Effect of Isotropic Free-stream Turbulence in Favorable Pressure Gradient Turbulent Boundary Layers over a Rough Surface

Abstract

Laser Doppler and Hotwire anemometry measurements were performed to study the effect of various conditions, namely free-stream turbulence (FST), favorable pressure gradient, and surface roughness, on turbulent boundary layers. Measurements were carried out at Re _θ ≤ 4,300 and free-stream turbulence levels of up to 7%, generated using an active grid. Results show that with the addition of FST, classical scaling laws are not able to collapse the profiles of mean velocity. Moreover, boundary layer parameters, including skin friction coefficient, confirm a complex interaction between the external conditions and the inner/outer flow. The discrepancy in the behavior of the stream-wise and wall-normal variances due to the presence of free-stream turbulence suggests that the addition of nearly isotropic free-stream turbulence promotes anisotropy in the body of the boundary layer. Second-order structure functions are examined to identify and quantify which turbulence length-scales contribute mostly to creating this discrepancy. The analysis demonstrates that the effect of FST resides in a wide range of length scales, and is not limited to the largest scales of the flow as in the case of ZPG flows.