Direct Numerical Simulations of the Effect of Wind-Driven Runback Droplets and Rivulets on Boundary Layer Transition

Small aircraft, unmanned aerial vehicles (UAVs) and urban air mobility vehicles (UAMs) operate at low altitudes and are thus more significantly impacted by inclement weather. The scale and speeds of these aircraft imply that boundary layer flow over wings and other aerodynamic surfaces are often laminar but can be disrupted by rain or icing. This study investigates the transient effects of wind-driven droplets and rivulets on the development of boundary layer airflow on a flat plate. A novel Digital Image Projection (DIP) technique was used to measure the dynamic shape changes of the wind-driven droplets/rivulets over a flat plate and is used as an input for immersed boundary direct numerical simulations (DNS). The DNS is used to study the disrupting effect of the droplets and rivulets on the initially laminar boundary layer airflow. The detailed mechanisms of transition to turbulence are examined using an instability and vorticity dynamics framework. This understanding between droplet and rivulet geometry and boundary layer transition is crucial for modeling both aerodynamics and potential icing that could occur in UAVs and UAMs.