Aerodynamics of scaled runback ice accretions

Run back ice accretions present a unique situation in iced-airfoil aerodynamics in that the airfoil typically has a clean leading edge before the ice accretion. To investigate the aerodynamic effects of runback ice accretions, simulations were scaled from accretions obtained in the NAS A Glenn Icing Research Tunnel for testing in the Illinois subsonic wind tunnel. Simple geometric scaling, based on airfoil chord, as well as boundary-layer scaling, based on estimated boundary-layer thickness, was used. The NACA 3415 and the NACA 23012 airfoils were tested at a Reynolds number of 1.8 7times; 10 ⁶ and Mach number of 0.18. Simple two-dimensional simulations were tested as well as three-dimensional simulations that more accurately simulated the features of the full-scale ice accretion. Significant aerodynamic penalties due to runback accretions were identified. In the worst case these penalties included a loss of over 0.75 in C_lmax and 7 deg in stalling angle of attack. In certain cases scaled runback accretions were found to increase the stalling angle of attack and maximum-lift coefficient. This phenomenon was investigated using boundary-layer measurements and fluorescent-oil flow visualization. It was concluded that the interaction between the boundary layer and the simulation was responsible for the phenomenon.