Turbulent Boundary Layer Control by Wall Oscillations

The objective of this paper is to examine the effect and the effectiveness of wall oscillation as a control scheme of drag reduction in order to better understand the mechanism of drag reduction. Two flow configurations were considered: constant flow rate and constant mean pressure gradient. The Navier-Stokes equations were solved using Fourier-Chebyshev spectral methods and the oscillation in sinusoidal form was enforced on the walls through boundary conditions for the spanwise velocity component. Results to be shown included the effects of oscillation frequency, amplitude, oscillation orientation, and peak wall speed at Reynolds number of 180 based on wall-shear velocity and channel half-width as well as the Reynolds number dependency s in both flow configurations. Comparison of effectiveness made between these two flow configurations has showed similarities as well as differences. Drag reduction as a function of peak wall speed was compared with both experimental and numerical data and the agreement was good in the trend and in the quantity. Comparison between these two flow configurations in the transient response to the sudden start of wall oscillation, turbulence statistics, and instantaneous flow fields was detailed and differences were clearly shown. Analysis and comparison are allowed to shed some light on the way that oscillations interact with wall turbulence.