Decay process of a manipulated large-scale horseshoe vortex in a turbulent boundary layer

Abstract

As a model of the coherent structure, an artificial horseshoe vortex was induced by injecting a pair of small pulsative jets into the bottom of a fully developed turbulent boundary layer from a flat plate beneath. After initial growth, the horseshoe vortex had a configuration similar to that of the natural coherent bulge and was broken up by a single manipulator plate installed parallel to the flow direction in the turbulent boundary layer. The ensemble-averaged data, conditionally sampled on the injection of the jets, gave velocity vector fields and shear stress contour maps around the nonmanipulated and manipulated vortices in each stage of their streamwise decay. The manipulated horseshoe vortex was sliced into two blocks that recombined in the downstream region, but with consequent suppression of the induced shear stress in the core of the manipulated horseshoe vortex. The "large-eddy breakup" method was verified as an efficient drag-reduction scheme. Nomenclature= streamwise span of manipulator plate = streamwise, normal, and spanwise scales of coherent vortices, respectively = overall value of induced shear stress = time from rising of driving pulse = period of injection -velocity components = local mean velocity = mean velocity of freestream = streamwise, normal, and spanwise coordinates, respectively = induced value = boundary-layer thickness = duration of driving pulse = rms value of the fluctuations = ensemble-averaged value (") = time-averaged value