Modelling Visibility in Particle-Fluid Flow Fields Generated by Helicopter Rotors

As a helicopter descends towards a bed of sand, a high velocity particle laden cloud can form around the helicopter body, a phenomenon known as “brownout”, and a consequence of which can potentially be a significant deterioration in visibility for the helicopter pilot. Here we consider a recently developed physically based rational mathematical model for the generation of wind-driven particle flow fields from otherwise static particle beds, one application of which is the scenario considered here. We 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 are varied. In particular, we demonstrate a counterintuitive result suggesting that, with specific yet potentially realistic parameter choices, pilot visibility may be improved in some viewing directions if the helicopter were hovering at a lower altitude.