Phillips, C. and Brown, R.E. (2008) The effect of helicopter configuration on the fluid dynamics of brownout. In: 34th European Rotorcraft Forum, 2008-09-16 - 2008-09-19, Liverpool, UK.
Brown's Vorticity Transport Model, coupled to an additional particle transport model, is used to simulate the development of the dust cloud that can form around a helicopter when operating in dusty or desert conditions. The flow field around a tandem rotor configuration is simulated during the final stages of landing. The time-averaged flow field around the helicopter is characterised by the existence of two stationary points immediately adjacent to the ground plane. Almost all entrainment of dust into the flow takes place forward of the rearmost stationary point; the dust initially remains in a thin, sheet-like layer above the ground. As the dust sheet approaches the forward stationary point, the layer thickens and forms a characteristic wedge-shaped 'separation zone'. The amount of sand that is subsequently drawn up away from the ground then appears to be critically dependent on the strength and position relative to the separation zone of strong regions of recirculation. VTM simulations suggest that, for a tandem rotor helicopter at least, the sudden growth of the dust cloud that is responsible for the onset of brownout may be due to a change in mode within the flow field surrounding the aircraft. At higher advance ratios the flow is dominated by a strong ground vortex that is created by the rear rotor. The forward extent of the resultant dust cloud is limited though by the absence of any strong recirculation within the flow below the front rotor of the system. At lower forward speed the ground vortex of the rear rotor is replaced by a strong vortex that lies just below the leading edge of the front rotor. This vortex is responsible for drawing a significant amount of dust out of the surface layer of entrained particulates to form a dense wall of dust some distance upstream of the helicopter. A study of the effect of blade twist on the strength and shape of the dust cloud formed in the flow surrounding helicopters with tandem rotors suggests that systems with smaller blade twist but the same disc loading might produce denser dust clouds than those with high blade twist.
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