Direct numerical simulations of boundary layer transition caused by impinging particles

Environmental effects such as rainfall can degrade airfoil performance through bypass transition to turbulence, which has significant implications to the safety of helicopters, urban air mobility, and unmanned aerial vehicles. This study examines the fundamental mechanisms by which dynamic impacts affect the flow through the lens of vorticity and local instability. Various scenarios of droplet impacts abstracted as particles were simulated as time-evolving body forces and studied using pseudo-spectral direct numerical simulations. These impact scenarios involve the creation and evolution of large-scale flow features such as turbulent spots, wedges, and low-speed streaks, and the interactions between them. This paper demonstrates that there exist meaningful interactions between “static” elements such as low-speed streaks and turbulent wedges, and transient disturbances such as impact initiated turbulent spots, with instances of localized suppression of turbulence being present. Also discussed are the effects of roughness Reynolds number Rekk associated with the part of the droplet that remains on the surface after impact. It is found that even impacts of droplets that are generally classified as “subcritical” can display a meaningful increase to the turbulence and time-averaged shear stress. These findings are expected to advance modeling and mitigation techniques of airfoil performance due to environmental factors involving transient impacts.