2021 Volume 35 Issue 1 Pages 43-50
The pressure measurement using a fiber optic prove hydrophone (FOPH) and the observation with a high-speed video camera have been conducted simultaneously to investigate the collapse of a laser-induced bubble near a rigid wall and the corresponding shockwave generation due to the bubble collapse. Prior to the experiment of the bubble collapse near a rigid wall, the experiment of a laser-induced bubble in infinite water has been conducted to check the applicability of the FOPH to the pressure measurement. The directivity of FOPH is evaluated by controlling a directional angle to the spherical shockwave and a distance from a collapsing bubble. The peak pressure Pmax obtained at each θ was normalized by the peak pressure P0max obtained at θ = 0, and the calibration formula was obtained. Also, the peak pressure of the shockwave decays in proportion to the -1st power of the distance, which shows the characteristic of a spherical wave propagation. In the experiment of the collapse of a laser-induced bubble near a rigid wall, the bubbles collapse accompanying with the translation toward the wall. Then, the micro-jet penetrates the bubble surface toward the wall. By introducing a stand-off parameter γ, the peak pressure by the first collapse was evaluated. Due to the calibration of the directional angle and the distance, the peak value of the pressure of the shockwave takes a local minimum when γ is around 1.
