A numerical model for raindrop splashback on airfoils in heavy rain

In recent years, microbursts have been implicated in several major aviation accidents. Since microbursts are often accompanied by very heavy rainfall, an interest has arisen in the study of airfoil and aircraft performance in rain. As raindrops impact the leading edge of an airfoil, small droplets are splashed back into the air flow field and an uneven water film forms on the airfoil surface. Both phenomena have been hypothesized to contribute to the degradation of airfoil performance. The splashed back droplets are accelerated by the air flow field, thus droplet drag acts as a momentum sink to slow the boundary layer, while the uneven water film effectively roughens the airfoil surface. This dissertation presents a fully two-way coupled Eulerian-Lagrangian particle tracking scheme to evaluate the effect of the former phenomenon as experienced by a NACA 62-210 airfoil section in cruise configuration. The air flow field is determined with a thin layer Navier-Stokes computational fluid dynamics (CFD) code and is input to a particle tracking scheme which determines the droplet drag distribution throughout the flow field. The air flow field is then recalculated, this time accounting the effect of droplet drag, then the particle trajectories are redetermined in the new air flow field. This process is repeated iteratively until a stationary solution is reached. Raindrops are assumed to be noninteracting, nondeforming, and nonevaporating spherical particles and are tracked through the curvilinear body-fitted grid used by the air flow code. A simple model is used to simulate raindrop impacts and the resulting splashback on the airfoil surface.; Results demonstrate that splashed back droplets have the capability of altering the airfoil boundary layer and thus contributing to a rain-induced performance loss. However, the magnitude of the simulated performance loss is less than that measured experimentally, indicating that alteration of the boundary layer by splashed back droplets may be a secondary effect and other mechanisms such as the effective roughening of the airfoil surface by the uneven water film must be included in a comprehensive model. The diameters and initial velocities of the splashed back droplets were found to influence the magnitude of the performance loss, thus the splashback must be correctly modeled if the performance loss is to be accurately predicted.