Through computational fluid dynamics simulations and wind tunnel tests this study examines a NACA63-218 airfoil in reverse flow at Re = 375,000, and demonstrates reduction in reverse flow drag through the introduction of reflex camber when the airfoil is pitched nose up in reverse flow. Of the three dominant sources of reverse flow 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 influences the first two, while leaving the third mostly unaffected. The change in trailing-edge geometry reduces exposure to ram drag on the upper surface while the suction on the lower surface rotates to result in a force opposite to the direction of the free stream. Although the 2D CFD simulations (URANS with SA turbulence model) had difficulty predicting the bluff body separation at the airfoil nose well, the change in flow at the trailing-edge was well captured yielding drag reductions of around 60% for a 10◦ reflex camber (compared to reductions of around 50% in the wind-tunnel test). Even greater percentage reductions in drag (up to 70%) were observed with a larger 15◦ reflex angle 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 signifcant promise for alleviating reverse flow drag on edgewise rotors at high advance ratio.
Proceedings of the 74th American Helicopter Society Annual Forum, Phoenix, Arizona, May 15-17, 2018.