Aerodynamic Simulation of Runback Ice Accretion

by National Aeronautics and Space Administration (NASA)

This report presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic tests were performed on a full-scale model using a high-fidelity, ice-casting simulation at near-flight Reynolds (Re) number. In addition, followon subscale tests were conducted with low-fidelity simulations on a quarter-scale model at low Reynolds number. The ice-casting simulation was attached to the leading edge of a 72-inch (1828.8-mm) chord National Advisory Committee for Aeronautics (NACA) 23012 airfoil model. Aerodynamic performance tests were conducted at the Office National d’Etudes et de Recherches Aérospatiales (French Aeronautics and Space Research Center) (ONERA) F1 pressurized wind tunnel over a Reynolds number range of 4.7×106 to 16.0×106 and a Mach (M) number range of 0.10 to 0.28. For Re = 16.0×106 and M = 0.20, the simulated runback ice accretion on the airfoil decreased the maximum lift coefficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1° to 15.0°. The pitching-moment slope was also increased and the drag coefficient was increased by more than a factor of two. In general, the performance effects were insensitive to Reynolds and Mach number changes over the range tested. The results from the full-scale tests were used to evaluate aerodynamic simulation methods for runback ice accretion. Aerodynamic tests were conducted on a quarter-scale NACA 23012 model (18-inch (457.2-mm) chord) at Re = 1.8×106 and M = 0.18, using low-fidelity, geometrically scaled simulations of the full-scale casting. Simple, two-dimensional simulations of the upper- and lower-surface runback ridges provided the best representation of the full-scale, high Reynolds number iced-airfoil aerodynamics. Higher-fidelity simulations of the runback ice accretion that included geometrically scaled three-dimensional features resulted in larger performance degradations than the full-scale model. At this time, it is not clear why a simple, two dimensional, geometrically scaled ridge simulation is required to best represent the fullscale, high Reynolds number, iced-airfoil aerodynamics. Reynolds number effects between the subscale model (1.8×106) and full-scale model (at 4.7×106) may be significant and more research is recommended. A new subclassification of spanwise ridge ice that distinguishes between short and tall ridges is proposed. This subclassification is based upon the flow field and resulting aerodynamic characteristics, regardless of the physical size of the ridge and the ice-accretion mechanism. Tall spanwise ridges have a profound effect on the airfoil flow field with a large (and often unsteady) separation bubble that grows rapidly with angle of attack and precipitates stall at a much lower lift coefficient and angle of attack than other ice accretions. In contrast, short spanwise ridges are characterized by a small, stable separation bubble formed in the immediate vicinity of the ridge. This small separation zone results in a limited effect on the airfoil pressure distribution relative to the clean configuration. Significant differences in surface pressure are only observed at the ridge location. The separation bubble does not significantly increase in size with angle of attack. Thus, the stall of the iced airfoil is generally governed by trailing-edge separation moving forward with increasing angle of attack (trailing-edge stall). In some cases, the airfoil may stall from the leading edge or from the ridge itself. More research is needed to determine the appropriate simulation methods for short ridges, particularly for Reynolds numbers less than 1.8×10^6.

52 pages, Paperback

Published November 1, 2010