The effect of boundary layer thickness on cavity flow over a backward-facing step

Ventilated cavities detaching from a backward facing step are investigated for a range of upstream boundary layer thicknesses in a cavitation tunnel. The upstream turbulent boundary layer thickness is varied by artificial thickening of the test section natural boundary layer using an array of transversely injected jets. Momentum thickness Reynolds numbers from 8.5 to 28×103 were tested giving boundary layer thickness to step height ratios from 1.25 to 3.8. A range of cavitation numbers (based on the cavity pressure) were obtained by variation of the ventilation flow rate for fixed freestream Reynolds and cavitation numbers. Cavity length to step height ratios from 15 to 60 were achieved. Cavity length was found to be linearly dependent on ventilation rate and to decrease with increasing boundary layer thickness. This result may have implications in the practical optimization of these flows which occur in applications such as drag reduction on marine hull forms.