Near-Wall Turbulence of Drag Reduction and Promotion in Unsteady Turbulent Pipe Flows

An unsteady behavior of the wall-drag, the velocity and the turbulence is experimentally investigated in the unsteady turbulent pipe flow, under several flow conditions with a rapid to slow time-variation and a large distortion of the flow-rate imposed stepwise. The instantaneous wall-shear stress and three-components of instantaneous velocities are simultaneously measured using the HWA technique of a surface hot-wire and a non-orthogonal triple-sensors probe. In the present study, to understand the unsteady behavior of the relation between the wall-drag and the velocity distribution, the unsteady response of the turbulence to the imposed unsteady flow is investigated by analyzing the relation between the unsteady Reynolds stress and the instantaneous velocity fluctuation during the accelerating and decelerating phase. It is found that, near the pipe-wall, the statistical parameter of the phase-average turbulence structure and the phase-average quadrant contribution rate of instantaneous velocity fluctuations to the Reynolds stress are almost similar to those in the steady turbulent flow, although the drag-reduction and the drag-promotion occurs during the accelerating and decelerating phase respectively.