A Computational Study on Rotor and Fuselage Configuration Effect on Rotorcraft Brownout

Rotorcraft brownout is an in-flight visibility restriction caused by clouds of sand and dust particles during landing, takeoff, and near-ground flight operations in arid desert terrain. This complex phenomenon is caused by the entrainment of dust, sand, and ground particles by rotor downwash and is compounded by fuselage geometry and its orientation with respect to the ground. Highly unsteady wind velocities are common in near-ground operations and play a significant role in the particulate cloud’s behavior that creates the brownout condition. Experiments and flight tests to understand brownout are challenging, expensive, and risky. Alternatively, computational fluid dynamics (CFD) has been used extensively over the past few decades to study rotorcraft aerodynamics. However, there are additional computational challenges associated with modeling the dust particle transport in a brownout. In this work, a computationally efficient Eulerian-based framework has been developed to model rotorcraft brownout. The flowfield is modeled by Reynolds averaged Navier–Stokes (RANS) equation and is solved using the SIMPLER algorithm. Turbulence properties are modeled using Realizable k – ε equations, while the rotor is modeled as a momentum source to focus on the global flowfield rather than the flow near the rotors. The Eulerian approach for both the flowfield and the dust transport allows computationally efficient and rapid analysis, taking an order of few hours in a single CPU to a fraction of an hour using GPU-based computation. In this work, results from two sets of experiments are presented. At first, a study on the fuselage’s effect on brownout with respect to height for a single-rotor configuration in hover is demonstrated. Next, a study on the brownout characteristics of three rotor–fuselage configurations in hover, namely single-rotor, tandem-rotor, and quad-rotor, is presented. It has been observed that the ground friction velocity profile and flowfield vorticity around the rotorcraft play a significant role in dust cloud behavior. Additionally, the body forces and interference due to the fuselage plays a vital role in the formation of brownout dust clouds. The experiments showed that the quad-rotor is worst affected in terms of size and height of the dust clouds. However, the tandem-rotor has been found to be worst in the terms of average dust density.