Aerodynamic and acoustic study of a small scale lightly loaded hovering rotor using large eddy simulation

Abstract

The aerodynamics and acoustics of a small-scale lightly loaded hovering rotor operating at low Reynolds number is investigated using high-fidelity numerical simulation. A canonical geometry is considered, which allows for different flow topologies typical of transitional flows to develop along the span. Specifically, regions with laminar attached flow, trailing edge laminar separation with prominently 2D structures, and successive laminar separation, transition and reattachment with prominently 3D structures are identified. Furthermore, the lightly loaded configuration is conducive to significant blade-wake interaction, that in turn affects the dynamics of the boundary layer. The acoustic footprint of these phenomena are analyzed by correlating pressure fluctuations in the flow to those at the blade surface. It is found that the radiated noise is characterized by tonal peaks at the blade passing frequency and harmonics, and by a broadband hump at higher frequencies, consistent with experimental observations. The broadband hump exhibits distinct peaks that are found to originate from the separated shear layers on both pressure and suction sides. In particular, two noise generation mechanisms arising at two different frequencies in the broadband hump are identified. One is due to the interaction between an instability transported in the tip vortex of one blade and the leading edge of the following blade, and another one is found to rely on the interaction between an instability transported in the tip vortex and in the wake of one blade scattered by the trailing edge of the following blade.