Secondary flows in statistically unstable turbulent boundary layers with spanwise heterogeneous roughness

Large-scale secondary motions are known to occur in turbulent flows over surfaces with spanwise roughness heterogeneity. Numerical studies often use adjacent high- and low-roughness longitudinal strips to investigate these secondary rolls in boundary layers without any thermal stratification. In the present study, the effect of unstable thermal stratification on secondary rolls in a very high-Reynolds-number turbulent flow with spanwise-heterogeneous roughness is investigated by means of large-eddy simulation. The strength of the unstable stratification is systematically changed from L/h = -20 to L/h = -1, where L and h are Monin-Obukhov length and boundary-layer height, respectively. This range covers the transition from neutral stratification to unstable stratification. The results show that the positive buoyancy associated with the unstable thermal stratification acts against the roughness-induced secondary rolls. In the case of unstable stratification, secondary rolls are completely canceled out by buoyancy and replaced by new stronger convection-induced rolls rotating in opposite directions. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics.

Automated summary

This study investigates the effect of unstable thermal stratification on secondary rolls in a very high-Reynolds-number turbulent flow with spanwise-heterogeneous roughness using large-eddy simulation. Results show that unstable thermal stratification cancels out secondary rolls induced by roughness and replaces them with new, stronger convection-induced rolls.