three-dimensional reconstruction to restore the multi-scale cavitation flow of the propeller.

[0020] The working method of the multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention includes the following steps:

[0021] Step 1, use the laser digital holographic acquisition system to record the bright and dark interference fringes generated by the object light and the reference light caused by the cavitation diffraction of the propeller at different phases at each moment. The amplitude and phase information of the diffracted light are recorded therein to obtain a multi-scale cavitation two-phase flow hologram, which specifically includes:

[0022] Step 1.1, install the experimental model into the experimental section, adjust the rotational speed of the propeller, adjust the incoming flow velocity and outlet pressure in the experimental section, so that obvious cavitation flow appears in the propeller in the experimental section, and determine the area where obvious cavitation occurs on the propeller as the holographic recording area.

[0023] Step 1.2, install a laser, a concave lens, a first convex lens, and a second convex lens on one side of the experimental section from far to near. The distance between the two convex lenses is the sum of their focal lengths. Adjust the height so that the laser and the lenses are at the same height. The laser generates uniformly bright parallel light after refraction and passes through the previously determined holographic recording area.

[0024] Step 1.3, set up a digital camera equipped with a lens and a neutral density filter on the other side of the experimental section, and adjust it to the same height as the laser optical path, so that the diffraction fringe spot can be completely recorded by the digital camera. Adjust the focal length of the lens to focus on the wall of the experimental section on the same side, and calibrate the magnification of the lens to determine the actual size corresponding to a single pixel. The calculation method is as follows:

[0025] d p22= d p / A 22

[0026] where d p22 is the actual size corresponding to the pixel, and A 22 is the lens magnification factor.

[0027] Step 1.4: Connect the triggering system and the data analysis system to the digital camera. After the cavitation flow of the propeller becomes stable, generate a rising edge signal through the trigger switch to start holographic image acquisition and store the holographic image.

[0028] Step 2: Perform image enhancement preprocessing on the recorded hologram to obtain a holographic enhanced image that more prominently shows the cavitation interference fringes, and avoid the influence on the calculation results of the target to be measured caused by the generation of diffraction fringes under laser irradiation due to the presence of minute particles such as dust or abrasion marks such as scratches on the lens, the lens, or the wall of the experimental section during the shooting process. Specifically, it includes:

[0029] Step 2.1: Calculate the time-averaged image. In the holographic image, the cavitation is in a flowing state and undergoes unsteady processes such as continuous decomposition and collapse, while the background noise composed of particles and scratches does not change with time during the shooting process. The background noise of the holographic image is obtained by time-averaging. The background noise calculation method is as follows:

[0030]

[0031] where (x, y) is the coordinate position of the pixel point, is the pixel brightness of the time-averaged image, M is the number of superimposed images, and f i is the pixel brightness of the i-th image.

[0032] Step 2.2: Use the time-averaged image as the background for noise removal. After noise removal, the holographic image only contains the interference fringes of the cavitation. The calculation method is as follows:

[0033]

[0034] where I k (x, y) is the pixel brightness of the holographic image after background noise reduction.

[0035] Step 2.3: After background noise removal, the pixel point brightness in the holographic image shows severe polarization, making it difficult to identify the interference fringes of the cavitation. Normalize the holographic image after background noise reduction to further highlight the target features. The calculation method is as follows:

[0036]

[0037] where max{I k (x, y)} and min{Ik (x, y) represent the maximum and minimum values of the pixel brightness in the background-removed image, and I H (x, y) is the pixel brightness of the normalized holographic image.

[0038] Step 3: Use the diffraction formula to perform reconstruction in the depth direction of the information-enhanced hologram preprocessed in Step 2 to obtain holographic reconstructed images at different depths, and identify the voids and determine the depth information of the void boundaries by the minimum brightness method. Specifically, it includes:

[0039] Step 3.1: Set the starting point, ending point, and step size of the holographic reconstruction, and use the Rayleigh-Sommerfeld diffraction formula to perform reconstruction in the depth direction. Restore the pixel brightness at different depths through the pixel brightness of the enhanced holographic image to obtain the reconstructed hologram.

[0040]

[0041] Among them, I(x, y, d) is the pixel brightness of the pixel point (x, y) on the reconstruction plane, d is the distance from the reconstruction plane to the recording plane in the optical system, j is the imaginary unit, k = 2π / λ is the wave number, λ is the wavelength of light, r is the vector radius from the point (x0, y0, 0) to the point (x, y, d), and the expression formula is θ is the angle between the vector radius r and the z-axis, and K(θ) = (cosθ + 1) / 2 is the tilt factor.

[0042] Step 3.2: Since the void edges are located at different positions in space, they will be focused at different depth positions during the reconstruction process. If the object is in the focused state, determine that the depth position of the hologram in the focused state is the real position of the void in space. When the void is in the focused state, the light-blocking effect on the light is stronger, and the light intensity at the position where the void is located is the weakest. Use the minimum brightness method to judge the focusing situation of the void in the holographic reconstructed images at different depths, determine the depth of the void, and combine the minimum brightness in the holographic reconstructed images at different depths to obtain a combined hologram, in which all pixel points are in the focused state. The calculation method of the minimum brightness method is as follows:

[0043]

[0044] Among them, I1(x, y) is the pixel brightness of the pixel point (x, y) of the combined holographic image.

[0045] Step 3.3: The brightness of the area where the void is located in the combined hologram is relatively low. Therefore, perform a binarization operation to identify the void, and calculate the average brightness of the combined hologram multiplied by a coefficient as the threshold for judging the void area, so as to separate the background area and the void area. The calculation method is as follows:

[0046]

[0047] Among them, I2(x, y) is the pixel brightness of the pixel point (x, y) in the binary image.

[0048] Step 4: Based on the spherical and ellipsoidal assumptions, use roundness as the judgment criterion to distinguish spherical voids and ellipsoidal voids, solve the size information and position information of both, and perform reconstruction in the three-dimensional space of the experimental section to obtain the three-dimensional information of multi-scale voids, specifically including:

[0049] Step 4.1: Based on the spherical and ellipsoidal assumptions, use roundness as the judgment criterion to determine whether the void is spherical or ellipsoidal. The calculation method is as follows:

[0050]

[0051] Among them, HCF is a dimensionless parameter used as the roundness judgment criterion, P is the perimeter of a circle with the same area as the void projection, and A is the area of the void projection.

[0052] Step 4.2: If it is determined that the void is spherical, calculate the diameter of the spherical void according to the projected area of the void on the combined hologram. The calculation method is as follows:

[0053]

[0054] Among them, d is the diameter of the spherical void.

[0055] Step 4.3: If it is determined that the void is ellipsoidal, obtain the depth of the major axis of the projected ellipsoidal void by depth interpolation of the projected void edge, and obtain the pitch angle α of the ellipsoid and the ellipsoid length a.

[0056] z(P0) = 0.5[z(P1) + z(P2)]

[0057] Among them, P1 and P2 are the intersection points of the straight line perpendicular to the major axis of the projected ellipsoidal void and passing through the point with the edge of the ellipsoidal void, and z is the depth of this point.

[0058] Fit the position of P0 on the major axis of the projected ellipsoidal void with its corresponding depth z(P0) to obtain the pitch angle α of the ellipsoid and the ellipsoid length a.

[0059]

[0060] Among them, l is the length of the major axis of the ellipsoid.

[0061] Step 4.4: According to the three-dimensional data of the void, perform three-dimensional reconstruction on the shape of the void to obtain the three-dimensional spatial diagram of all voids within the window. The three-dimensional data of the void includes the center point coordinates, Euler angles, and three-dimensional dimensions.

[0062] Step 4.5: Project the three-dimensional reconstructed image obtained by fitting onto the x-y plane, and perform error analysis on the fitting effect.

[0063]

[0064] Among them, E is the error metric, N2 is the number of pixel points occupied by the voids in the projected image, and N1 is the number of pixel points occupied by the voids in the image generated by the combined hologram.

[0065] It further includes Step Five: Perform statistical analysis on the diameters and quantities of multi-scale voids based on the void scale information and spatial information obtained in Step Four, analyze the changing trends of various parameters over time, and obtain the specific evolution process of multi-scale voids. Specifically, it includes:

[0066] Step 5.1: Convert the diameter of the three-dimensional reconstructed voids to their actual sizes according to the pixel size and magnification factor.

[0067] Step 5.2: For all voids within the window, use the Sauter mean value to characterize the average bubble diameter of the voids. The calculation method is as follows:

[0068]

[0069] where d i represents the bubble diameter of each void.

[0070] Step 5.3: Select equally spaced bubble diameter ranges, calculate the probability density of the multi-scale void bubble diameters, analyze the distribution law among multi-scale voids, and suppress the cavitation noise of the propeller by controlling the initial formation of voids according to the distribution law among multi-scale voids.

[0071] Beneficial effects:

[0072] 1. The multi-scale void measurement system for propeller two-phase flow based on laser digital holography disclosed in the present invention utilizes the principles of light interference and diffraction to record phase and amplitude information, and reconstructs the multi-scale void image of propeller two-phase flow through the diffraction formula. Compared with traditional intrusive probe measurements, it can achieve high-precision measurement without disturbing the void flow.

[0073] 2. The multi-scale void measurement system for propeller two-phase flow based on laser digital holography disclosed in the present invention determines the measurement parameters based on the optimal sampling distance for recording the void interference fringe hologram by a digital camera, rationally utilizes pixels to record a larger flow area under the condition of satisfying the sampling theorem, and fully utilizes the spatial resolution of the digital camera.

[0074] 3. The multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention uses a holographic image information enhancement method. By using the time-averaging method, it extracts the background noise information that does not change with time in the hologram, and normalizes the pixel brightness of the background-removed hologram, significantly enhancing the contrast of the bright and dark interference fringes of the cavitation, so as to more accurately identify the boundary information of the cavitation.

[0075] 4. The multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention identifies the cavitation position through the minimum brightness method of the cavitation holographic reconstruction diagram. By comparing the brightness of a single pixel point in different depth directions, it judges the focusing state of the cavitation in the holographic reconstruction diagrams in different depths, and combines the minimum brightness pixels in all holographic reconstruction diagrams to generate a combined hologram, ensuring that the pixel points of each cavitation in the diagram are in a focused state, so as to accurately determine the actual depth information of the cavitation.

[0076] 5. The multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention is based on the multi-scale cavitation three-dimensional reconstruction method of the propeller. It distinguishes spherical cavitation or ellipsoidal cavitation through roundness judgment, and combines the cavitation edge position information to obtain the geometric parameters and spatial position parameters of the spherical cavitation and the ellipsoidal cavitation respectively, realizing the accurate reconstruction of the multi-scale cavitation field in the three-dimensional space of the experimental section.

[0077] 6. The multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography disclosed by the present invention uses the interference recording and diffraction reconstruction technology to realize the non-contact measurement of the propeller two-phase flow field, restores the real-time cavitation morphology and distribution of the flow field, and analyzes its distribution law by calculating the probability density of the multi-scale cavitation bubble diameter. Furthermore, based on the recognition result of the cavitation inception critical condition, it suppresses the propeller cavitation noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 is a three-dimensional schematic diagram of the multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography of the present invention.

[0079] Among them, 1 - high-speed camera, 2 - lens, 3 - neutral density filter, 4 - propeller, 5 - propeller shaft, 6 - experimental section, 7 - second convex lens, 8 - first convex lens, 9 - concave lens, 10 - laser.

[0080] Figure 2 is a method flow chart of the multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography of the present invention.

[0081] Figure 3 is the holographic image processing flow of the multi-scale cavitation measurement system for propeller two-phase flow based on laser digital holography of the present invention.

[0082] Among them,Figure 3 (a) is the original hologram, Figure 3 (b) is the background image, Figure 3 (c) is the background-removed hologram, Figure 3 (d) is the normalized hologram, Figure 3 (e) is the combined hologram, Figure 3 (f) is the binary hologram, Figure 3 (g) is the three-dimensional reconstruction diagram. Detailed implementation mode

[0083] The present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0084] Example 1

[0085] As Figure 1 shown, the propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography technology disclosed in this embodiment mainly consists of a high-speed camera 1, a lens 2, a neutral density filter 3, a propeller 4, a propeller shaft 5, an experimental section 6, a second convex lens 7, a first convex lens 8, a concave lens 9, and a laser 10.

[0086] The high-speed camera 1 is a SpeedSence M310, with a CMOS sensor as the recording medium. Its acquisition frequency is set to 4000fps, the exposure time is 30μs, the total acquisition duration is 1s, and the acquisition resolution is 1152*1080.

[0087] The lens 2 is a Nikon 105mm lens.

[0088] The neutral density filter 3 is an OD2 full-wavelength absorption filter, and the optical signal passing rate is 1%.

[0089] The propeller 4 is an HSP large skewed propeller model with a model diameter of 0.25m, supported by the propeller shaft 5 and providing a rotational torque.

[0090] The experimental section 6 is a transparent cuboid structure, 2.6m long, and both the width and height are 0.6m.

[0091] The diameters of the second convex lens 7 and the first convex lens 8 are 150mm and 50mm respectively, and the focal lengths are 200mm and 50mm respectively. The diameter of the concave lens 9 is 100mm, and the focal length is -160mm.

[0092] The laser 10 is a Rayleigh continuous laser with a wavelength of 527nm, and the diameter of the laser spot generated is 3mm.

[0093] The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography disclosed in this embodiment is specifically implemented as follows:

[0094] Step 1: Use a laser digital holographic acquisition system to record multi-scale cavitation two-phase flow holograms at different phases of the propeller at each moment, specifically including:

[0095] Step 1.1: Install the test model into the experimental section, adjust the rotational speed of the propeller 4 to 21 r / s, adjust the incoming flow velocity in the experimental section 6 to 5 m / s, and adjust the outlet pressure to a cavitation number of 2.5, so that obvious cavitation flow appears in the propeller in the experimental section, and determine the area where obvious cavitation occurs in the propeller as the holographic recording area.

[0096] Step 1.2: Install the laser 10, concave lens 9, first convex lens 8, and second convex lens 7 from far to near on one side of the experimental section. The distance between the two convex lenses is 250 mm. Adjust the height of the connecting rod so that the optical elements are at the same height. The laser is refracted to generate parallel light with uniform brightness, which passes through the previously determined holographic recording area.

[0097] Step 1.3: Set up a high-speed camera 1 equipped with the lens 2 and neutral density filter 3 on the other side of the experimental section, adjust it to the same height as the laser optical path, so that the diffraction fringe spot can be completely recorded by the high-speed camera. Adjust the acquisition frame rate of the high-speed camera to 2520 Hz, the acquisition duration to 1 s, the acquisition resolution to 1400×1400, and the exposure time to 15 μs. Adjust the lens focal length to focus on the wall of the experimental section on the same side, calibrate the lens magnification, and the actual size corresponding to a single pixel is 25 μm. Thus, the distance between the camera and the wall of the experimental section can be determined to be 1.66 m according to the optimal sampling distance.

[0098] Step 1.4: Connect the trigger system and data analysis system to the digital camera. After the cavitation flow of the propeller is stable, generate a rising edge signal through the trigger switch, start holographic image acquisition and store the holographic image. The hologram acquired at a certain moment is as Figure 3 (a) shown.

[0099] Step 2: Perform image enhancement preprocessing on the recorded hologram to obtain a holographic enhanced image that more prominently shows the cavitation interference fringes, and avoid the influence on the calculation results of the target to be measured due to the generation of diffraction fringes by the presence of dust and other small particles or scratches and other wear marks on the lens, lens, or the wall of the experimental section during the shooting process. Specifically including:

[0100] Step 2.1: Calculate the time-averaged image. The cavitation in the holographic image is in a flowing state and undergoes non-steady processes such as continuous decomposition and collapse, while the background noise composed of particles and scratches does not change with time during the shooting process. The background noise of the holographic image can be obtained by time averaging. The obtained background noise image is as Figure 3 (b) shown.

[0101] Step 2.2: Use the time-averaged image as the background for noise removal. After denoising, the holographic image only contains the interference fringes of the voids. The background-removed image is as shown in Figure 3 (c).

[0102] Step 2.3: After background noise removal, the pixel brightness in the holographic image is severely polarized, making it difficult to identify the interference fringes of the voids. Normalize the background-denoised holographic image to further highlight the target features. The normalization result is as shown in Figure 3 (d).

[0103] Step 3: Use the diffraction formula to perform void depth-direction reconstruction on the information-enhanced hologram preprocessed in Step 2, obtain holographic reconstruction diagrams at different depths, and identify the voids and determine the depth information of the void boundary by the minimum brightness method. Specifically, it includes:

[0104] Step 3.1: Set the starting point, ending point, and step size of the holographic reconstruction, and use the Rayleigh-Sommerfeld diffraction formula to perform depth-direction reconstruction. Restore the pixel brightness at different depths based on the pixel brightness of the enhanced holographic image to obtain the reconstructed hologram.

[0105] Step 3.2: Since the void edges are located at different positions in space, they will be focused at different depth positions during the reconstruction process. If the object is in the focused state, it can be considered that the depth position where the focused-state hologram is located is the real position of the void in space. When the void is in the focused state, the light-blocking effect on the light is stronger, and the light intensity at the position of the void is the weakest. Use the minimum brightness method to judge the focused state of the void in the holographic reconstruction diagrams at different depths, determine the depth of the void, and combine the minimum brightness in the holographic reconstruction diagrams at different depths to obtain a combined hologram, where all pixel points are in the focused state. The combined hologram is as shown in Figure 3 (e).

[0106] Step 3.3: The brightness of the area where the void is located in the combined hologram is relatively low. Therefore, perform binarization operation to identify the voids. Calculate the average brightness of the combined hologram multiplied by a coefficient as the threshold for judging the void area, so as to separate the background area and the void area. The binarization result is as shown in Figure 3 (f).

[0107] Step 4: Based on the spherical and ellipsoidal assumptions, use roundness as the judgment criterion to distinguish spherical voids and ellipsoidal voids, solve the size information and position information of both, and perform reconstruction in the three-dimensional space of the experimental section to obtain the three-dimensional information of multi-scale voids. Specifically, it includes:

[0108] Step 4.1: Based on the spherical and ellipsoidal assumptions, use roundness as the judgment criterion to judge whether the void is spherical or ellipsoidal.

[0109] Step 4.2: If it is determined that the cavitation bubble is spherical, calculate the diameter of the spherical cavitation bubble based on the projected area of the cavitation bubble on the combined hologram.

[0110] Step 4.3: If it is determined that the cavitation bubble is ellipsoidal, the depth of the major axis of the projected ellipsoidal cavitation bubble can be obtained by depth interpolation of the projected edge of the cavitation bubble, and the pitch angle α of the ellipsoid and the length a of the ellipsoid are obtained.

[0111] Step 4.4: According to the three-dimensional data of the cavitation bubble, perform three-dimensional reconstruction on the shape of the cavitation bubble to obtain the three-dimensional spatial map of all cavitation bubbles within the viewing window. The three-dimensional data of the cavitation bubble includes the center point coordinates, Euler angles, and three-dimensional dimensions. The result of the three-dimensional reconstruction is as Figure 3 (g) shown.

[0112] Step 4.5: Project the three-dimensional reconstruction map obtained by fitting onto the x-y plane, and the error analysis of the fitting effect can be performed.

[0113] It further includes Step Five, perform statistical analysis on the diameters and quantities of multi-scale cavitation bubbles according to the cavitation bubble scale information and spatial information obtained in Step Four, analyze the changing trends of each parameter over time, and obtain the specific evolution process of the multi-scale cavitation bubbles. Specifically, it includes:

[0114] Step 5.1: Convert the diameter of the three-dimensional reconstructed cavitation bubble to its actual size according to the pixel size and magnification factor.

[0115] Step 5.2: For all cavitation bubbles within the viewing window, use the Sauter mean value to characterize the average bubble diameter of the cavitation bubbles.

[0116] Step 5.3: Select equally spaced bubble diameter ranges, calculate the probability density of the multi-scale cavitation bubble diameters, and analyze the distribution law among the multi-scale cavitation bubbles.

[0117] This embodiment can realize the quantitative monitoring and calculation of the three-dimensional structure of the multi-scale cavitation bubbles of the propeller, be used for the real-time monitoring and related statistical analysis of the morphological changes and three-dimensional spatial positions of the multi-scale cavitation bubbles of the propeller under the cavitation state, provide an important basis for the evolution process and related research of the multi-scale cavitation bubbles of the propeller, and analyze its distribution law by calculating the probability density of the multi-scale cavitation bubble diameters, providing data support for suppressing the cavitation noise of the propeller based on the critical conditions of cavitation inception.

[0118] The above specific description further elaborates on the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography, characterized by: It includes an experimental section, an experimental model, a supporting shaft, a polarized laser, a concave lens, a first convex lens, a second convex lens, a digital camera, a lens, a neutral density film, a synchronous trigger system and a data analysis system; The polarized laser emits a linearly polarized laser, which is expanded by a concave lens and then converged by a first convex lens. The converged light converges at the focus of the first convex lens and then crosses and collimates through a second convex lens to form parallel light that irradiates the cavitation area generated by the propeller in the experimental section. Part of the light diffracts when passing through the cavitation bubble and is offset to form object light, while the light that does not pass through the cavitation bubble forms reference light. The light and parallel light interfere with each other to form light and dark stripes that are refracted by a neutral density filter and a lens and recorded on the film of a digital camera. The synchronous trigger system is connected to the digital camera, and the camera is triggered by a step signal to start the interference fringe acquisition. The digital camera saves the acquired image to the data analysis system, and restores the multi-scale cavitation flow of the propeller through information enhancement, depth restoration, and three-dimensional reconstruction. The polarized laser is used to emit polarized laser light with temporal coherence and spatial coherence, so that the light of the laser light after passing through the lens group is parallel light.

2. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 1, characterized in that: The distance between the first convex lens and the second convex lens satisfies the following conditions: s=f1+f2 Where s is the distance between the first convex lens and the second convex lens, and f1 and f2 are the focal lengths of the first convex lens and the second convex lens.

3. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography as claimed in claim 2, characterized in that: The concave lens and the first convex lens are appropriately spaced apart so that the laser light can be fully expanded by refraction through the concave lens and then converged, thereby obtaining parallel light with more uniform brightness.

4. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography as claimed in claim 2, characterized in that: The propeller is installed on a supporting shaft in the experimental section, and the propeller plane is perpendicular to the water flow direction in the experimental section; the supporting shaft fixes the propeller and realizes the rotation of the propeller.

5. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 2, characterized in that: The lens is installed at the front end of the high-speed camera and is mainly used to adjust the focal length and aperture size. The dimming film is installed on the lens to absorb the interference fringe light intensity generated by cavitation to prevent direct laser irradiation from damaging the digital camera.

6. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 5, characterized in that: The digital camera is a high-speed camera that records holographic images at a higher frame rate; The real pixel size recorded by the digital camera should be less than half of the minimum holographic interference fringe spacing to restore the high-frequency signal of the object light. In addition, the real pixel size should not be too small to avoid too sparse distribution of diffracted light. The distance between the digital camera film and the wall of the experimental section should meet the following conditions: Where z0 is the distance between the digital camera film and the wall of the experimental section, N is the number of pixels on the hologram, and d p is the pixel size, and λ is the laser wavelength.

7. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 1, characterized in that: The trigger system uses a synchronous data acquisition instrument to control the imaging system to start acquisition by sending a trigger signal. The data analysis system adopts a holographic image processing integration method to save the collected holographic images, and perform information enhancement, depth restoration, and three-dimensional reconstruction to restore the multi-scale cavitation flow conditions of the propeller.

8. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 1, 2, 3, 4, 5, 6 or 7, characterized in that: The working method includes the following steps: Step 1: Use a laser digital holographic acquisition system to record the bright and dark interference fringes produced by the object light and the reference light generated by the cavitation diffraction at different phases of the propeller at each moment, which records the amplitude and phase information of the diffracted light, and obtain a multi-scale cavitation two-phase flow hologram; Step 2: Perform image enhancement preprocessing on the recorded hologram to obtain a holographic enhanced image that highlights the cavitation interference fringes, so as to avoid the generation of diffraction fringes under laser irradiation due to the presence of tiny particles such as dust or scratches on the lens, lens or experimental section wall during the shooting process, which will affect the calculation results of the target to be measured; Step 3, reconstructing the information enhanced hologram preprocessed in step 2 in the cavitation depth direction using the diffraction formula, obtaining holographic reconstruction images at different depths, identifying cavitation using the minimum brightness method and determining the depth information of the cavitation boundary; Step 4: Based on the spherical and ellipsoidal assumptions, the roundness is used as the judgment standard to distinguish between spherical cavitation and ellipsoidal cavitation, and the size and position information of the two are solved, and reconstructed in the three-dimensional space of the experimental section to obtain the three-dimensional information of multi-scale cavitation.

9. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 8, characterized in that: Step 1 is implemented as follows: Step 1.1, installing the experimental model in the experimental section, adjusting the propeller speed, the incoming flow velocity and the outlet pressure in the experimental section, so that the propeller in the experimental section has obvious cavitation flow, and determining that the place where the propeller produces obvious cavitation is the holographic recording area; Step 1.2, install the laser, concave lens, first convex lens, and second convex lens on one side of the experimental section from far to near. The distance between the two convex lenses is the sum of their focal lengths. Adjust the height so that the laser and the lens are at the same height. After refraction, the laser generates parallel light with uniform brightness and passes through the previously determined holographic recording area. Step 1.3: Build a digital camera equipped with a lens and a neutral density filter on the other side of the experimental section and adjust it to the same height as the laser light path so that the diffraction fringe spot can be completely recorded by the digital camera. Adjust the lens focal length to focus on the wall of the experimental section on the same side and calibrate the lens magnification to determine the actual size corresponding to a single pixel. The calculation method is as follows: d p22 =d p / A 22 where d p22 is the actual size corresponding to the pixel, A 22 is the lens magnification; Step 1.4, connect the trigger system and the data analysis system to the digital camera, and after the propeller cavitation flow is stable, generate a rising edge signal through the trigger switch to start holographic image acquisition and store the holographic image; Step 2 is implemented as follows: Step 2.1, calculate the time average image. The cavitation in the holographic image is in a flowing state and undergoes unsteady processes such as continuous decomposition and collapse. The background noise composed of particles and scratches does not change with time during the shooting process. The background noise of the holographic image is obtained by time averaging. The background noise calculation method is as follows: Among them, (x, y) is the pixel coordinate position, is the pixel brightness of the time-averaged image, M is the number of superimposed images, and f i is the pixel brightness of the i-th image; Step 2.2: Use the time-averaged image as the background to remove noise. The denoised holographic image only contains the interference fringes of the cavitation bubble. The calculation method is as follows: Among them, I k (x, y) is the pixel brightness of the holographic image after background noise reduction; Step 2.3: After background noise is removed, the brightness of pixels in the holographic image is severely differentiated into two levels, and it is difficult to identify the interference fringes of the cavitation bubble. The background noise-reduced holographic image is normalized to further highlight the target features. The calculation method is as follows: Among them, max{I k (x,y)} and min{I k (x, y)} represent the maximum and minimum pixel brightness in the background-removed image, respectively. H (x, y) is the pixel brightness of the normalized holographic image; Step 3 is implemented as follows: Step 3.1, set the starting point, end point and step size of the holographic reconstruction, and use the Rayleigh-Sommerfeld diffraction formula to reconstruct the depth direction, and restore the pixel brightness at different depths by enhancing the hologram pixel brightness to obtain the reconstructed hologram; Where I(x, y, d) is the pixel brightness of the reconstructed plane pixel point (x, y), d is the distance from the reconstruction plane to the recording plane in the optical system, j is the imaginary unit, k = 2π / λ is the wave number, λ is the wavelength of light, r is the radius vector from point (x0, y0, 0) to point (x, y, d), and the expression formula is θ is the angle between the radius vector r and the z-axis, and K(θ)=(cosθ+1) / 2 is the tilt factor; Step 3.2, since the edge of the cavitation bubble is located at different positions in space, it will be focused at different depths during the reconstruction process. If the object is in a focused state, the depth position of the hologram in the focused state is determined to be the real position of the cavitation bubble in space; when the cavitation bubble is in a focused state, the light blocking effect is stronger, and the light intensity at the position of the cavitation bubble is the weakest. The minimum brightness method is used to judge the focusing condition of the cavitation bubble in the holographic reconstruction image at different depths, determine the depth of the cavitation bubble, and combine the minimum brightness in the holographic reconstruction images at different depths to obtain a combined hologram. All pixels in the image are in a focused state; the calculation method of the minimum brightness method is as follows: Wherein, I1(x, y) is the pixel brightness of the combined hologram pixel (x, y); Step 3.3: The brightness of the area where the cavitation is located in the combined hologram is low, so a binarization operation is performed to identify the cavitation. The average brightness of the combined hologram is calculated and multiplied by a coefficient as the threshold for judging the cavitation area, thereby separating the background area from the cavitation area. The calculation method is as follows: Where I2(x, y) is the pixel brightness of the binary image pixel (x, y); Step 4 is implemented as follows: Step 4.1, based on the spherical and ellipsoidal assumptions, use roundness as a criterion to determine whether the cavitation bubble is spherical or ellipsoidal; the calculation method is as follows: Among them, HCF is a dimensionless parameter used as a criterion for roundness, P is the circumference of a circle with the same area as the cavitation projection, and A is the area of ​​the cavitation projection; Step 4.2: If the cavitation bubble is judged to be spherical, the diameter of the spherical cavitation bubble is calculated according to the projection area of ​​the cavitation bubble on the combined hologram; the calculation method is as follows: Where d is the diameter of the spherical cavitation bubble; Step 4.3, if the cavitation is determined to be an ellipsoid, the depth of the major axis of the projected ellipsoid cavitation is obtained by interpolating the depth of the projected cavitation edge, and the pitch angle α and the length a of the ellipsoid are obtained; Among them, P1 and P2 are the intersection points of the straight line perpendicular to the long axis of the projected ellipsoid cavitation and passing through the point and the edge of the ellipsoid cavitation, and z is the depth of the point; The position of P0 on the long axis of the projected ellipsoid cavitation bubble is fitted with its corresponding depth z(P0) to obtain the pitch angle α and length a of the ellipsoid; Where l is the length of the major axis of the ellipsoid; Step 4.4, reconstructing the shape of the cavitation bubble in three dimensions according to the cavitation bubble three-dimensional data to obtain a spatial three-dimensional image of all cavitation bubbles in the viewing window; the cavitation bubble three-dimensional data includes center point coordinates, Euler angles and three-dimensional dimensions; Step 4.5, project the three-dimensional reconstruction image obtained by fitting onto the xy plane, and perform error analysis on the fitting effect; Among them, E is the error metric, N2 is the number of pixels occupied by cavitation in the projection image, and N1 is the number of pixels occupied by cavitation in the image generated by the combined hologram.

10. The propeller two-phase flow multi-scale cavitation measurement system based on laser digital holography according to claim 8 or 9, characterized in that: The method further includes step 5, performing statistical analysis on the diameter and quantity of multi-scale cavitation according to the cavitation scale information and spatial information obtained in step 4, analyzing the variation trend of each parameter over time, and obtaining the specific evolution process of multi-scale cavitation; the specific implementation method is: Step 5.1, converting the three-dimensional reconstructed cavitation diameter to its actual size according to the pixel size and magnification; Step 5.2: For all cavitation bubbles in the window, the average diameter of the cavitation bubbles is characterized by the Sauter average value; the calculation method is as follows: Among them, d i Indicates the diameter of each cavitation bubble; Step 5.3, select an equally spaced bubble diameter range, calculate the probability density of the multi-scale cavitation bubble diameter, analyze the distribution law between the multi-scale cavitation bubbles, and control the cavitation initiation according to the distribution law between the multi-scale cavitation bubbles to suppress the cavitation noise of the propeller.

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