Computational and Experimental Investigation of Camber-Morphing Airfoils for Reverse Flow Drag Reduction on High-Speed Rotorcraft

Abstract

Through computational fluid dynamics simulations and wind tunnel tests, this study examines a NACA 63-218 airfoil in reverse flow at Rec=375,000 and demonstrates reduction in reverse flow drag through the introduction of reflex camber. Of the three contributors to drag—ram pressure on the upper surface near the trailing edge, suction on the lower surface near the trailing edge, and bluff body separation at the rounded nose—reflex camber (where the camber line near the trailing edge of the airfoil is deflected upward) influences the first two, reducing exposure to ram drag on the upper surface while rotating the suction on the lower surface away from the direction of drag. Particle image velocimetry and surface pressure measurements were utilized in experiment to directly compare with the results obtained through simulation. As expected, the flow was dominated by separation over the sharp trailing edge, where at moderate angles of attack (α <190°), a separation bubble was observed; the use of reflex camber reduced the extent of this separation. The simulations (unsteady Reynolds-averaged Navier–Stokes with and without the Spalart–Allmaras turbulence model) captured the reduction in separation at the trailing-edge well, as there was good agreement between the velocity fields when compared to experiments. This yielded maximum drag reductions near 60% for a 10° reflex camber, compared to reductions near 50% in experiments. Even greater percentage reductions in drag (up to 70%) were observed with a larger 15° reflex angle (not tested experimentally) for nose-up pitch angles greater than 5°in reverse flow. With simulations at a higher Reynolds number (1.5 million) showing very similar drag reductions, using reflex camber over inboard blade sections appears to have significant promise for alleviating reverse flow drag on edgewise rotors at high advance ratio.