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.