The influence of nucleation on cavitation inception in turbulent shear layers

The influence of nucleation on cavitation inception in a high Reynolds number turbulent shear layer in the wake of a backward facing step was investigated using a controlled nuclei population. The flow was seeded with near monodisperse nuclei through a single injection port upstream of the step. The spatial distribution of the nuclei concentration within the resulting plume was characterized using a volumetric nuclei measurement technique based on Mie-Scattering Imaging. Incipient cavitation events were captured using two high-speed cameras, mounted to the side and below the tunnel test-section, triggered simultaneously with acoustic measurements. Seeding the flow with large nuclei addressed the issue of secondary re-nucleation in the step re-circulation zone and onset of developed cavitation observed for the flow with the natural nuclei population. The ability to discern individual incipient events enabled examination of the effect of cavitation number and the number of injected nuclei on the inception event rate. The event rate was found to follow a power law with cavitation number and to be linear with the injection rate. The inception events were mainly detected within the boundaries of the nuclei plume, however, a considerable number of out of plume events were observed. This was linked to capture of injected nuclei and/or generated cavitation products in the step re-circulation zone and their dispersion across the tunnel span due to flow three dimensionality. This suggests that the use of a more two-dimensional geometry with limited volume of re-circulation to capture and store nuclei might be beneficial. Notwithstanding these issues, mapping of spatial distribution of cavitation susceptibility, obtained by combining the spatial distribution of cavitation events and nuclei concentration, compare favourably with those reported for the same geometry in the literature. The current work provides a valuable dataset for development of computational tools for modelling of cavitation inception in nucleated flows.