Bubble dynamics and cavitation inception in a turbulent separating flow

The bubble dynamics and cavitation inception were studied in the shear layer behind two-dimensional backward facing steps. The steps of height 1 and 3 mm were cut at two streamwise distances from the leading edge of a Rankine body. The flow approaches the step at the two locations is turbulent. Observations of cavitation inception and bubble dynamics were made under stroboscopic light and by both flash photography and high-speed motion pictures. The cavitation inception downstream of the step was found to depend on the step height and the location of the step on the body surface, i.e. the mean separating pressure coefficient. The high-speed motion pictures showed that the first traces of cavitation appear in the free shear layer directly downstream of the step edge suggesting that the cavitation begins in the small-scale vortex cores. This observation and the high audible sound associated with the inception imply that the cavitation is vaporous. The pictures also showed that the gas diffusion may he occur in the separation zone, where some bubbles were observed to grow at a fixed location in this zone. The cavitation inception indices revealed a decreasing trend with the step Reynolds number, Re-h, which is consistent with the general trend of the turbulent shear layers.