This paper presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic testing was performed on a full-scale, 72-in.-chord (1828.8-mm-chord), NACA 23012 airfoil model over a Reynolds number range of 4:7 10 6 to 16:0 10 6 and a Mach number range of 0.10 to 0.28. A high-fidelity ice-casting simulation of a runback ice accretion was attached to the model leading edge. For Re ˆ 16:0 10 6 and M ˆ 0:20, the artificial ice shape decreased the maximum lift coefficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1 to 15.0 deg. In general, the iced-airfoil performance was insensitive to Reynolds and Mach number changes over the range tested. Aerodynamic testing was also conducted on a quarter-scale NACA 23012 model [18 in. (457.2 mm) chord] at Re ˆ 1:8 10 6 and M ˆ 0:18, using low-fidelity geometrically scaled simulations of the full-scale casting. It was found that 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 those measured on the full-scale model. Based upon this research, a new subclassification of spanwise-ridge ice is proposed that distinguishes between short and tall ridges. This distinction is made in terms of the fundamental aerodynamic characteristics as described in this paper. Nomenclature C d = drag coefficient C l = lift coefficient C l;max = maximum lift coefficient, coincident with stall C l; = lift-curve slope C m = quarter-chord pitching-moment coefficient c = airfoil chord length k = ice-roughness height or thickness M = freestream Mach number n = wall-normal distance above airfoil surface Re = Reynolds number based on chord U = mean streamwise velocity U 1 = freestream velocity u RMS = root-mean-square of fluctuating streamwise velocity x = chordwise position along airfoil y = normal position from airfoil chord line = airfoil angle of attack stall = stalling angle of attack, coincident with C l;max C d;rms = percent root-mean-square difference in C d

The objective of this experimental investigation was to determine the geometric simulation fidelity required to accurately model the aerodynamics of a horn-ice accretion in a wind tunnel. A casting and a 2-D smooth simulation with variable horn geometry were constructed to model a horn-ice accretion on a NACA 0012 airfoil. Several simulations of differing fidelity, including a casting, were constructed to model a horn-ice accretion on a NACA 23012 airfoil. Aerodynamic testing was performed in the University of Illinois 3 4 ft wind tunnel at a Reynolds number of 1:8 10 6 and a Mach number of 0.18. Minor changes to the upper-horn geometry of the NACA 0012 2-D smooth simulation were found to have notable impacts on drag and maximum lift. Therefore, spanwise variations in the ice accretion geometry must be carefully examined so that an appropriate cross section can be chosen from which to generate a tracing for a 2-D simulation. Such a 2-D smooth simulation, as was constructed for the NACA 23012 airfoil, can model maximum lift to within 1% of that of the casting. This type of simulation can also provide an estimate of drag that is within the uncertainty of the casting due to spanwise variation, although it does not reproduce three dimensionality in the iced-airfoil flowfield. Nomenclature C d = drag coefficient C d;min = minimum drag coefficient C l = lift coefficient C l;max = maximum lift coefficient C m = quarter-chord pitching-moment coefficient C p = pressure coefficient c = airfoil chord length k = feature height M = freestream Mach number Re = freestream Reynolds number, based on the airfoil chord length s = airfoil model coordinate along the surface length x = coordinate in the airfoil model chordwise direction = airfoil angle of attack = ice-shape horn angle with respect to the chord line

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This paper presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic testing was performed on a full-scale, 72-in.-chord (1828.8-mm-chord), NACA 23012 airfoil model over a Reynolds number range of 4:7 10 6 to 16:0 10 6 and a Mach number range of 0.10 to 0.28. A highfidelity ice-casting simulation of a runback ice accretion was attached to the model leading edge. For Re 16:0 10 6 and M 0:20, the artificial ice shape decreased the maximum lift coefficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1 to 15.0 deg. In general, the iced-airfoil performance was insensitive to Reynolds and Mach number changes over the range tested. Aerodynamic testing was also conducted on a quarter-scale NACA 23012 model [18 in. (457.2 mm) chord] at Re 1:8 10 6 and M 0:18, using low-fidelity geometrically scaled simulations of the full-scale casting. It was found that simple two-dimensional simulations of the upper-and lowersurface 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 threedimensional features resulted in larger performance degradations than those measured on the full-scale model. Based upon this research, a new subclassification of spanwise-ridge ice is proposed that distinguishes between short and tall ridges. This distinction is made in terms of the fundamental aerodynamic characteristics as described in this paper.