Fluid mechanics of cavitation inception in rectangular slots

In order to better understand the fluid mechanics associated with cavitation inception in slots, the flow over a rectangular slot along a flat wall was studied in a non-cavitating fluid, namely air. Flow visualization experiments revealed the highly unsteady nature of the vortex-like structure(s) that developed inside the slot. Flow visualization of the slot entrance region showed that mass is transferred into the slot from the wall boundary layer; however, this mass does not appear to be drawn into the core of the primary vortex inside the slot. Direct pressure measurements inside the slot provided time-averaged contour plots of the pressure coefficient. These isobars were then used to obtain the location of the primary vortex, and also to document the minimum pressure coefficient as a function of aspect ratio. The measured pressure coefficients inside the slot are an order of magnitude lower than those for which cavitation has been observed in previous water tunnel experiments. These measurements therefore lead to question the interpretation of the previous water tunnel slot cavitation experiments. Specifically, it is likely that the previously reported cavitation was not vaporous cavitation, as had been assumed.