Passive drag reduction contol of a d-type rough wall boundary layer with longitudinal thin ribs placed within the transverse grooves

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 show the effectiveness of the present drag reduction technique using the longitudinal ribs. Maximum drag reduction rates to d-type rough wall flow and smooth wall flow are -10% and -3%, respectively. The drag reduction rate can be reasonably expressed in terms of the rib Reynolds number. Compariston of some mean flow properties between the modified flow and the d-type rough wall flow provide experimental facts that the present control device reduces the turbulent energy production rate.