Investigation of Relative Humidity and Induced-Vortex Effects on Aircraft Icing

Abstract

Two new mechanisms for downstream ice growth (i.e., downstream of the primary ice shape) in aircraft icing scenarios were investigated. The first mechanism is local variation of relative humidity with its potential for water deposition due to supersaturation. The second mechanism is induced-vortex effects due to their potential impact on droplet paths. It was shown that for rough surfaces with an extended period of exposure, relative humidity effects can lead to additional growth. The resultant frost is a sandpaperlike roughness that can severely degrade the aerodynamic performance of the wings. It was also shown that the vortices induced by the existing ice-shape features are capable of altering the droplet paths. As a result, impingements occur beyond the limits predicted by the methods in other icing prediction codes.Nomenclature a x , a y = nondimensional acceleration component in the x and y directions a 0 , a 0 = acceleration component in the 0 and 0 directions a = reference value for the nondimensionalization of acceleration C p = pressure coefficient, P P 1 = 1 2 air V 2 1 c = chord length D = diffusivity of water vapor in air, m 2 =s e, e = actual and saturation vapor pressure, respectively, mbar h = height of the roughness element K = thermal conductivity of water, J=m s K L = latent heat of vaporization for water, 2:5 10 6 J=kg LER = leading-edge radius, m LWC, VWC = liquid and vapor water content, respectively, in kilograms of water per cubic meter of air MVD = median volumetric diameter, m N = number of droplets in 1 m 3 of air P = static pressure, Pa (unless otherwise stated) R v = specific gas constant for water vapor, 461 J=kg K RH w , RH ice = relative humidity with respect to liquid water and ice, respectively r = radius of the droplet, m S = cross-sectional area of a sphere, m 2 s d = separation between two neighboring droplets, m T = static temperature, K t = reference value for the nondimensionalization of time V 1 = velocity of the approaching flow v x , v y = nondimensional velocity component in the x and y directions v 0 , v 0 = velocity component in the 0 and 0 directions w = mixing ratio of water vapor and dry air = ratio of the specific heats air , v = density of dry air and water vapor, respectively, kg=m 3 water = density of liquid water, kg=m 3 = stream function