The Influence of Free-Stream Turbulence on an Axisymmetric Turbulent Boundary Layer in, and Relaxing from, an Adverse Pressure Gradient

Abstract

This paper describes an experimental investigation of an axisymmetric boundary layer with a high and almost constant, level of free-stream turbulence (Tu δ≈8%) in the streamwise direction. The boundary layer develops initially in an adverse pressure gradient and then relaxes downstream into a constant pressure region. Transition occurs at the onset of the adverse pressure gradient raising the skin friction, before it decreases due to the pressure rise. This effect causes a breakdown of the logarithmic law of the mean velocity, in that the constant C is lower and the profile takes a long distance to recover to its equilibrium value. The free-stream turbulence promotes the transition process and affects the development of the boundary layer without changing the overall pressure recovery. With high free-stream turbulence the minimum skin friction is higher, i.e. the flow is less prone to separation, and the shape parameter H 12 is smaller. The thickness of the boundary layer doubles in size compared with the flow with low (Tu δ≤1.7%) free-stream turbulence.