On the friction drag reduction effect by the control of large-scale turbulent structures

Direct numerical simulation (DNS) of a feedback controlled turbulent channel flow at Re,=640 is carried out. In order to seek a possibility of employing fewer sensors and actuators of larger sizes for real applications, the control performance is assessed through either the small scale or large scale components of wall normal velocity fluctuation, of which threshold is defined lobe the spanwise wavelength of 300 viscous wall units. '1' he present numerical results reveal that the control of small scale fluctuations generally leads to larger drag reduction than that of large scale fluctuation. In the former case, the contribution of small scales to the friction drag is reduced, while that of large scales remains unchanged. In contrast, when only the large scale fluctuation is damped, the contribution the small scale fluctuation to the friction drag is found drastically increased due to the reduced pressure strain correlation, which acts as a major destruction mechanism of the Reynolds shear stress.