Influence of Nonspherical Bubble Collapse on Decomposition and Luminescence

通过考虑内部气体的van der Waals状态方程，采用边界单元法和有限体积法相结合的方法计算了环形气泡的坍塌。结果表明，由于真实气体的影响，热边界层变薄，最大内压变高。给出了用来计算气泡径向运动的Rayleigh-Plesset方程中压力项的物理意义。此外，我们还改进了Ghost流体方法(GFM)，利用Riemann解对边界节点的速度、压力和密度进行了修正，避免了气液界面附近的数值振荡。利用改进的GFM耦合和混合粒子能级集方法，成功地捕捉到了水中激波-气泡相互作用的尖锐界面。结果表明，边界节点的修正提高了可压缩气液两相流动GFM的数值稳定性。用离界面最近的实节点的速度来计算非黎曼解的切向速度是有效的。设计了小型聚焦超声振荡器。我们测试了曲率半径分别为8 mm和5.77 mm的两种压电陶瓷。结果表明，曲率半径较大的陶瓷具有较好的聚焦效果。实验和数值结果表明，由于陶瓷边缘的影响，聚焦位置向陶瓷方向移动。我们还考察了气体扩散对微泡稳定性的影响。结果表明，当表面活性剂浓度达到最大值时，由于表面积的压缩，表面张力降低，从而稳定了微泡。

We calculated the collapse of toroidal bubbles using the boundary element method combined with the finite volume method by considering the van der Waals equation of state for the internal gas. It was shown that the thermal boundary layer becomes thin and the maximum internal pressure becomes higher than that of ideal gas due to the real gas effect. We also showed the physical meaning of the pressure terms in the Rayleigh-Plesset equation that is used to calculate the radial motion of bubbles in an averaged equation for cavitating flows. Also, we have improved the Ghost Fluid Method(GFM) by correcting velocities, pressure and density at boundary nodes using the Riemann solutions to avoid numerical oscillations near the gas-liquid interface. We have succeeded in capturing the sharp interface for the shock-air bubble interaction in water by using the improved GFM coupling with the hybrid particle level set method. The results showed that the correction of boundary nodes improves the numerical stability of the GFM for compressible gas-liquid flows. It is effective to give the tangential velocity, which is not obtained from the Riemann solution, by the velocity at the nearest real node to the interface.Small focusing ultrasonic oscillators were devised. We tested two kinds of piezoelectric ceramics whose radius of curvature were 8 mm and 5.77 mm. It was shown that the better focusing is realized by the ceramics with the long radius of curvature. The experimental and numerical results showed that the position of focusing is moved toward the ceramics due to the influence of the edge of the ceramics. We also investigated the influence of gas diffusion on the stability of microbubbles. It was shown that the decrease of the surface tension due to the compression of the surface area at the maximum surfactant concentration stabilize the microbubbles.