Turbulent boundary layers over smooth and rough forward-facing steps

The present work explores the impact of the roughness on the turbulent boundary layers over forward-facing steps. The roughness topography on the top surface of the rough step is replicated from a realistic turbine blade and embodies three-dimensional and highly irregular topographical features. High spatial resolution particle image velocimetry measurements are performed in the x-y planes at two different spanwise positions in turbulent boundary layers over both smooth and rough steps of the same mean heights at Re-h=3450 and delta/h=8. Comparison of mean flow structures, Reynolds normal and shear stresses, quadrant analysis of instantaneous shear stress contributing events, and average spanwise vorticity reveals that the separated flow after the step is weakened by the surface roughness on top of the step while the flow ahead of the step is invariant to the surface conditions. The characteristics of the coherent spanwise vortices such as the numbers, size, and circulation distributions are also found to be significantly modified by the roughness topography. (C) 2011 American Institute of Physics. [doi:10.1063/1.3576911]