Turbulent boundary-layer flow and structure on a convex wall and its redevelopment on a flat wall

Abstract

Two experiments (δ/R = 0.05 and 0.10) were performed to determine how boundary-layer turbulence is affected by strong convex curvature. The flow passed from a flat surface, over convex surface with 90° of turning, and then onto a flat recovery surface. The pressure gradient along the test surface was forced to be zero.After the sudden introduction of curvature, the shear stress in the outer part of the boundary layer is sharply diminished. The wall shear also drops off quickly downstream. When the surface suddenly becomes flat again, the wall-shear and the shear-stress profiles recover very slowly towards flat-wall conditions. The shear-stress profiles in the curved region for both experiments collapse when is plotted vs. distance from the wall normalized on wall radius, n/R. The strong-curvature data of So & Mellor also fall on the same curve. Thus suggests an asymptotic state for the shear-stress profiles of strongly curved boundary layers where R rather than boundary-layer thickness controls the active turbulence lengthscales. In this asymptotic region, the active shear-layer thickness is less than its initial value at the start of curvature. In the recovery region, the width of the active shear layer regrows slowly within the original velocity-gradient boundary layer, like a developing boundary layer under a free stream with a velocity gradient normal to the wall.