A simple pressure-based feedback control strategy for wall-transpiration control of incompressible unsteady 2D channel flow was recently investigated by Aamo, Krstic, & Bewley (2001). Nonlinear 2D channel flow simulations which implemented this control strategy resulted in flow transients with instantaneous drag far lower than that of the corresponding laminar flow. The present note examines the physical mechanism by which this very low level of instantaneous drag was attained. It then explores the possibility of achieving sustained drag reductions to below the laminar level by initiating such low-drag transients on a periodic basis. All attempts at sustaining the mean flow drag below the laminar level fail, providing further evidence in favor of the conjecture that the laminar state provides a fundamental “performance limitation” in such flows.

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