EXPERIMENTAL STUDY OF A LASER-GENERATED CAVITATION BUBBLE

This work reports the first results of a study on the pressure and velocity fields generated by a laser- induced cavitation bubble. Although extensively studied (Lauterborn & Vogel, Shockwaves 2013), the initial stages of the energy driven formation of the bubble, as well as the collapse phase, require a deeper investigation. The same lack of understanding applies to the collapse instants. In order to address these two extremes of the bubble life, a test chamber was designed to investigate the effect of laser energy and of static pressure. Two configurations are investigated, free bubble and bubble/wall interaction. The set-up consists of a stainless steel chamber (120x120x120 mm 3 ), equipped with quartz windows for optical access and designed to be pressurized (10 bar maximum). The laser source is a Nd:YAG pulsed laser (Litron NanoPIV), with a pulse duration of 6 ns and a maximum energy of 30 mJ per pulse. The beam is expanded, collimated and focused using a parabolic mirror immersed in high- ohmic water. This design reduces optical aberrations and forms a highly symmetric bubble (Obreschkow et al, Exp Fluids 2013). A measurement of the input energy is also performed, and a LED bulb provides background illumination for high-speed shadow-imaging (Photron Mini UX100 camera). The pressure induced by the shock wave at energy release and at collapse, is measured through two approaches. The first one uses a specifically designed fiber optic hydrophone for a local pressure measurement, by detection of the change in refractive index of the water due to pressure gradients. The second method is based on interferometry and gives a global pressure measurement across the volume. Finally, the fluid velocity in the bubble near field is performed with micro-PIV, using 4.5 ?m fluorescent polystyrene particles as tracers. The figure represents the bubble sequence at 54.000 fps. The maximum radius is about 800 ?m for a collapse time of 74 ?s. The 125-?m fiber sensor is also shown next to the cavitation bubble.