Effective use of the streamwise waviness in the control of turbulent separation

Abstract It is commonly known that for a sufficiently high Reynolds number, for flow over a rough surface and other previously examined wall topologies, turbulent boundary layer separation generally occurs earlier, and the separation bubble is noticeably larger due to a greater momentum deficit caused by wall irregularities. The recently discovered amplitude modulation effect has provided an improved understanding of the momentum transport that occurs from the outer to the inner part of the boundary layer and its subsequent effect on skin friction. At sufficiently large Reynolds numbers, large-scale motion first causes an increase and then decrease in the flow velocity near the wall. This results in an increased net convection velocity of small structures in the streamwise direction, and consequently an increase in wall shear stress. In this work, we demonstrate that a wavy wall, with a streamwise waviness with a carefully selected amplitude and period, can effectively enhance the amplitude modulation effect and ensure an increase in the wall shear stress, thereby postponing turbulent separation. The experimental results are presented in normalized general form using suitable flow scaling. The research goal is to find values of the amplitude, period, and length (always with the total number of periods) of the wavy wall for which the highest rise in skin friction is gained at a fixed point downstream of the wavy wall. The most effective wavy wall geometry investigated ensured a 13% increase in skin friction (relative to the value at zero-pressure-gradient inlet flow) at the location where flow separation occurred on an unmodified surface (i.e. on a flat plate).