Viscous Drag Reduction by Periodic Acceleration & Deceleration

Abstract

An experimental wind tunnel study was performed to investigate the viability of incompressible turbulent boundary layer skin friction drag reduction by temporal acceleration and deceleration, motivated by similar pipe and channel flows studies. The acceleration and deceleration phases were achieved by rapidly opening and closing a set of downstream louver vanes, and it was observed that wind tunnel compressibility effects are substantial despite the nominally incompressible conditions. Deceleration produced an undershoot of the corresponding quasi-steady flow, but did not subsequently overshoot like in pipe flows due gradual reduction of the deceleration. The acceleration phase exhibited a skin friction coefficient overshoot, but did not subsequently produce the well-known relaminarization undershoot, due to a weaker and insufficiently-sustained acceleration. Energetic savings can potentially be achieved by intermittent propulsion that produces rapid acceleration over a small fraction of the cycle, followed by relatively slow deceleration for the remainder of the cycle.