Optimum-wavelength forcing of a bluff body wake

Abstract

Three-dimensional forcing of the wake of a circular cylinder was studied experimentally to determine the optimal spatial-forcing-wavelength for drag reduction. Dielectric-barrier-discharge plasma actuators were mounted on a cylinder in a square-wave pattern to create the three-dimensional forcing. Six spatial wavelengths (1d–6d) and two blowing ratios (Uj/U∞ = 0.2 and 0.6) were tested at a Reynolds number of 4700. For most spatial wavelengths and blowing ratios, the segmented plasma actuators produced streamwise vorticity that altered the wake development, formation length, and drag. A spatial wavelength of 4d emerged as the optimum wavelength for the high-blowing-ratio case. Forcing with this optimum wavelength significantly attenuated vortex shedding, leading to maximum drag reduction in the high-blowing-ratio case. This optimum wavelength of 4d exists because longer wavelengths reduce the spatial extent of induced velocity and shorter wavelengths inhibit the development of streamwise vorticity.