Predicting Brownout with Coarse-Grained Discrete Element Method: Effects of Interphase Coupling

In desert flight, rotor-induced downwash flow can stir up dense sand particle clouds, i.e., brownout, severely degrading visibility and threatening flight safety. Traditional methods struggle to balance the accuracy and cost of numerical simulations. A full-scale tracking of particles is computationally prohibitive, while a reduced particle number only allows qualitative analysis. To address this, we apply a coarse-grained discrete element method (CG-DEM) with a four-way coupling scheme in predicting brownout for the first time, which incorporates both particle collisions and particle-to-fluid momentum feedback. We perform a systematic investigation of the role of fluid–particle interphase coupling in rotor-induced flow and particle cloud evolution. The results show that the flowfield and brownout development are critically influenced by the interphase coupling. The airborne particles attenuate flow velocity, wall-shear stress, and turbulence, which thickens the boundary layer by about 50% and suppresses vortex pairing. For sand particles, the interphase coupling yields approximately 140% fewer suspended particles and slows down the development of the particle cloud. Moreover, the mean particle size is smaller in the four-way coupling simulation due to a stronger sieving effect. The present study identifies the critical role of the interphase coupling in accurate simulation of brownout and also clarifies the pivotal physical mechanisms.