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In recent years, with the rise of new computational methods, Smoothed Particle Hydrodynamics SPH has been gradually applied to the study of bubble dynamics. Zhang 28 established an axisymmetric SPH numerical model for bubble dynamics, simulated the underwater explosion bubble combined with Boundary Element Method BEM , and obtained jets and shock waves that are highly consistent with the experimental results. As the SPH computational method is gradually applied to the research field of fluid—structure interactions 29 , this new computational method will become a new way to solve complex bubble dynamics problems and bubble-wall interactions.
Based on the above-mentioned researches on two cavitation bubbles and multi-cavitation bubbles, this paper qualitatively discussed the surface wave phenomena, the development of internal jets, and the development of two bubble shock waves that occur during the interaction of two cavitation bubbles with a big difference in size. And the evolution characteristics of two cavitation bubbles induced by the two systems were compared.
Studying interactions of two cavitation bubbles from the mesoscopic level requires a multi-cavitation-inducing system. The jet and shock wave phenomena appearing in the interaction between the cavitation bubbles need to be observed by means of a high-speed dynamic acquisition and analysis system. In this paper, the method of laser-induced cavitation is mainly used for the jet phenomenon in the two cavitation bubbles interactions.
The principle is shown in Fig. The method of using laser underwater focusing to induce cavitation bubble is shown in Fig. The use of laser is advantageous as it is highly accurate in controlling the bubble nucleation spot and it is nonintrusive to the dynamics of two cavitation bubbles. By replacing the half-transmitting mirrors of different transmittances and adjusting the angles of the corresponding focusing mirrors, the relative size and relative position of the cavitation bubbles in the water can be precisely adjusted.
The method of inducing cavitation bubbles through low-voltage underwater discharge is shown in Fig.
Jet and Shock Wave from Collapse of Two Cavitation Bubbles | Scientific Reports
When the capacitor is fully charged, switch to the discharge circuit. In the discharge circuit, the current is instantaneously discharged through the contact electrode inside the tank. The contact point of the electrode generates a large amount of heat, and the water body near the electrode contact point is instantaneously vaporized to form a bubble while the end of the electrode contact point is fused.
In order to minimize the influence of the electrode on the evolution characteristics of the bubbles, in our experiment, a copper wire with a diameter of 0. The evolution cycle of the cavitation bubbles is very short. In order to observe the cavitation dynamics process, a high-speed dynamic acquisition and analysis system must be adopted. The system consists of a high-speed camera, a macro lens, and a light source. The shock waves can change the density of the water before and after the wave.
The intensity of the parallel light that is projected into the lens after the water with the density variations will change, so we can observe the shock wave on the image. In the study of this paper, the radius R max is used as the characteristic parameter of the cavitation bubble when it expands to the maximum volume, t represents the time during which the bubbles developed, wherein the subscripts represent different bubble numbers; the distance of two bubbles in the interaction process is represented as L.
In Fig. The pictures in Fig. The sizes of the two bubbles developed to the maximum volume are 0. Interactions of two cavitation bubbles generated at the same time with different sizes. Frame-rate: fps, Exposure time: 2. As the pulse of the laser energy ends, the bubble gradually expands outward. During the period of It is obvious from Fig. According to the studies of Obreschkow et al.
It can be seen that in the free field, a single cavitation bubble is slightly disturbed by the surroundings, which will change the collapse characteristics in its free field. By adjusting the transmittance of the half-transmitting mirror in the laser-induced cavitation bubble system and the angle of the corresponding focusing mirror to change the relative position and relative size of the two cavitation bubbles.
The distance between the two at the inception time is 1. Since the jet develops inside the cavitation bubble, it moves very fast, the velocity of the jet is about When piercing the far-end surface of the bubble, the velocity of the jet decreases sharply to It can be seen from Fig. In addition, it is apparent from Fig. Chew 31 and Rui 32 studied the interactions of two cavitation bubbles of different sizes through low-voltage discharging method. The two cavitation bubbles described herein are induced by the contactless laser, so there are the following differences: 1 the relative sizes and relative distances of the two bubbles obtained in this paper are far less than the range studied in the literature and the bubbles are generated at the same time, and there is no phase difference ; 2 under the condition of no external interference, we found that when two cavitation bubbles generated at the same time have a big difference in size and the distance between the two bubbles is very close, the small bubble only forms a surface wave towards the big one while the big bubble forms a jet that is far away from the collapse of the small bubble.
It should be noted that due to the limitations of the experimental technique, we have not been able to obtain the critical value at which point that the extremely small bubble exerts no influence on the big one.
Interactions of two cavitation bubbles generated at the same time with same sizes. Frame-rate: fps, Exposure time: 4. When expanded to the maximum volume, the middle layer becomes a very thin liquid film, but as can be seen from the image, the two bubbles did not fuse. During the shrinking process, the adjacent surfaces of the bubbles still appear planar, while the surfaces away from each other violently shrink toward the center of the respective bubbles.
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And during the shrinking process, there was no obvious fusion between the two, as well as no jets. At this stage, the surface of the bubble appears to be non-smooth. During this rebound expansion and contraction, the two bubbles are finally integrated into one.
During the process, the two bubbles are not interfered by each other, and no obvious deformation forms appear on the adjacent sides. During this phase, the adjacent surfaces exhibit a slight mutual attraction effect, and the bubbles take on an ellipsoidal shape, as shown in Fig. And during the synchronous shrinkage process, the surface contraction speed away from each other is faster than the adjacent sides, and finally the jet emerges when the bubble shrinks to the minimum distance, as shown in Fig.
During this phase, both bubbles shoot jets to each other as shown in Fig. After the fusion, the two bubbles move more vigorously on the upper and lower surfaces, and finally become tabular as in Fig. By comparing the experiments in this paper and numerical results in the literature, we can find that although the experimental methods and the sizes of the bubbles are different, the form of the fusion of the two bubbles in this paper is basically consistent with the simulated pattern in the literature.
Especially, in the late stage of cavitation bubble collapse, the surface forms of the upper and lower cavitation bubbles are basically symmetrical see Fig. It can be seen that for two cavitation bubbles of the same size that are generated at the same time, in the case where the distance between the two bubbles is very small, the collapsed form is similar to that of a single cavitation bubble in the boundless domain. Due to the limitations of experimental technique, this paper only qualitatively analyzed the fusion process of two bubbles of the same size.
The collapse strength after the fusion of the two cavitation bubbles is often the most concerned issue in practical engineering, which requires more in-depth quantitative researches. There are two typical bubble dynamics are presented during the collapse process of cavitation bubbles, namely jets and shock waves. In the previous section, we focused on the jet phenomenon exhibited by the interactions of two laser-induced cavitation bubbles.
In this part, underwater low-voltage discharge technology is used to induce two cavitation bubbles, so as to study the differences between shock waves resulting from collapse of two cavitation bubbles and the ones from single cavitation bubble. The differences between the jets resulting from collapse of spark-induced two cavitation bubbles and the ones from laser-induced two cavitation bubbles are also analyzed and compared. The frame-rate of the two groups is , fps. In order to obtain a clear bubble collapse shock wave, the exposure time is reduced to 0.
In addition to the images of the shock waves, the other images in Fig. The time for the appearance of the shock wave has been time stamped in Fig. Interactions of two cavitation bubbles. Frame-rate: fps, Exposure time: 0. The distance between the two at the inception time is Subsequently, the two bubbles are in the stage of contraction and collapse.
Rui 32 studied the characteristics of the micro-jets formed when the two cavitation bubbles collapse with the same test method as this paper. Therefore, this paper mainly discusses the shock waves from the collapse of two cavitation bubbles. To capture the shock wave, the exposure time must be reduced to 0. It can be seen from the waveform structure of the wave that the development of the shock wave is not completed once, but the second wave has been developed in a very short time after the first shock wave has been developed, as shown in Fig.
The bubble center distance is about It can be seen from the image that the two bubbles are not fusing. It can be clearly seen from Fig. We used the frame-rate of fps in the experiment, and only three pictures were captured during the entire bubble collapse process. Under this condition, the collapse shock wave of the bubble comes from the bubble of larger size. The main manifestation is that the bubble collapse shock wave is not released once, but following waves appears successively, and the wave superposition appears in some areas.
As shown in Fig. The collapse shock wave of the earlier cavitation bubbles in Fig. Its form and development are similar to those of a single bubble collapse. For collapse shock wave of the later cavitation bubbles, the wave is not released once, but two shock waves appear in a short period of time, and in the process of outward development, wave superposition occurred in some areas. As shown in the wave shape in the lower part of Fig.
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For cavitation bubbles generated at the same time with different size, the shock wave is mainly caused by the large bubble collapse. For cavitation bubbles generated at different time, the two bubbles have shock waves from the collapse one after another. The shock waves in both cases are completely different from the single spherical shock wave of the single bubble collapse, there are continuous waves appear in a short time.
And due to different speeds, these may have waves superposition in some areas. For the jet and shock wave phenomena that occur in multiple bubble collapses, the above analysis is carried out by using different experimental systems.
However, for the utilization and prevention of multi-cavitation interactions, it is necessary to consider the influence of boundary conditions around multi-cavitation bubbles. This section only studies the collapse modes of the two bubble center lines parallel to the wall. As a comparative experiment, Fig. During the second expansion and collapse of the cavitation bubble, the cavitation bubble moves toward the wall surface quickly. For Fig. However, due to the presence of a small cavitation bubble around, the larger one is penetrated in its first stage of expansion-contraction by the jet from the small bubble, which disturbs the law of the development of the large cavitation itself to the wall surface.
The main manifestations are: 1 under the condition of a single cavitation bubble, the bubble shrinks to the minimum volume. The time when the bubble shrinks to the minimum volume is Under the condition of a single bubble, the surface of the bubble away from the wall surface shrinks faster. And when there is disturbance of a small cavitation bubble, due to the small bubble forms a jet that impacts the large cavitation bubble, so that the shape development of the large one is no longer asymmetrical, as shown in Fig.
In the study of this paper, the details of the internal jet of the small cavitation bubble on the large one and those of the small cavitation bubble suppressing the formation of the micro-jet by the large one to the wall surface can be clearly seen. Internal jets also appear in the literature. The two bubble center line is perpendicular to the wall surface in the literature while the two bubble center line is parallel to the wall surface in this paper.
By comparison, it is found that the formation mechanism of the internal jet in the literature is caused by the existence of the wall surface.