Scaling laws for testing airfoils under heavy rainfall

Abstract

Subscale test data have shown that airfoils operating in a simulated heavy-rain environment can experience significant performance penalties. The physical mechanism resulting in this performance penalty has yet to be conclusively identified. Therefore, the extrapolation of subscale data to full-scale conditions must be undertaken with extreme caution since complete scaling laws are unknown. This paper discusses some of the technical issues that must be addressed and resolved prior to extrapolating the performance of full-scale airfoils from subscale test data. A set of scaling laws is suggested based on the neglect of thermodynamic interactions between the droplets and the air/water vapor phase.heat at constant volume D =drop diameter hj g = latent heat of vaporization of water £ = mean distance between droplets m = mass M =Mach number n(D) = raindrop size spectrum n 0 =8xl0 3 m-3 mm-1 =« 0/5 N = aerodynamic force ND = droplet number density p = pressure R = rainfall rate, mm/h, or gas constant T = temperature Uj = velocity vector U x = flight speed V = volume or droplet impact velocity V T = drop terminal velocity W L = liquid water content, g/m 3 We = Weber number x,y,z = Cartesian coordinate system a. = angle of attack 13 = impact angle 7 = ratio of specific heat 0 = contact angle A = reciprocal of rain spectrum scale JJL = absolute viscosity u = kinematic viscosity p = density o = surface tension r = shear stress $ = velocity potential Subscripts a fs ss s V w = air = full scale = subscale = solid = vapor = water Presented as Paper 85-0257 at

Aerodynamic study of aerofoil and wing in simulated rain environment via a two-way coupled Eulerian-Lagrangian approach

“…In wind tunnel experiments or numerical simulations, the scaled models of aerofoils and wings are usually adopted for our analysis of rain effects. In 1987, Bilanin (22) addressed the subject of scaling laws for model tests of aerofoils in simulated rain and determined that variables such as kinematic viscosity, surface tension interaction, mean raindrop spacing, air and raindrop density, volumetric mean droplet diameter and drop velocity are necessary parameters for scaling rain effects on aerofoil aerodynamic performance. As scaled rain model is used in our simulation, scaling laws are implemented given by:…”

“…It is assumed that the rain droplets in present study are non-evaporating, non-interacting and non-deforming spheres. The former assumption of non-interacting droplets has been justified by Bilanin (22) . According to his study even for high rainfall of 1,872mm/h and for an average raindrop diameter of 4mm, the mean distance between raindrops will be of the order of 70mm or 17•5 times the droplet diameter.…”

Numerical Simulation of Airfoil Aerodynamic Penalties and Mechanisms in Heavy Rain

“…and it represents the probability of finding drops of diameter in a sample of splashed raindrops. Bilanin [28] has investigated the evaporation of the particles near the surface and found that evaporation does not affect the airfoil aerodynamic efficiency, so in our study we ignore the vaporization of water film. Since the film particle vaporization is ignored, only the momentum and energy conservation equations remain in the conservation equations for wall-film particles.…”

Numerical Study of Aerodynamic Efficiency of a Wing in Simulated Rain Environment

“…We assume that the rain particles are noninteracting. Bilanin [30] showed in his study that, even for extremely high rainfall of 1872 mm∕h, for an average raindrop diameter of 4 mm, the mean distance between raindrops should not be less than 70 mm or 17.5 times the drop diameter. Therefore, raindrop collisions cannot occur frequently enough, and we can neglect the collisions of the particles in our rain model.…”