Laser-induced control of a cavity bubble behind a sinking sphere in water entry: Dependency on the surface temperature and impact velocity

We investigate the effect of continuous-wave laser irradiation on the cavity evolution behind a sphere in water entry. By tuning the irradiation time, the surface temperature (T) of the sphere before the impact varies in 105-355 °C. We change the radius and impact velocity of the sphere, by which both the shallow and deep seals are considered. Compared to the reference case (the sphere was roughened to have a cavity initially), we find that the cavity expands or shrinks depending on T. Overall, for all cases, the cavity bubble expands to the maximum size and shrinks steeply with increasing T. At higher T, the cavity is destroyed significantly, even smaller than the reference case. However, the detailed interaction between the cavity and laser-induced cavitation bubbles is quite different. In a shallow-seal case, nucleate boiling occurs on the sphere surface and vapor bubbles merge into the cavity, resulting in the expansion of the cavity. At a highly subcooled condition, on the other hand, the vapor bubble collapses into microbubbles as soon as it contacts water, resulting in the cavity reduction. As the impact speed increases (for a deep-seal condition), the flux of entrained air becomes dominant and the stage of cavity expansion is quite narrow. As T increases, the heated cavity collapses into microbubbles and almost 90% is destroyed. Finally, we investigate the effects of modified cavity on hydrodynamic forces on the sphere. While the temporal variation of hydrodynamic forces is complex, the drag reduction over 40% is achieved.