Effects of local forcing on a turbulent boundary layer

Abstract

A numerical study was made to delineate the effects of local forcing on a turbulent boundary layer by carrying out direct numerical simulation (DNS) and large eddy simulation (LES). Localized blowing through a spanwise slot was adopted as a local forcing due to its possibility of practical actuation in the flow control. Both steady and unsteady forcing were tested. Comparison between steady and unsteady blowing was made. In unsteady blowing, an effective blowing frequency was observed for augmentation of the near-wall turbulent structures. A most effective blowing frequency is obtained at $f^+$=0.03, where the maximum increases of Reynolds shear stress, streamwise vorticity fluctuations and energy redistribution are made. The phase-averaged stretching and tilting terms are studied for analyzing the increase of streamwise vorticity fluctuations which is closely related to turbulent coherent structures. LES of Re=1410 also shows that a most effective blowing frequency for enhancement of Reynolds shear stress and streamwise vorticity fluctuations is observed at $f^+$=0.03, similarly Re=300.