Numerical Simulation to Investigate Interactions of Generated Underwater Micro Shock Waves and Micro Bubbles by Focusing Femtosecond Pulse Laser

The purpose of this study is to elucidate the mechanism of propagation of the laser-induced micro shock waves under condition where the micro bubbles are generated. In this paper, effects of generated micro bubbles on propagation of the laser-induced micro shock waves were investigated by CFD (computational fluid dynamics). Firstly, the two models (1-D model and 1-D spherical symmetric model) were computed for comparison of the peak pressure variation of the shock waves with propagation. As for governing equations for the propagation of the shock waves, continuity equation, Euler’s momentum equation and Tait’s state equation are used. From the computation, it is confirmed that attenuation of pressure of the 1-D spherical symmetric model was earlier than the 1-D model. In addition, the attenuation of the 1-D spherical symmetric model agreed with the laser-induced shock waves obtained experimentally. However, the peak pressure and duration time of the shock wave was not the same as the experimental result. Then, the bubble behavior was included in the computation of the shock wave propagation. As for the bubble behavior, Rayleigh-Plesset equation is used. From this computation, pressure wave was obtained which superposed the pressure of the shock wave on the internal pressure of the bubble. Although the duration time of the pressure wave was close to the experimental result, the value of the pressure was almost the same as atmospheric pressure. It is suggested that there is a possibility that phenomenon other than the bubbles is generated such as plasma when the shock wave is generated by focusing the femtosecond pulse laser