Modeling Visibility and Surface Deformation in Helicopter Rotor Generated Particle-Fluid Flow Fields

Abstract

As a helicopter descends toward a bed of sand, a high-velocity particle-laden cloud can form around the helicopter body, a phenomenon known as brownout, a consequence of which can potentially be a significant deterioration in visibility for the helicopter pilot. This paper considers a physically based rational mathematical model for the generation of wind-driven particle flow fields from otherwise static particle beds, one application of which is this scenario. The authors introduce a directional opacity measure, defined for each observation angle from the helicopter cockpit, and show how visibility may vary in the model as certain parameters (the height of the helicopter, ratio of swirl velocity to downdraft velocity, and balance between lift and gravity in the interfacial layer between the sand bed and the fluidized region) vary, demonstrating a counterintuitive result suggesting that there exist potentially realistic parameter regimes for which pilot visibility may be improved in some viewing directions if the helicopter were hovering at a lower altitude. The paper also calculates the associated deformation of the upper surface of the particle bed and show how certain surface deformation features, including vertical depth and horizontal extent of the deformation, may be sensitive to variation of key parameters.