Contributions Toward Understanding the Effects of Rotor and Airframe Configurations On Brownout Dust Clouds

Abstract

Brownout dust cloud simulations were conducted for rotorcraft undergoing representative landing maneuvers, primarily to examine the effects of different rotor placement and rotor/airframe configurations. The flow field generated by a helicopter rotor in ground effect operations was modeled by using an inviscid, incompressible, time-accurate Lagrangian free-vortex method, coupled to a semi-empirical approximation for the boundary layer flow near the ground. A surface singularity method was employed to represent the aerodynamic influence of a fuselage. A rigorous coupling strategy for the free-vortex method was developed to include the effects of rotors operating at different rotational speeds, such as a tail rotor. For the dispersed phase of the flow, particle tracking was used to model the dust cloud based on solutions to a decoupled form of the Basset-Boussinesq-Oseen equations appropriate to dilute gas particle suspensions of low Reynolds number Stokes flow. Important aspects of particle mobility and uplift in such vortically driven dust flows were modeled, which included a threshold-based model for sediment mobility and bombardment effects when previously suspended particles impact the bed and eject new particles. Various techniques were employed to reduce the computational cost of the dust cloud simulations, such as particle clustering and parallel programming using graphics processing units. The predicted flow fields near the ground and resulting dust clouds during the landing maneuvers were analyzed to better understand the physics behind their development, and to examine differences produced by various rotor and airframe configurations. Metrics based on particle counts and particle velocities in the field of view were developed to help quantify the severity of the computed brownout dust clouds. The presence of both a tail rotor and the fuselage was shown to cause both local and global changes to the aerodynamic environment near the ground and also influenced the development of the resulting dust clouds. Studies were also performed to examine the accuracy of self-induced velocities of vortex filaments by augmenting the straight-line vortex segments with a curved filament correction term. It was found that while curved elements can accurately recover the self-induced velocity in the case of a vortex ring, there existed bounds of applicability when extended to three-dimensional rotor wakes. Finally, exploratory two-dimensional and three-dimensional studies were performed to examine the effects of blade/particle collisions. The loss in particle kinetic energy during the collision was adopted as a surrogate metric to quantify the extent of potential blade erosion.