Direct numerical simulations of low Reynolds number turbulent channel flow with EMHD control

Abstract

We present results of numerical simulations of turbulence control in saltwater 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 studied two main secondary flow patterns which we denote by UV and WV (i.e., predominantly streamwise/normal and spanwise/normal) induced by both static and pulsed EM forcing. We have observed low net drag reduction, with a maximum of approximately 1%. This may be within the uncertainty of our computations. However, we have also found regions of localized 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 force did not result in any observable resonance effects, at the low Reynolds numbers of this study. The mean turbulence intensities appear to be only weakly correlated with the reduction in viscous drag. The change in net drag does not appear to scale linearly on the magnitude of the EM forcing in the cases we have considered. Flow visualizations in the both the UV and WV cases indicate that the mean secondary flow above the actuators consists of a pair of near-wall oppositely oriented streamwise vortices which induce a flow where the normal velocity is wall-ward and is accompanied by strong spanwise wall jets.