Collaborative Investigation of the Aerodynamic Behavior of Airfoils in Reverse Flow

Two (static and dynamic) fundamental models of the flow over airfoils in the reverse flow region of a helicopter in forward flight are investigated experimentally and computationally at Reynolds numbers of O(10(5)). The first model examines the time-averaged and unsteady flow resulting from a two-dimensional NACA 0012 airfoil held at a static angle of attack. Computational tools successfully predict the presence of three unsteady wake regimes and time-averaged airloads measured experimentally at the University of Maryland (UMD). A second model is investigated by pitching a two-dimensional NACA 0012 airfoil through deep dynamic stall in reverse flow. Both experimental and computational results reveal flow separation at the sharp leading edge for shallow angles of attack, leading to the early formation of a reverse flow dynamic stall vortex. Subsequent flow features in the pitching cycle (i.e., a trailing edge vortex and a secondary dynamic stall vortex) are also captured by the numerical simulation, although the timing and strength of some of these features do not align completely with experiment. This work gives fundamental insight into the aerodynamic behavior of airfoils in reverse flow, improves understanding of the complex nature of the reverse flow region, and demonstrates a promising new computational tool for simulating this unique flow regime.