Computational Study of Rotorcraft Aerodynamics in Ground Effect and Brownout

When helicopters operate close to the ground in desert conditions, the rotor wake can entrain large amounts of dust into the flow field surrounding the aircraft. This entrainment of dust can result in the potentially dangerous condition known as brownout where the pilot loses situational awareness. Understanding the physics that governs the entrainment of dust from the ground may eventually allow the condition of brownout to be avoided completely. To enable the formation of dust clouds around helicopters to be investigated, Brown’s Vorticity Transport Model (VTM) has been enhanced to include the ability to model the entrainment of dust from the ground and the transport of this dust once in the flow field. Comparison of the predictions of the VTM with experimental results has shown the VTM to be capable of capturing the general characteristics of the dust clouds. Close examination of the formation of the dust clouds revealed that the general physics that governs the entrainment process is the same for different rotors and a universal model of this process is described. Differences in the size and density of the dust clouds that form around different rotors result from differences in the overall behaviour of the wakes that are generated. The design of a rotor can have a significant effect on the size and density of the dust cloud that is produced. The tip vortices have been identified as the main cause of the changes to the dust cloud. However, the behaviour of these tip vortices, when the rotor is operating in ground effect, is dependent on the rotor design and also on the advance ratio of the rotor. Thus, to determine the size and density of the dust cloud that would form around any particular rotor, the behaviour of the wake of that rotor must first be known.