Effects of rainfall on laminal flow airfoils examined through the lens of turbulent spots and streaks

Rainfall has been found to degrade airfoil performance through a variety of factors. This study focuses on one significant effect of raindrop impacts – the potential to cause early bypass transition within laminar flow near the leading edge. We examine this possibility through the lens of turbulent spots, wedges, and low-speed streaks, caused by static and transient disturbances in the boundary layer. Multiple impact scenarios are considered, including droplets rebounding from the airfoil, dispersing mass in the impact, and being absorbed into a water film. The complexity of each of these raindrop impact scenarios is abstracted as appropriate time-evolving body forces in our pseudo-spectral direct numerical simulations. The focus is interactions between a turbulent spot caused by the initial raindrop impact and the following disturbance, which may be a turbulent wedge, low-speed streak, or gradually evolve with time, depending on impact regime. The different scenarios are analyzed from both a local instability and vorticity dynamics points of view. Utilizing the results for improved modeling and potential mitigation of airfoil performance degradation due to rain or other transient phenomena will be discussed.