Characteristics of Wind-Driven Runback Droplet/Rivulet Flows: Part 2 - Direct Numerical Simulations of Boundary Layer Transition

The prediction and modeling of the effect of rainfall on boundary layer transition is of interest to ensuring the safe and efficient operation of aircraft, UAVs and rotorcraft in inclement weather conditions. This requires understanding the coupling between the evolution of droplet shapes driven by wind and their impact on the boundary layer flow. Our previous simulations suggest that idealized droplet or particle-like disturbances can disrupt the boundary layer by creating turbulent spots, wedges and/or unstable low-speed streaks. With this background, the evolution of droplets into rivulets and their effect on the surrounding airflow on a flat-plate boundary layer are studied using a combination of experiments and simulations. A sequence of wind driven runback droplet geometries, experimentally measured using digital image projection (as discussed in part 1), are introduced in direct numerical simulations using immersed-boundary methods to study their effect on boundary layer transition. The differing timescales of droplet and flow evolution enables a quasi-static analysis using instability and vorticity viewpoints to reveal the structure of the flow as a function of droplet geometry and the associated roughness Reynolds numbers.