Cavitating Structures at Inception in Turbulent Shear Flow

Cavitation inception in the near field of turbulent shear layers occurs along secondary quasi-streamwise vortices developing between the primary spanwise vortices. To observe these incepting structures and understand the flow conditions under which they form, the present paper compares the flow statistics to results of high speed imaging performed in the shear layer behind a backward facing step. Simultaneous top and side imaging captures the location and spatial arrangement of the cavitating structures at varying cavitation indices. Two-dimensional PIV measurements characterize the mean velocity and Reynolds stress profiles, for Reynolds numbers in the separating boundary layer of Re_τ = 807, 1504, 2345. Being concentrated within the quasi-streamwise strings, the cavitation events are preferentially located in the near field of the shear layer, where the unsteady flow is dominated by coherent structures, upstream of the peaks in turbulence level. They move more upstream with increasing Re and decreasing pressure. Furthermore, the probability of cavitation increases by 3-5 times when the velocity is increased from 10.5 to 16 m/s. These indicate that the pressure fluctuation peaks causing the cavitation do not follow the same trends as the Reynolds stresses which collapse when scaled by the freestream velocity.