During the past 30 years, near-wall streamwise vortices in
turbulent boundary layers have been the object of many investigations, and
they
have been found to play a major role in the dynamics of turbulent boundary
layers.
Recent studies have shown that the high skin-friction regions in turbulent
boundary layers are also closely related to the near-wall streamwise vortices.
These regions of high skin-friction on the wall are created by the inrush of
high-speed fluid toward the wall (sweep), which is induced by a strong
streamwise vortex.
Choi et al. (JFM, 1994) showed that a significant drag reduction is possible
when the surface boundary condition is modified to suppress the near-wall
streamwise vortices.
It has also been shown that the magnetohydrodynamic (MHD) force can increase the stability of a flow and thus can prevent its transition to turbulence. It has been postulated that the MHD force can also significantly reduce turbulent drag by altering turbulence structures. The mechanism responsible for the reduction, however, is not well understood.
In this study, we investigate the possibility of controlling streamwise
vortices
in conducting fluids in a turbulent channel flow by means of uniform magnetic
fluxes using the direct numerical simulation technique.
The present study considers two types of electromagnetic forcing which are
induced from a uniform magnetic flux in the streamwise and spanwise
directions.
An improved understanding of the effect of such momentum forcing on the
near-wall streamwise vortices should be key to better boundary-layer control
strategies.