Generic Transport Aft-body Drag Reduction using Active Flow Control

Abstract

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 aeroplanes, 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 aeroplane/helicopter configuration. The experimental study provided surface pressures, direct drag measurements, surface and smoke flow visualization. The baseline flow was numerically analyzed, using finite volume solutions of the RANS equations. 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. As the model incidence is decreased, the pair of vortex systems also penetrates deeper towards the centerline of the ramp, decreasing the pressure and increasing the drag. As expected, the ramp lower corner bubble was highly 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 ramp-corner bubble is eliminated.

Wind-tunnel tests of a heavy-class helicopter optimised for drag reduction

“…The experimental activities conducted in these frameworks demonstrated that a significant drag reduction could be obtained over a wide range of angles of attack through the use of different flow control strategies. Similar results were found in the numerical/experimental study by Ben-Hamou et al (4) , which aimed to investigate the effect on drag reduction produced by piezo-fluidic actuators situated at the back-ramp lower corner of a generic transport helicopter fuselage.…”

“…The drag reduction value (of the order of 2%) obtained at cruise attitude using such passive devices, requiring very simple modification to existing helicopters, can be considered a useful result leading to a non-negligible benefit in terms of fuel saving. In fact, recent literature has shown that slightly higher values of drag reduction (an average of 10% of the baseline fuselage drag only) can be obtained by employing active flow control strategies as air-jet blowing actuators positioned at the back-ramp region (3,4) . However, it must noticed that this latter solution would produce apparent drawbacks on existing helicopters related to installation, power requirements and maintenance of such active actuators.…”

Review on the use of piezoelectric materials for active vibration, noise, and flow control

“…In the study [233,234] conducted to determine the flow conditions for optimal performance of a delta wing, an array of five autonomous piezoelectric actuators were mounted on the leading edge of the semispan delta wing. For the study on the drag reduction and unsteady load alleviation [235], resulting from the separated flow at the aft part of a generic part of the helicopter/transport plane body, twenty four individually operated, compact cavity controlled piezo-fluidic actuators were positioned around the ramp. With these actuators, a total drag reduction of 3%-11% was reported.…”