A cavitation morphology acquisition system for a model entering water with an adjustable entry angle

Abstract

The present invention discloses a cavitation morphology acquisition system during a model entry process with an adjustable entry angle, and belongs to the technical field of cavitation dynamics. The system includes a launch system, an observation system, and an image acquisition and processing system; the launch system launches the model into the observation system at a high speed in the form of a variable angle, and after the model is ejected from the launch system, the launch system can block the high-speed airflow that provides the launch power from affecting the free liquid surface; the observation system provides a test environment for the model to enter the water and synchronizes the launch system, recording the entire launch process of the model by video; the image acquisition and processing system processes the image acquired by the observation system based on Matlab software and acquires the falling speed, acceleration, and cavitation volume of the model in the observation system. The present invention can realize the observation and analysis of the water entry cavitation formed during the entry of models of different scales at different angles and different water entry speeds.

Description

Technical Field

[0001] The invention belongs to the technical field of cavitation dynamics, and in particular relates to an experimental device for collecting, measuring and analyzing cavitation morphology during a model entering water. Background Art

[0002] The water entry process refers to the process in which a model (rotating body or sphere) enters water from the air through the gas-liquid interface at a certain initial velocity. Starting from the moment the model touches the free liquid surface, the free liquid surface is disturbed by the model and propagates around until the model enters the water and moves stably to the bottom in the water. The model entry process is a short transient process, but this process involves high impact loads caused by medium mutations, multi-scale vortices caused by high-speed motion, and phase changes of water. Therefore, the water entry process largely determines whether the model can stably enter water from the air and the model's entry posture. The water entry process has a wide range of applications in various fields. In the engineering field, the study of the model entry process is of great significance for solving the entry of air-dropped torpedoes, anti-submarine missiles, and the recovery of spacecraft. During the water entry process, when the model touches the free liquid surface, complex physical phenomena occur: the interaction between air, water, and the model, the interaction between the free liquid surface and cavitation, and the instantaneous increase in the impact load of the model impacting the free liquid surface.

[0003] At present, theoretical research on water entry problems mainly focuses on using numerical simulation methods to analyze the cavitation formed during the model's entry into water. However, since the process of entering water is accompanied by a large number of complex flow phenomena such as turbulence, phase change, compressibility, medium mutation, instantaneous extreme loads, and moving contact lines, and involves the interaction between the free liquid surface and the cavitation, and the interaction between the cavitation and the model, the prediction of the model's motion posture is extremely complex. In engineering, the effects of viscosity and compressibility are usually ignored and approximate calculations are used. In addition, the identification and capture of the gas-liquid interface are highly dependent on the accuracy and number of grids, resulting in a large discrepancy between the cavitation contour and the model's motion posture and the actual process. Therefore, experimental observation of the model entering water is particularly important. Summary of the invention

[0004] In view of this, the present invention provides a cavitation morphology acquisition system for a model entering water with an adjustable water entry angle, which can observe and analyze the water entry cavitations formed when models of different scales enter water at different angles and different water entry speeds.

[0005] A cavitation morphology acquisition system for a model entering water with an adjustable water entry angle, the system comprising a launch system, an observation system and an image acquisition and processing system;

[0006] The launching system launches the model into the observation system at a high speed in a variable angle. After the model is ejected from the launching system, the launching system can block the high-speed airflow providing the launching power from affecting the free liquid surface.

[0007] The observation system provides a test environment for the model to enter the water and synchronizes with the launch system to record the entire launch process of the model by video;

[0008] The image acquisition and processing system processes the images acquired by the observation system based on Matlab software and acquires the falling speed, acceleration and cavitation volume of the model in the observation system.

[0009] Furthermore, the launch system includes a bracket, a support plate, a compressor, a safety valve, a solenoid valve, a launch tube and a wake blocker;

[0010] The bracket and the compressor are fixedly installed on the test bench; the side frame of the bracket is provided with adjustment holes of different angles; one end of the support plate is movably installed on the bracket, and the other end is limited by a fixed shaft arranged in the adjustment hole of the bracket; the launch tube is installed on the support plate, and the water entry angle is adjusted by changing the angle between the support plate and the free liquid surface; the wake blocker is coaxially installed at the front end of the launch tube; when the model passes through the wake blocker, the wake blocker uses the pressure difference to block the high-speed airflow ejected from the launch tube; the compressor is connected to the solenoid valve through the air pipe, and the solenoid valve is connected to the safety valve and the launch tube in turn.

[0011] Furthermore, the wake blocker is a soft film tube with a notch, and the notch runs through the front and rear ends of the soft film tube.

[0012] Furthermore, a pressure diaphragm is installed inside the launch tube to limit the movement of the model under the action of gravity during tilted launch.

[0013] Furthermore, the main structure of the pressure diaphragm is a disk with a central hole, and four ribs are evenly distributed along the circumference of the central hole on one side of the disk. The ribs are perpendicular to the plane of the disk, and the ends of the ribs are bent inward; the diameter of the circle where the four ribs are located is consistent with the aperture size of the launch tube.

[0014] Furthermore, the observation system includes an experimental water tank, a light source, a high-speed camera and a synchronization device; the experimental water tank is located obliquely below the transmitting tube, the light source and the high-speed camera are located on both sides of the water tank, and the synchronization device is used to synchronously trigger the high-speed camera 9 and the solenoid valve.

[0015] Furthermore, the steps of processing the image by the image acquisition and processing system are as follows:

[0016] Step 1: crop the original image to obtain the flow field image around the cavitation bubble and save it as an image;

[0017] Step 2: Process the image in grayscale; calculate the grayscale gradient and edge direction of the image; calculate the grayscale gradient of the image by Sobel operator to obtain the edge strength, and use a 3×3 convolution matrix to calculate the grayscale gradient values ​​in the horizontal and vertical directions, using G x , G y Indicates that the calculation formula of the gray gradient value G of each pixel is:

[0018]

[0019] Use θ to describe the direction of the gradient: θ = a × tan2 (G x , G y ), where a is a constant, and the gray value of each point in the image is replaced by the gray gradient value G;

[0020] Step 3: Binarize the image to extract the morphology of the cavitation bubble; perform double threshold determination: select two appropriate thresholds to define the upper and lower limits, set the pixels with grayscale values ​​greater than the upper threshold to white (255), and set the pixels with grayscale values ​​less than or equal to the lower threshold to black (0), and obtain the outline of the cavitation bubble;

[0021] Step 4: Calculate the model's falling velocity, acceleration and cavitation volume based on the contour of the cavitation bubble.

[0022] Furthermore, the process of calculating the falling velocity and acceleration of the model in step 4 is as follows: the extracted cavitation contour is placed in the Cartesian coordinate system to measure the position of the model, and after the displacement and time data of the model are clarified, a displacement and time relationship curve is drawn, and a high-order polynomial fitting is performed on the curve to obtain a functional relationship between displacement and time, ensuring that the correlation index R 2 =0.99, and then take the first and second order derivatives of the function to obtain the velocity and acceleration of the sphere.

[0023] Furthermore, the process of calculating the model cavitation volume in step 4 is as follows: measure the characteristic length d of the model in the figure and the actual characteristic degree D of the model to obtain the proportional relationship Further measurement of the maximum diameter of the cavitation bubble in the figure c According to the proportional relationship between the actual maximum diameter D of the cavitation bubble and the cavitation bubble length hc, c With height H c , the calculation formula is:

[0024]

[0025]

[0026] Further, the actual volume V of the water-entering cavitation bubble is obtained. c for:

[0027]

[0028] Where V m is the actual volume of the model.

[0029] Furthermore, the launch tube is connected to the safety valve via a flange, and a sealing gasket is installed at the connection portion of the flange.

[0030] Furthermore, a steel plate and a foam plate are sequentially installed on the wall surface of the water tank perpendicular to the launching direction and the bottom of the water tank, and the wall surface of the water tank parallel to the launching direction is made of an acrylic plate.

[0031] Beneficial effects:

[0032] 1. The cavitation morphology acquisition system of the present invention is composed of a launch system, an observation system, and an image acquisition and processing system. The launch system launches the model into the observation system at a high speed in a variable angle. After the model is ejected from the launch system, the launch system can block the high-speed airflow that provides the launch power from affecting the free liquid surface; it can realize the experimental observation of the process of launching models of different scales into water at different angles and speeds, extract the evolution process of the cavitation morphology when entering water, and the effect of the impact load on the model. During the experiment, the electromagnetic valve and the high-speed camera are controlled to open synchronously through the program and the automatic control device to achieve precise control of the model's entry into water, which provides a strong basis for analyzing the entry into water problem.

[0033] 2. A wake blocker is installed at the outlet of the launch tube in the launch system of the present invention. When the model passes through, under the action of high-speed airflow, the pressure between the wake blockers is lower than the pressure on the outside. The wake blockers are tightly attached together under the action of the pressure difference, blocking the high-speed airflow and ensuring that the free liquid surface is not destroyed. The adverse effects of aerodynamic launch disturbing the free liquid surface can be effectively avoided, and the analysis accuracy of the free liquid surface evolution process when the model enters the water is improved.

[0034] 4. A pressure diaphragm is installed inside the launch tube of the launch system of the present invention to limit the movement of the model under the action of gravity during tilted launch. The main structure of the pressure diaphragm is a disk with a central hole. Four ribs are evenly distributed along the circumference of the central hole on one side of the disk. The ribs are perpendicular to the plane of the disk, and the ends of the ribs are bent inwards. The diameter of the circle where the four ribs are located is consistent with the aperture size of the launch tube. The above structure enables the model to always remain in a relatively fixed position during the angle change of the launch tube. When the airflow drives the model to launch, the pressure diaphragm will not generate a large resistance to affect the launch of the model.

[0035] 5. The image acquisition and processing system of the present invention can process the experimental images and automatically extract the cavitation contour. The image acquisition and processing system can finally extract the contour, position and volume of the cavitation through grayscale processing, double threshold determination, binarization processing and boundary recognition. Combined with the time calibration of the synchronous instrument, it can obtain important information such as the speed and acceleration of the model movement.

[0036] 6. The launch tube of the present invention is connected to the safety valve through a flange, and the launch of models of different scales can be achieved by replacing the caliber of the launch tube. A sealing gasket is installed at the flange to ensure the sealing of the device.

[0037] 7. The water tank of the present invention is provided with steel plates and foam plates on the wall perpendicular to the launching direction and the bottom of the water tank, and the wall parallel to the launching direction is made of acrylic plate. The steel plates and foam plates can prevent the model from hitting the wall of the water tank, causing damage to the water tank or splashing the model to cause danger to the experimenter. At the same time, by installing a foam plate at the bottom of the water tank, the model can fall into it, so as to collect the model. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the composition of the cavitation morphology collection system of the present invention;

[0039] Figure 2 The schematic diagram of the launch system structure is as follows;

[0040] Figure 3 This is a schematic diagram of the connection between the wake blocker and the launch tube;

[0041] Figure 4 It is a schematic diagram of the wake blocker structure;

[0042] Figure 5 This is a schematic diagram of the working principle of the wake blocker;

[0043] Figure 6 This is a schematic diagram of the position of the model and the pressure diaphragm in the launch tube;

[0044] Figure 7 It is a structural schematic diagram of the pressure diaphragm;

[0045] Figure 8 This is a comparison chart of the effects before and after the wake blocker is used;

[0046] Fig. 9 This is the cavitation outline after the water-entering cavitation image is processed.

[0047] Among them: 1-air pipe, 2-solenoid valve fixing seat, 3-solenoid valve, 4-safety valve, 5-launching tube, 6-launching tube fixing seat, 7-wake blocker, 8-computer, 9-high-speed camera, 10-water tank, 11-light source, 12-compressor, 13-bracket, 14-support plate, 15-fixed shaft, 16-adjustment hole. DETAILED DESCRIPTION

[0048] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0049] As attached Figure 1 As shown, the present invention provides a system for collecting cavitation morphology during a model entry into water with an adjustable entry angle. The system includes a launching system, an observation system, and an image acquisition and processing system.

[0050] As attached Figure 2 As shown, the launch system includes a bracket 13, a support plate 14, a compressor 12, a safety valve 4, a solenoid valve 3, a launch tube 5 and a wake blocker 7.

[0051] The bracket 13 and the compressor 12 are fixedly installed on the test bench. The compressor provides high-pressure gas as the power to drive the model to move at high speed. A pressure gauge and a pressure regulating valve are installed on the side of the compressor 12 to monitor and control the outlet pressure. The compressor 12 delivers high-pressure gas to the solenoid valve 3 through the air pipe 1. The safety valve 4 and the launch tube 5 are connected by a flange to facilitate the replacement of the launch tube. A position pressure diaphragm for fixing the model is installed inside the launch tube. The solenoid valve 3 and the solenoid valve fixing seat 2 are connected by a flange, and a sealing rubber sheet is installed between the solenoid valve 3 and the solenoid valve fixing seat 2 to play a buffering and sealing role. The solenoid valve fixing seat 2 and the support plate 14 are fixed by bolts. The solenoid valve 3 and the safety valve 4 are connected by a flange, and a sealing gasket is installed between the flanges. The launch tube fixing seat 6 and the support plate 14 are also connected by bolts, and a rubber sheet is padded between the launch tube 5 and the launch tube fixing seat 6 to play a buffering role. Before the model is launched, the compressor 12 is started to increase the pressure to a certain level, and then the pressure regulating valve is adjusted to make the outflowing high-pressure gas reach the rated pressure. Then, the solenoid valve 3 is opened through the synchronization device and the high-speed camera in the observation system is triggered synchronously to shoot. Under the action of the high-pressure gas, the pressure diaphragm is deformed and the model is released. The expansion of the high-pressure gas pushes the model to accelerate to the specified launch speed and enter the water, completing the model entry process.

[0052] The side frame of the bracket 13 is provided with adjustment holes 16 of different angles. One end of the support plate 14 is movably mounted on the bracket 13 through a bearing, and the other end is limited by a fixed shaft 15 set in the bracket adjustment hole. At the same time, the fixed shaft 15 supports the support plate 14, and the two ends of the fixed shaft 15 are fixed by nuts. The angle of the model entering the water is adjusted by changing the angle between the support plate 14 and the free liquid surface.

[0053] As attached Figure 3 As shown, the wake blocker 7 is coaxially installed at the front end of the launch tube 5. When the model passes through the wake blocker 7, the wake blocker uses the pressure difference to block the high-speed airflow ejected from the launch tube.

[0054] As attached Figure 4 As shown, the wake blocker is a soft film tube with a notch of equal width, and the notch runs through the front and rear ends of the soft film tube. When the high-speed moving model is ejected from the launch tube 5, the high-speed airflow will directly act on the free liquid surface of the water body, causing a great impact on the model's entry into the water. According to the Bernoulli equation, after the wake blocker 7 is installed, when the model passes through, under the action of the high-speed airflow, the pressure between the wake blockers is less than the outside pressure, and the soft film tubes are tightly attached together under the action of the pressure difference, blocking the high-speed airflow and ensuring that the free liquid surface is not destroyed. The specific principle is as follows Figure 5 shown.

[0055] As attached Figure 6 and 7 As shown, the main structure of the pressure diaphragm is a disk with a central hole. Four ribs are evenly distributed along the circumference of the central hole on one side of the disk. The ribs are perpendicular to the plane of the disk, and the ends of the ribs are bent inward. The diameter of the circle where the four ribs are located is consistent with the aperture size of the launch tube.

[0056] The observation system includes a water tank 10, a light source 11, a high-speed camera 9 and a synchronization device; the experimental water tank is located obliquely below the launch tube, the light source 11 and the high-speed camera 9 are located on both sides of the water tank, and the synchronization device is used to synchronously trigger the high-speed camera 9 and the solenoid valve. The water tank 10 is sequentially installed with steel plates and foam plates on the wall perpendicular to the launch direction to prevent the model from hitting the water tank wall to damage the water tank or causing the model to splash and cause danger to the experimenters. At the same time, the foam plate is installed to facilitate the collection of the model. A soil buffer layer and a protective net are laid at the bottom of the water tank. The wall of the water tank 10 parallel to the launch direction is made of acrylic plate, and the parallel light source increases the brightness to achieve a better observation effect.

[0057] The following is an example of a specific implementation method using a model with a cylinder volume of 25L, a launch tube length of 0.4m, a launch speed of 60m / s, a diameter of 20mm, and a mass of 32.88g:

[0058] Step 1: Determine the model launch angle, taking 30° as an example, rotate the support plate to this angle, and then insert the support shaft to fix the position of the support plate and the launch device.

[0059] Step 2: Use a compressor to increase the pressure according to the speed-pressure relationship. First, determine the pressure required for the model to be launched. The relationship between the model speed and pressure is:

[0060]

[0061] Where v is the velocity at the model outlet, P1 is the pressure required by the compressor, is the model mass correction coefficient, V is the volume of the compressor cylinder, M dan is the model mass, L is the launch tube length, A is the cross-sectional area of ​​the launch tube which is determined by the model size, P0 is the atmospheric pressure, and k is the adiabatic index.

[0062] The calculated pressure required for the compressor is 665294Pa.

[0063] When the internal pressure of the high-pressure cylinder reaches the calculated pressure, the inflation stops.

[0064] Step three: Control the solenoid valve and high-speed camera through the program to capture the entire process.

[0065] Step 4: extract the original image obtained by the high-speed camera and obtain the focus area.

[0066] Step 5: Process the image in grayscale. Calculate the image grayscale gradient and edge direction; calculate the image grayscale gradient using the Sobel operator to obtain edge strength, and use a 3×3 convolution matrix to calculate the grayscale gradient values ​​in the horizontal and vertical directions, using G x , G y The calculation formula of the gray gradient value G of each pixel is:

[0067]

[0068] Use θ to describe the direction of the gradient: θ = a × tan2 (G x , G y ), where a is a constant, and the grayscale value of each point in the image is replaced by the grayscale gradient value G.

[0069] Step 6: Binarize the image to extract the morphology of the cavitation bubble. Perform double threshold determination: Determine two grayscale thresholds to define the upper and lower limits of the image grayscale value, set the pixels with grayscale values ​​greater than the upper threshold to white (255), and set the pixels with grayscale values ​​less than or equal to the lower threshold to black (0) to obtain the cavitation bubble outline.

[0070] Step 7: Calculate the model's falling velocity, acceleration, and cavitation volume. Place the extracted cavitation contour in the Cartesian coordinate system to measure the model's position. After obtaining the model's displacement and time data, perform a high-order polynomial fit on the data to ensure that the correlation index R 2 =0.99, and then the first and second order derivatives of the fitting curve are taken to obtain the velocity and acceleration of the sphere.

[0071] To launch models of different sizes, it is only necessary to replace the launch tube according to the size of the model.

[0072] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A system for collecting cavitation morphology during a model's entry into water with an adjustable entry angle, characterized in that: The system includes a launch system, an observation system, and an image acquisition and processing system; The launching system launches the model into the observation system at a high speed in a variable angle. After the model is ejected from the launching system, the launching system can block the high-speed airflow providing the launching power from affecting the free liquid surface. The observation system provides a test environment for the model to enter the water and synchronizes with the launch system to record the entire launch process of the model by video; The image acquisition and processing system processes the images acquired by the observation system based on Matlab software and obtains the falling speed, acceleration and cavitation volume of the model in the observation system; The launch system includes a bracket, a support plate, a compressor, a safety valve, a solenoid valve, a launch tube and a wake blocker; The bracket and the compressor are fixedly mounted on the test bench; the side frame of the bracket is provided with adjustment holes of different angles; one end of the support plate is movably mounted on the bracket, and the other end is limited by a fixed shaft arranged in the adjustment hole of the bracket; the launch tube is mounted on the support plate, and the water entry angle is adjusted by changing the angle between the support plate and the free liquid surface; the wake blocker is coaxially mounted on the front end of the launch tube; when the model passes through the wake blocker, the wake blocker uses the pressure difference to block the high-speed airflow ejected from the launch tube; the compressor is connected to the solenoid valve through the air pipe, and the solenoid valve is connected to the safety valve and the launch tube in turn; The wake blocker is a soft film tube with a notch, and the notch runs through the front and rear ends of the soft film tube.

2. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 1, characterized in that: A pressure diaphragm is arranged inside the launch tube to limit the movement of the model under the action of gravity during tilted launch.

3. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 2, characterized in that: The main structure of the pressure diaphragm is a disc with a central hole. Four ribs are evenly distributed along the circumference of the central hole on one side of the disc. The ribs are perpendicular to the plane of the disc, and the ends of the ribs are bent inwards. The diameter of the circle where the four ribs are located is consistent with the aperture size of the launch tube.

4. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 1, characterized in that: The observation system includes an experimental water tank, a light source, a high-speed camera and a synchronization device; the experimental water tank is located obliquely below the transmitting tube, the light source and the high-speed camera are located on both sides of the water tank, and the synchronization device is used to synchronously trigger the high-speed camera and the electromagnetic valve.

5. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 1, characterized in that: The steps of image acquisition and processing system for processing images are as follows: Step 1: crop the original image to obtain the flow field image around the cavitation bubble and save it as an image; Step 2: Process the image in grayscale; calculate the grayscale gradient size and edge direction of the image; calculate the grayscale gradient size of the image through the Sobel operator to obtain the edge strength, and use the 3×3 convolution matrix to calculate the grayscale gradient value in the horizontal and vertical directions, respectively. G x , G y Indicates the grayscale gradient value of each pixel G The calculation formula is: Use θ to describe the direction of the gradient: ,in a is a constant, and the grayscale value of each point in the image is composed of the grayscale gradient value G replaced by; Step 3: Binarize the image to extract the morphology of the cavitation bubble; Perform double threshold determination: select two appropriate thresholds to define the upper and lower limits, set pixels with grayscale values ​​greater than the upper threshold to white (255), and pixels with grayscale values ​​less than or equal to the lower threshold to black (0), and obtain the outline of the cavitation bubble; Step 4: Calculate the model's falling velocity, acceleration and cavitation volume based on the contour of the cavitation bubble.

6. The cavitation morphology collection system during the model entry into water with adjustable entry angle as claimed in claim 5, characterized in that: The process of calculating the falling velocity and acceleration of the model in step 4 is as follows: the extracted cavitation contour is placed in the Cartesian coordinate system to measure the position of the model, and after the displacement and time data of the model are clarified, a displacement and time relationship curve is drawn, and a high-order polynomial fitting is performed on the curve to obtain a functional relationship between displacement and time, ensuring that the correlation index R 2 =0.99, and then take the first and second order derivatives of the function to obtain the velocity and acceleration of the sphere.

7. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 6, characterized in that: The process of calculating the volume of the model cavitation bubble in step 4 is as follows: measure the characteristic length of the model in the figure d The actual feature level of the model D , and obtain the proportional relationship , and further measured the maximum diameter of the cavitation bubble in the figure d c and cavitation length hc , the actual maximum diameter of the cavitation bubble is calculated based on the proportional relationship D c With height H c , the calculation formula is: Further obtain the actual volume of the water-entering cavitation bubble for: In the formula is the actual volume of the model.

8. The cavitation morphology collection system during the model entry process with adjustable entry angle as claimed in claim 4, characterized in that: The water tank has a wall perpendicular to the launching direction and a bottom thereof with a steel plate and a foam plate installed in sequence, and the wall parallel to the launching direction is made of an acrylic plate.

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