水中衝撃波背後の可燃性気泡の挙動

抄録

In the present work, an underwater shock wave was applied to an explosive bubble, which then generated a spherical shock wave after shrinking process. A special care was taken for arrangement of the bubble and a pressure transducer so as to measure pressure behind the spherical shock wave originating from single bubble expansion. The bubble was made of a stoichiometric ethylene-oxygen mixture and its initial equivalent radius ranged from 1.0 mm to 2.2 mm. An incident underwater shock wave (ISW) was driven by gaseous detonation which propagated towards water surface, giving a peak pressure behind ISW (P_i) from 6 MPa to 23 MPa. Shadowgraph images of the bubble show that it starts to shrink after passage of ISW and then emits a light indicating combustion during the shrinking phase. When the bubble turns to expansion, it generates a shock wave (BSW) propagating spherically into the surrounding water. The experimental results reveal that the non-dimensional maximum pressure behind BSW (P_peak/P_i) is almost inversely proportional to the non-dimensional measurement distance from the bubble based on the initial radius of the bubble. The energy conversion efficiency from the bubble energy to the shock energy is dependent on the momentum acquired by water around the bubble, which is estimated based on the concept of the Kelvin impulse. The measured shrinking time of the bubble is found to be in good agreement with the Rayleigh collapse time.