Method and device for observing internal structure of cavitation bubble group based on plane wave ultrasonic imaging

Abstract

The invention discloses a cavitation bubble group internal structure observation method and device based on plane waves in the technical field of cloud bubble measurement, and the method comprises the steps: carrying out a hydrofoil experiment in a water tank, and generating to-be-observed cloud bubbles; adjusting the ultrasonic imaging probe array through computer programming, and adjusting time delays of different array elements to generate a plurality of plane waves with different deflection angles; collecting plane wave echoes received by each array element, and storing echo data corresponding to each deflection angle; coherent superposition is carried out on image data obtained at different deflection angles, the contrast ratio and the signal-to-noise ratio of imaging are improved, and the effect equivalent to focusing imaging is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of cloud-like bubble measurement, and in particular to a method and a device for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging. Background Art

[0002] Cavitation is an important phenomenon in hydrodynamics, which refers to the process in which the local pressure in the liquid is too low, which induces phase change to produce gas phase. Therefore, cavitation usually occurs in the low-pressure area on the surface of high-speed rotating bodies, hydrofoils, and high-speed propeller blades. The cavitation phase change will lead to the generation of a large number of bubbles, forming cavitation groups. When the cavitation groups gather in a cloud-like form, it is called cloud cavitation.

[0003] As a non-steady-state flow field structure, cavitation groups may collapse after the surrounding local pressure increases. For cloud-like cavitation, its non-steady-state evolution, especially the collapse phenomenon, will not only produce noise and vibration, seriously affect the hydrodynamic performance of the navigation body, but can even erode the surface of the structure and cause damage. Therefore, the study of cloud-like cavitation is of great significance to the safety of navigation bodies. Since cloud-like cavitation contains many cavitations of different sizes, the size of the internal cavitations, the distance between different cavitations, the number density of cavitations, etc. directly affect the non-steady-state evolution of the appearance of cloud-like cavitation. It can be seen that obtaining the internal structural characteristics of cloud-like cavitation is the core basis for studying cloud-like cavitation.

[0004] At present, the research on the non-steady evolution of cloud cavitation is mainly based on numerical simulation and experimental observation. In numerical simulation, the time-averaged cavitation model is usually used to simulate the collapse of cloud cavitation, but the calculation accuracy of this method is relatively low;

[0005] In terms of experimental observation, optical means are mainly used to image and analyze cloud-like bubbles. However, due to the large number of internal cavitation bubbles and the widespread existence of gas-liquid interfaces, a large amount of light reflection causes blurred vision, making it impossible to clearly image the internal structural characteristics of cloud-like cavitation bubbles.

[0006] Some domestic and foreign scholars have also inserted optical fiber sensors and endoscopes into the cloud-like cavitation to observe the internal structural characteristics. However, as a contact measurement method, this method will interfere with the evolution of the cloud-like cavitation; in addition, the time resolution of this method is very low, and it can only obtain the time average, and cannot accurately capture the pulsation characteristics of small cavitation.

[0007] In summary, among the existing research methods for cloud-like cavitation, the confidence of the calculation results of the numerical simulation method needs to be discussed; the contact experimental method will interfere with the evolution of cloud-like bubbles, and its own time resolution is limited; the non-contact experimental method based on optics will not affect the evolution of cloud-like cavitation, but the field of view is blurred due to light reflection, and clear imaging cannot be achieved. Therefore, it is necessary to consider developing a non-contact experimental method with good imaging effect and high time resolution.

[0008] like Figure 1 As shown in the figure, traditional ultrasonic imaging uses the method of emitting focused waves multiple times for imaging; however, for an entire target area, the time cost of imaging increases because the array elements need to be excited sequentially. On this basis, researchers proposed using plane waves as the medium for ultrasonic imaging. Since plane waves only need to control the ultrasonic transducer array and excite the array elements once to image the entire target area, the temporal resolution of imaging can be significantly improved. However, since plane waves do not have a focusing effect, the contrast and signal-to-noise ratio of the imaging are low, and the imaging quality is poor. Summary of the invention

[0009] The purpose of the present invention is to provide a method and device for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging, so as to solve the technical problems in the prior art that the plane wave has no focusing effect, the imaging contrast and signal-to-noise ratio are low, and the imaging quality is poor.

[0010] In order to solve the above technical problems, the present invention specifically provides an observation device for the internal structure of a cavitation group, comprising:

[0011] A water tank, used for conducting a hydrofoil experiment to generate cloud-like cavitation to be observed, and one side wall of the water tank is equipped with an acoustically transparent window;

[0012] An ultrasonic imaging probe array comprises a plurality of array elements, which are fixedly arranged in sequence in a straight line on the outer side wall of the acoustic transparent window, and are used to generate plane waves to image the cloud-like cavitation in the shooting area.

[0013] In order to solve the above technical problems, the present invention further provides a method for observing the internal structure of a cavitation group, using the above observation device, comprising the following steps:

[0014] Conduct hydrofoil experiments in a water tank to generate cloud-like bubbles to be observed;

[0015] The ultrasound imaging probe array is adjusted by computer programming to adjust the delay of different array elements to generate multiple plane waves with different deflection angles;

[0016] Collect the plane wave echoes received by each array element and store the echo data corresponding to each deflection angle;

[0017] The image data obtained at different deflection angles are coherently superimposed to improve the contrast and signal-to-noise ratio of the imaging, thereby obtaining an effect equivalent to focused imaging.

[0018] As a preferred solution of the present invention, the straight line where the multiple array elements are located is taken as the x-axis. If the array element located at the origin is excited at time zero, the distance x from the origin is n The calculation formula for the array element transmission delay at t is: d =x n sinα / c;

[0019] Among them, c represents the propagation speed of the plane wave in the plane, and α represents the deflection angle of the wavefront of the plane wave compared to the coordinate axis.

[0020] As a preferred solution of the present invention, during the propagation of a plane wave, if there is a scatterer at a certain point (x, z), then:

[0021] The time it takes for a plane wave to propagate to the scatterer is (zcosα+xsinα) / c;

[0022] The scattered point is reflected to the origin at a distance x n The time of the array element at is

[0023] Distance from origin x n The total time of the propagation process When α is 0, there is no deflection.

[0024] As a preferred solution of the present invention, after the plane wave is backscattered by the scatterer, it propagates along the path to the array element and is received by it to form echo data. The method of beamforming the echo data is as follows:

[0025] For the distance x from the origin n The relationship between the received echo data RF and the time it takes for the plane wave backscattered by the scatterer to propagate to the position is: RF = F(x n ,t(x n ,x,z)), where RF is the radio frequency signal.

[0026] By integrating along the array direction and performing beam synthesis, the superimposed signal of each array element can be obtained, and then the scatterer can be imaged; if the aperture of the array element is 2a, the scatterer imaging data can be expressed as

[0027] Different deflection angle α i The images obtained under (i=1,2,...,n) are superimposed, and the final imaging data

[0028]

[0029] As a preferred solution of the present invention, the water tank adopts a low turbulence circulating water tank to reduce the noise generated when the water from the water tank flows through the pipeline;

[0030] Sound insulation materials are arranged around the water tank to separate the water tank and the ultrasonic imaging probe array from the incoming flow equipment, so as to reduce the influence of noise on the ultrasonic imaging probe array.

[0031] As a preferred solution of the present invention, the low-frequency noise in the plane wave echo is filtered to remove the background noise of cloud-like bubbles generated by cavitation on the hydrofoil surface.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention is based on the experimental foundation of hydrodynamics and acoustics, and applies plane wave ultrasonic imaging to the experimental scene of cloud bubble observation. The ultrasonic imaging probe array is adjusted through computer programming, and the delay of different array elements is adjusted to generate multiple plane waves with different deflection angles. The plane wave echoes received by each array element are collected, and the echo data corresponding to each deflection angle are stored. The image data obtained under different deflection angles are coherently superimposed to improve the contrast and signal-to-noise ratio of the imaging, and obtain an effect equivalent to focused imaging. The problem of blurred imaging due to interface light reflection in traditional optical non-contact observation methods is overcome, and a new non-contact observation method is formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0035] Figure 1 Schematic diagram of conventional focused wave ultrasound imaging (left) and plane wave ultrasound imaging (right);

[0036] Figure 2 Schematic diagram of plane wave delay deflection in an embodiment of the present invention;

[0037] Figure 3 Schematic diagram of a plane wave without deflection (left) and a plane wave with a deflection angle of α (right) in an embodiment of the present invention (wherein the dotted line represents the plane wave);

[0038] Figure 4 is a schematic diagram of an observation device in an embodiment of the present invention;

[0039] Figure 5 for Figure 4 Schematic diagram of the left side;

[0040] Figure 6 This is a schematic diagram of the interior of a cloud-like bubble;

[0041] Figure 7 Schematic diagram of observing cloud-like bubbles through plane waves with different deflection angles.

[0042] The numbers in the figure represent the following:

[0043] 1- water tank, 2- acoustic transparent window, 3- ultrasonic imaging probe array, 4- cloud-like cavitation, 5- plane wave, 6- hydrofoil. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] The present invention specifically provides a device for observing the internal structure of a cavitation group, comprising:

[0046] A water tank 1 is used for conducting a hydrofoil experiment to generate a cloud-like cavitation bubble 4 to be observed, and one side wall of the water tank 1 is equipped with an acoustically transparent window 2;

[0047] The ultrasonic imaging probe array 3 includes a plurality of array elements, which are fixedly arranged in sequence in a straight line on the outer wall of the acoustic transparent window 2 to generate a plane wave 5 to image the cloud-like cavitation 4 in the shooting area.

[0048] The schematic diagram of the experimental setup is as follows: Figure 5 As shown, the schematic diagram of the cloud-like cavitation bubble 4 and the shooting area are as follows Figure 6 As shown, Figure 7 A schematic diagram of the principle of observing the internal structure of a cavitation group based on plane waves is shown. The time resolution of this method can reach 30,000 frames per second.

[0049] In order to solve the above technical problems, the present invention further provides a method for observing the internal structure of a cavitation group, using the above observation device, comprising the following steps:

[0050] Conducting a hydrofoil experiment in water tank 1 to generate cloud-like bubbles to be observed;

[0051] The ultrasonic imaging probe array 3 is adjusted by computer programming to adjust the delay of different array elements to generate a plurality of plane waves 5 with different deflection angles;

[0052] Collect the plane wave echoes received by each array element and store the echo data corresponding to each deflection angle;

[0053] The image data obtained at different deflection angles are coherently superimposed to improve the contrast and signal-to-noise ratio of the imaging, thereby obtaining an effect equivalent to focused imaging.

[0054] The part about plane wave 5 includes three steps: plane wave 5 emission, plane wave 5 propagation, and beam synthesis.

[0055] During the transmission process, the delay is calculated according to the tilt angle, and the delay of different array elements is adjusted by controlling the ultrasonic transducer array to achieve deflection of the plane wave 5 at different angles.

[0056] like Figure 3 As shown, the plane wave 5 propagates in the plane at a wave speed c. If the array element at the origin is excited at time zero, in order to ensure that the wave surface of the plane wave 5 deflects an angle α relative to the coordinate axis, the distance from the origin x n The array element transmission delay t d =x n By controlling the transmission delay of each array element, even if the placement of the ultrasonic transducer array is not deflected, an equivalent plane wave 5 deflection can be achieved.

[0057] During the propagation of plane wave 5, Figure 4 As shown in the figure, if there is a scatterer at a certain point (x, z), the time it takes for the plane wave 5 to propagate to the scatterer is (zcosα+xsinα) / c, and it is reflected from the scattering point to a point x away from the origin. n The time of the array element at is ( That is, for the distance x from the origin n For the array element at When α is 0, there is no deflection.

[0058] It should be noted that Figure 4 The deflected plane wave 5 shown by the dotted line in the middle right figure is not a real plane wave 5. Since the ultrasonic transducer array is fixed at the x-axis, the advancing wavefront of the plane wave 5 is always located in the half-plane of the positive half-axis of the z-axis. The figure shows an equivalent treatment of deflecting the ultrasonic transducer array for the convenience of geometric analysis.

[0059] After being backscattered by the scatterer, the plane wave 5 propagates along the path to the array element and is received by it, forming echo data, which needs to be beamformed. nThe received echo data RF is related to the time it takes for the plane wave 5 backscattered by the scatterer to propagate to this position, that is, RF = F(x n ,t(x n ,x,z)), where RF is the radio frequency signal, indicating that the radio frequency signal RF is x n and t(x n ,x,z), but this function does not have a definite function expression, so it is expressed in this form.

[0060] By integrating along the array direction and performing beam synthesis, the superimposed signal of each array element can be obtained, and then the scatterer can be imaged. If the aperture of the array element is 2a, the scatterer imaging data can be expressed as

[0061] Different deflection angle α i The images obtained under (i=1,2,...,n) are superimposed, and the final imaging data Before superposition, no nonlinear processing is performed on the sub-images corresponding to each deflection angle, so the superposition process is a correlation superposition. By adopting the correlation superposition of different deflection angles, the image quality can be significantly improved.

[0062] In summary, the present invention chooses to use plane wave ultrasonic imaging as a non-contact measurement method to develop a quantitative measurement method for the internal structure of cloud-like cavitation. Relying on the experimental basis of hydrodynamics and acoustics, plane wave ultrasonic imaging is applied to the experimental scenario of cloud-like bubble observation, which overcomes the problem of blurred imaging due to interface light reflection in traditional optical non-contact observation methods and forms a new non-contact observation method.

[0063] Compared with traditional optical non-contact methods and contact methods, the acoustic method requires a large number of ultrasonic probes and requires signal matching with a computer; in order to ensure the propagation of plane waves, the wall of the experimental water tank needs to be acoustically transparent, and the flow turbulence needs to be guaranteed when cloud cavitation is generated.

[0064] Furthermore, the water tank 1 adopts a low-turbulence circulating water tank to reduce the noise generated when the water from the water tank 1 flows through the pipeline;

[0065] Sound insulation materials are arranged around the water tank 1 to separate the water tank 1 and the ultrasonic imaging probe array 3 from the incoming flow equipment, so as to reduce the influence of noise on the ultrasonic imaging probe array 3 .

[0066] Furthermore, the low-frequency noise in the plane wave echo is filtered to remove the background noise of cloud-like bubbles generated by cavitation on the surface of the hydrofoil 6.

[0067] The reason for adopting the above-mentioned noise reduction method is that cloud-like cavitation is generated by flowing through the inside of the water tank onto the surface of the hydrofoil 6, and strong background noise will be generated in the process. Since the acoustic detection method is adopted, the background noise problem is a difficulty that needs to be overcome.

[0068] The background noise comes from two aspects: the noise generated by the flow from the water tank flowing through the pipeline; and the noise of cloud-like cavitation bubbles generated by cavitation on the surface of the hydrofoil 6.

[0069] In view of the noise generated by the water flow from the water tank through the pipeline, the turbulence of the water flow in the pipeline needs to be controlled to reduce the noise generated during the flow process. In conventional experiments, the low-turbulence circulating water tank can produce a flow condition with a turbulence of less than 5%, which greatly reduces the water flow noise in the pipeline. At the same time, sound insulation materials need to be arranged around the water tank to separate the water tank and the probe from the rest of the parts, so as to avoid background noise interference from the pipeline during the observation process to the greatest extent.

[0070] The noise of cloud-like bubbles produced by cavitation on the surface of the hydrofoil 6 cannot be reduced by physical means, but the frequency of this noise is low. By sampling the noise under a fixed incoming flow velocity and filtering the low-frequency noise in the plane wave echo, a better background noise removal effect can be achieved.

[0071] The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present application.

Claims

1. A device for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging, characterized in that: include: A water tank (1) is used for conducting a hydrofoil experiment to generate cloud-like cavitation bubbles (4) to be observed, and one side wall of the water tank (1) is equipped with an acoustically transparent window (2); An ultrasonic imaging probe array (3), the ultrasonic imaging probe array (3) comprising a plurality of array elements, the plurality of array elements being fixedly arranged in sequence in a straight line on the outer side wall of the acoustic transparent window (2), and being used to generate a plane wave (5) for imaging the cloud-like cavitation (4) in the shooting area.

2. A method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging, characterized in that: Using the observation device according to claim 1, comprising the following steps: Conducting a hydrofoil experiment in a water tank (1) to generate cloud-like bubbles to be observed; The ultrasonic imaging probe array (3) is adjusted by computer programming to adjust the delay of different array elements so as to generate a plurality of plane waves (5) with different deflection angles; Collect the plane wave echoes received by each array element and store the echo data corresponding to each deflection angle; The image data obtained at different deflection angles are coherently superimposed to improve the contrast and signal-to-noise ratio of the imaging, thereby obtaining an effect equivalent to focused imaging.

3. The method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging according to claim 2, characterized in that: The straight line where multiple array elements are located is the x-axis. If at time zero, the array element at the origin is excited, the distance from the origin x n The calculation formula for the array element transmission delay at t is: d =x n sinα / c; Wherein, c represents the propagation velocity of the plane wave (5) in the plane, and α represents the deflection angle of the wavefront of the plane wave (5) relative to the coordinate axis.

4. The method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging according to claim 3, characterized in that: In the propagation process of plane wave (5), if there is a scatterer at point (x, z), then: The time it takes for the plane wave (5) to propagate to the scatterer is (zcosα+xsinα) / c; The scattered point is reflected to the origin at a distance x n The time of the array element at is Distance from origin x n The total time of the propagation process When α is 0, there is no deflection.

5. The method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging according to claim 4, characterized in that: After being backscattered by the scatterer, the plane wave (5) propagates along the path to the array element and is received by it, forming echo data. The method of beamforming the echo data is as follows: For the distance x from the origin n The relationship between the received echo data RF and the time it takes for the plane wave (5) backscattered by the scatterer to propagate to the position is: RF = F(x n ,t(x n ,x,z)), where RF is the radio frequency signal; By integrating along the array direction and performing beam synthesis, the superimposed signal of each array element can be obtained, and then the scatterer can be imaged; if the aperture of the array element is 2a, the scatterer imaging data can be expressed as Different deflection angle α i The images obtained under (i=1,2,...,n) are superimposed, and the final imaging data 6. The method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging according to claim 1, characterized in that: The water tank (1) adopts a low-turbulence circulating water tank to reduce the noise generated when the water from the water tank (1) flows through the pipeline; Sound insulation material is arranged around the water tank (1) to separate the water tank (1) and the ultrasonic imaging probe array (3) from the incoming flow equipment, thereby reducing the impact of noise on the ultrasonic imaging probe array (3).

7. The method for observing the internal structure of a cavitation group based on plane wave ultrasonic imaging according to claim 1, characterized in that: The low-frequency noise in the plane wave echo is filtered to remove the background noise of cloud-like bubbles generated by cavitation on the surface of the hydrofoil (6).

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