The effect of opposing unsteady vorticity on turbulent wall flow

A cylinder and a thin plate were placed close together in a flow adjacent to a wall to study the effects on the turbulent boundary layer. Different spacings of the cylinder and plate within the shear flow were investigated to assess the possibility of lowering the production of fluctuating vorticity in the boundary layer by generating a fluctuating vorticity of opposite sign. Streakline photographs visualized changes in the flow induced by alterations in the cylinder/plate separation distance, the flow velocity and the angle of attack of the thin plate. Drag data were also acquired with varying thicknesses of the thin plate and diameters of the cylinder. Downstream skin friction reductions were obtained with the production of unsteady control vortices with a low turbulence boundary layer. Up to 4 percent drag reduction was also observed when the cylinder was sufficiently far from the wall.