The rotorcraft community has a growing interest in the development of high-speed helicopters, but one barrier to the design of such helicopters is the lack of understanding of the aerodynamic behavior of retreating rotor blades that operate in reverse flow. This work considers two fundamental models of the complex unsteady flow environment encountered by rotor blades in the reverse flow region of a rotor disk: static and oscillating airfoils in reverse flow. Knowledge of the time-averaged and unsteady airloads for both of these models is needed to provide insight for the design of rotor blades with improved aerodynamic performance and mitigation of component fatigue and vibrations during high-speed flight. To this end, two-dimensional wind tunnel tests have been performed on four airfoil sections: two featuring a sharp geometric trailing edge (NACA 0012 and NACA 0024) and two featuring a blunt geometric trailing edge. Time-averaged airloads were measured on static airfoils in reverse flow using a custom-built force balance system. Flow field measurements were captured using time-resolved particle image velocimetry (TR-PIV). Airfoils with a blunt geometric trailing edge were found to delay flow separation to greater angles of attack. This leads to a decrease in drag, but also an increase in downward-acting lift and pitching moment. Three unsteady flow regimes were identified: slender body vortex shedding, turbulent wake, and deep stall vortex shedding. Unsteady airloads were measured in these three regimes using unsteady pressure transducers mounted beneath the airfoil surface. The magnitude of the unsteady airloads has been found to be greatest in the turbulent wake regime; the unsteady airloads in deep stall are moderate in magnitude and periodic due to vortex-induced-vibrations.
Anya Jones is an Assistant Professor in the Department of Aerospace Engineering. She received her PhD in experimental aerodynamics from the University of Cambridge, United Kingdom, her S.M. in aeronautics and astronautics from MIT, and her B.S. in aeronautical and mechanical engineering from Rensselaer Polytechnic Institute. Her research is focused on the experimental fluid dynamics of unsteady, three-dimensional, and separated flows. Her current projects focus on unsteady low Reynolds number aerodynamics, vortex dynamics, flapping wings, separated and reverse flow rotor aerodynamics, and wind and tidal turbines. She is currently chair of a NATO Research Technology Organization task group on unsteady aerodynamics, and a member of the AIAA Applied Aerodynamics Technical Committee, the University of Maryland Energy Research Center, and the Alfred Gessow Rotorcraft Center.
Date(s) - Sep 17, 2015
12:00 pm - 1:00 pm