Impact of Wake Vortices on Particle Trajectories of a Forward-Flight Helicopter

Helicopters operating in sandy, dusty, or snowy environments face severe risks due to the particle-induced brownout phenomenon. This study numerically examines flow evolution and particle distribution in forward flight to clarify fluid–particle interactions. Particles are observed to couple strongly with wake vortices, accumulating beneath the rotor disk and around the blade tips. Asymmetric tip vortices expand the brownout region on the advancing side, while centrifugal forces expel particles from vortex cores, revealing a critical vorticity threshold for accumulation. The streamwise extent of brownout increases nearly linearly with the advance ratio, whereas the spanwise extent decreases, offering guidance for safe formation flight. A semi-analytical model shows that time-averaged drag, pressure gradient, and gravity govern the particle motion, with drag and pressure dominating at higher advance ratios or for smaller particles, thereby delaying downwash and enlarging the brownout region.