Probable sites of cavitation inception and associated pressure field in a turbulent shear layer

Experiments performed downstream of a backward facing step, where the mean flow and turbulence statistics are well characterized, examine the early phases of cavitation events, and correlate them with the corresponding pressure field. Cavities detected using high speed silhouette imaging at cavitation indices of 0.45-0.55 and speeds of 10.5 and 16 m/s are used to compute probability maps that depict their location and frequency. These events are more likely to occur between 45% - 75% of the reattachment length, well upstream of the peaks in turbulence level. Occurring primarily in the braids between the spanwise vortices dominating the near field of the shear layer, the correlation between probability of cavitation and the turbulence scales in the shear layer is not evident. Hence, the dynamics of these quasi streamwise vortices is measured using 3D time-resolved tomographic PIV, enhanced by particle tracking. The spatial resolution of these measurements is sufficient for characterizing the ~1mm diameter cores of the secondary vortices. The corresponding evolution of the instantaneous pressure distributions is derived by integrating the material acceleration. Conditional spatial sampling is used for correlating the pressure field with the cavitation events.