Turbulence in Accelerating Boundary Layers

Abstract

In this study we focus on the effect of mean flow acceleration on the near wall structures within turbulent boundary layers. 2D PIV measurements in streamwise-wall normal and streamwise-spanwise planes have been performed upstream of and within a sink flow for inlet Reθ of 6326 and 3071, and at constant acceleration parameters of K = 0.6×10−6 and 1.1×10−6, respectively. Due to the imposed favorable pressure gradient (FPG), the Reynolds stresses normalized by the local freestream velocity decrease over the entire boundary layer. However, when scaled by the inlet freestream velocity, stresses increase close to the wall and decrease in the outer part of the boundary layer. This is caused by the confinement of the structures in the near-wall region in the accelerating flow. The weaker normalized strength of the vortical structures and the substantial negative wall-normal mean velocity in the FPG region are identified as the likely contributors to this trend. Data in the wall parallel planes dissecting the large scale structures shows their signatures in the form of “swirling” patterns and low speed streaks. For the higher Reynolds number flow, the high near-wall ∂U/∂y in the accelerating region decreases the angle of inclination of the large eddies. Consequently, their signature in the x-z plane is weaker and elongated in the streamwise direction. In the FPG region, the small scale structures tend to occur in streamwise aligned groups and almost all of them are observed in the low speed streaks, which are the regions of ejection induced by large eddies. Due to the lower turbulence levels in the outer parts, the high momentum regions, indicating sweep events, contain very few small scale structures. This distinction between the low and high momentum regions is much weaker in the zero pressure gradient (ZPG) area which has high turbulence levels in the outer layer.