Pressure Gradient and Surface Curvature Effect on the Turbulent/Non-Turbulent Interface

The turbulent boundary layer (Re_τ=2700) over a generic submarine shape (Suboff) was measured in a wind tunnel using particle image velocimetry. The study focused on the turbulent/non-turbulent interface from 70% to 95% of the body length, where pressure gradient and wall curvature effects were significant. To this end, a local kinetic energy criterion was modified to better demarcate the irrotational flow in boundary layers with longitudinal streamline curvature. A study of the interface location revealed that the mean interface was shifted closer to and further from the wall by favorable and adverse pressure gradients (FPG, APG), respectively. In contrast, the variance of the interface location exhibited the opposite trend, showing an increase with the FPG and a decrease with the APG. Analysis of the interface geometry suggested that small-scale “nibbling” on the order of the Taylor microscale was invariant, whereas the large-scale motions on the order of the boundary layer thickness gradually increased with downstream distance.