Simulation of rotor vortex interactions with a particle-laden turbulent boundary layer

Rotorcraft operation in austere environments can result in difficult operating conditions, particularly in the vicinity of sandy areas. The uplift of sediment by rotorcraft downwash, a phenomenon known as brownout, hinders pilot visual cues and may result in potentially dangerous situations. Brownout is a complex multi-phase flow problem that is not unique and depends on both the characteristics of the rotorcraft and the sediment. The lack of fundamental understanding constrains models and limits development of technologies that could mitigate its adverse effects. This provides the over-arching motivation of the current work focusing on models of particle-laden sediment beds. The particular focus of the current contribution is numerical modeling of near-surface fluid-particle interactions in turbulent boundary layers. Simulations are performed with and without coherent vortices superimposed on the background flow in order to understand the influence of rotorcraft downwash. The objectives are to gain insight into the fluid-particle dynamics that influence transport near the bed by analyzing the competing effects of the vortices, inter-particle collisions, and gravity. The computations are performed using an Euler-Lagrange approach in which a fractional-step method is applied to the fluid and with the particulate phase modeled using Discrete Particle Simulation. Following the introduction of coherent vortices into the domain, the structures substantially distort the boundary layer, increasing the wall shear stress and raising kinetic energy levels of the turbulence. The fluid phase then returns to equilibrium as the vortices dissipate further downstream. The particle phase displays an analogous return to equilibrium following significant interactions with the fluid flow. The recovery of the dispersed phase is slower than for the fluid and is sensitive to the particle response time. The effects of inter-particle collisions are relatively strong and apparent throughout the boundary layer. Gravitational settling results in higher particle concentrations near the wall and correspondingly increases in collision rates.