Dependence of skin-friction reduction on the geometric parameters of blowing jet array

Abstract

Unsteady wall-normal blowing through one array of streamwise slits has been recently demonstrated to reduce substantially skin-friction drag in a turbulent boundary layer [X. Q. Cheng et al., “Skin-friction reduction using periodic blowing through streamwise slits,” J. Fluid Mech. 920, (2021)]. This work aims to understand the dependence of skin-friction drag reduction (DR) on the parameters of the slit array, including the slit width b, length l, and number N or actuation width. The control performance exhibits a dependence on the three parameters in terms of DR, downstream drag recovery, and net-energy saving. It has been found for the first time that DR achieved under high-frequency unsteady blowing may exceed that under steady blowing given the same blowing mass flow rate. Physical mechanisms behind are discussed. The scaling of the drag reduction is also examined empirically, providing valuable insight into the flow physics behind control.