Effects of upstream roughness and Reynolds number on separated and reattached turbulent flow

Abstract

This paper presents experimental investigation of upstream roughness and Reynolds number effects on the recirculation region over a smooth forward facing step. The upstream rough wall was produced from 1.5 mm sand grains and the Reynolds number based on step height, Re_h, was varied from 2040 to 9130 for both the upstream smooth and rough walls. For the smooth wall, the reattachment length increased monotonically with Re_h to an asymptotic value of 2.2 step heights for Re_h ≥ 6380. Upstream roughness reduced the reattachment length by 44% because of larger momentum deficit and higher turbulence level in the rough wall boundary layer. The mean velocities and Reynolds stresses were also reduced by roughness. The Reynolds shear stress and production of turbulent kinetic energy showed high negative values at the leading edge of the step indicating counter-gradient diffusion. The implications of these results for standard eddy viscosity models are discussed.

Keywords:

• separated and reattached flow
• forward facing step
• wall roughness
• Reynolds number
• particle image velocimetry

Acknowledgements

The financial support of this work by the Natural Sciences and Engineering Research Council of Canada (NSERC-DG) to Mark F. Tachie is gratefully acknowledged. The authors acknowledge Dr Karen Flack of United States Naval Academy for the contour plots of the rough wall topography.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Natural Sciences and Engineering Research Council of Canada (NSERC-DG).