Turbulent drag reduction of a d-type rough wall boundary layer with longitudinal ribs placed with in the grooves. Reduction rate and time mean properties.

Abstract

In order to control the turbulent energy production and transport processes due to the coherent vortices associated with the bursting phenomenon in a d-type rough wall turbulent boundary layer, longitudinal thin ribs were placed within the transverse grooves with a suitable spanwise spacing. Direct measurements of the local skin friction coefficient evidently show the effectiveness of drag reduction using the longitudinal ribs. Maximum drag reduction rates to d-type rough wall flow and to smooth wall flow are -10% and -3%, respectively. The drag reduction rate can be reasonably expressed in terms of the rib Reynolds number. Comparisons of some mean flow properties between the modified flow and the d-type rough wall flow provide evidence that the present passive control device reduces the turbulent energy production rate.