DIRECT NUMERICAL SIMULATIONS OF LOW-REYNOLDS-NUMBER TURBULENT CHANNELFLOW WITH EMHD CONTROL

We present results of numerical simulations of turbulence control in s altwater channel flows using electromagnetic (EM) forces. The control actuators are millimeter-sized micro-tiles flush mounted in the lower channel wall. This arrangement closely models one of the experimental designs proposed and developed by Bandyopadhyay at NUWC. We have studi ed two main secondary flow patterns which we denote by UV and WV (i.e. , predominantly streamwise/normal and spanwise/ normal) induced by bot h static and pulsed EM forcing. We have observed low net drag reductio n, with a maximum of approximately 1%. This may be within the uncertai nty of our computations. However, we have also found regions of locali zed reduction/increase in wall shear stress as high as +/- 11% versus the uncontrolled flow. Also, in every simulation with control we have observed a consistent (albeit small) reduction in skin friction which increases confidence in the results. The method of pulsing the EM forc e did not result in any observable resonance effects, at the low Reyno lds numbers of this study. The mean turbulence intensities appear to b e only weakly correlated with the reduction in viscous drag. The chang e in net drag does not appear to scale linearly on the magnitude of th e EM forcing in the cases we have considered. Flow visualizations in t he both the UV and WV cases indicate that the mean secondary flow abov e the actuators consists of a pair of near-wall oppositely oriented st reamwise vortices which induce a flow where the normal velocity is wal l-ward and is accompanied by strong spanwise wall jets. (C) 1998 Ameri can Institute of Physics.