Generic transport aft-body drag reduction using active flow control

Active flow separation control is an effective and efficient mean for drag reduction and unsteady load alleviation resulting from locally or massively separated flow. Such a situation occurs in configurations where the aerodynamic performance is of secondary importance to functionality. The performance of heavy transport helicopters and planes, having a large, and almost flat, aft loading ramp suffer from the poor aerodynamics of the aft body. Hence, a combined experimental and numerical investigation was undertaken on a generic transport plane/helicopter configuration. The experimental study was composed of surface pressures and direct drag measurements and surface as well as smoke flow visualization. The baseline flow was numerically analyzed, using finite volume solutions of the RANS equations, in steady and time-accurate modes. The baseline flow around the model was insensitive to the Reynolds number in the range it was tested. The flow separating from the aft body was characterized by two main sources of drag and unsteadiness. The first is a separation bubble residing at the lower ramp corner and the second is a pair of vortex systems developing and separating from the sides of the ramp. Apparently, a secondary bubble on the ramp causes increased suction and elevated drag as the model incidence is being reduced from positive to negative angles. As the incidence decreases the pair of vortex systems also penetrates deeper towards the centerline of the ramp. As expected, the ramp lower corner bubble was very receptive to periodic excitation introduced from four addressable Piezo-fluidic actuators situated at the ramp lower corner. Total drag was reduced by 3-11%, depending on the model incidence. There are indications that the flow in the wake of the model is also significantly steadier when the bubble at the lower ramp corner is eliminated. The vortex system is tighter and steadier when the bubble is eliminated.