On initial stage of bubble cavitation zone dynamics in pulse rarefaction waves

This paper presents experimental results of studying peculiarities of the initial stage of bubble cavitation evolution in liquid near a free surface when a microsecond duration shock wave reflects from it. Use of independent diffraction and electromagnetic techniques, as well as laser beam intensity variation, allowed the threshold effect evolution to be observed with an increase in the incident shock wave amplitude and solvability of these effects to be analyzed. Correlation of dynamics of the free surface and cavitation zone, predicted earlier on the basis of numerical studies performed [V. K. Kedrinskii and S. I. Plaksin, Proc. of the 11th ISNA (1984)], is confirmed experimentally. For the experiments, a transparent small hydrodynamic shock tube filled with distilled water was used. The shock wave was generated by a membrane occupying the central tube bottom part and excited by pulse magnetic field. The system geometry was chosen in order to exclude the effect of the tube walls on the wave field structure and cavitation evolution process. The experiments showed that, with the shock amplitude increase, the following effects can be observed: (1) the jump-like increase in light intensity scattered on cavitation micronuclei, (2) considerable velocity retaining by the free surface for a long time after the rarefaction wave has passed, and (3) its complete absorption by developing cavitation zone. Attention is paid to hysteresis-type effects of the free-surface dynamics that can be due to both the structure of nuclei and peculiarities of their attaining the “visible” zone (zone of detection).