DRAG REDUCTION OF BLUFF-BODIES THROUGH MOMENTUM INJECTION

Effectiveness of the moving surface boundary-layer control in increasing lift and/or reducing drag at a subcritical Reynolds number is studied, using a two-dimensional wedge-shaped airfoil and a flat plate at high angles of attack, through an extensive wind-tunnel test program. Results suggest that injection of momentum, achieved here by introduction of bearing mounted, motor driven, hollow cylinders, can significantly delay separation of the boundary layer, resulting in a narrow wake and the associated reduction in the pressure drag. Results show that both the cylinder surface velocity as well as the surface roughness have significant effect on the boundary-layer control. The wedge airfoil showed an increase in C(L)/C(D) from 2 to 80, whereas the flat plate at 90 deg showed a reduction in drag coefficient by around 75%. A flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinyl chloride tracer particles, complements the wind-tunnel tests. It shows, rather dramatically, the effectiveness of the moving surface boundary-layer control.