Observation of Shock Wave Generation in a Cavitating Jet Focusing on Changes in Image Brightness

The aim of the present study is to create new technologies for solving problems such as ship fouling control and ballast water treatment in the field of maritime sciences. We report the investigation on underwater shock waves generated by the collapse of fine bubbles in cavitating flow fields. The cavitating jets are generated by four types of orifices and a triple-plunger pump. The schlieren method is used to clearly capturing the underwater shock waves. The schlieren images reveal that the local high-brightness region appeared in the shadow of the bubble flow moves downstream accompanied with shock wave generation. The relationship between the movement of the high-brightness region and the shock wave generation is correlated to the frequency of the chained collapse of cavitation bubbles. It is suggested that the movement of the high-brightness region depends on the jet flow velocity, the number density of bubbles, and the bubble contraction rate determined from the bubble diameter, the internal pressure of bubbles, and the strength of external shock wave pressures.