Numerical experiments on feedback EMHD control of large scale coherent structures in channel turbulence

The current work focusses on the spatio-temporal evolution of large scale c oherent structures in the turbulent boundary layer of a plane channel both with and without microtile-based EMHD control. The heuristic concept behind the microtile designs that we have simulated apparently does not yield a s uccessful drag reduction strategy (for the open-loop case [1]). In this wor k we investigate the flow response when the Lorentz force is applied with f eedback conditioned on the advection of large-scale flow structures (e.g. h airpin vortices). We performed a long-time simulation conditioned on the pa ssage of strong ejection events but obtained no reduction in skin friction. Based on short-time simulations we found that the near-wall flow structure s undergo merely a spatial phase-shift when advecting above a single Lorent z force actuator: the structures are simply decelerated or accelerated with little change in their appearance, based on flow visualization. During thi s interaction of the applied Lorentz force with the flow, the Reynolds stre ss is unchanged.