Local relaminarization mechanism induced by a dynamic free-slip boundary

Abstract

Applying a dynamic free-slip boundary in a turbulent boundary layer has been shown recently to shift outward the near-wall transverse vorticity away from the wall and reduces the wall skin friction by more than 40%. Herein we present a local relaminarization mechanism induced by the dynamic free-slip boundary, from the perspective of energy exchange and transportation. The spatial evolution of the energy components associated with the mean motion, turbulent motion, and a shear-free oscillatory motion is presented. An analysis of the average energy exchange process in the near-wall region suggests that the energy of turbulence is transferred to the mean motion, against the canonical downward turbulent energy cascade. A considerable amount of energy is supplied to the shear-free motions, which displaces the highly turbulent and shear motions away from the wall. The relaminarization mechanism is associated with outward-shifted transverse vorticity and the depletion of the shear motions near the wall. As an effective method to manipulate the critical region for wall shear stress generation, the dynamic free-slip boundary produces a much stronger effect than the conventional relaminarization process, which can be employed for efficient drag reduction and boundary layer control.