Sensitivity of Laminar Separation Noise from a Rotor to Inflow Turbulence

Abstract

The goal of this paper is to deepen our fundamental knowledge of rotor aeroacoustics in transitional boundary layer regimes and, more specifically, in the presence of a laminar separation bubble. A numerical study is conducted by using a scale-resolving wall-modelled lattice Boltzmann solver and the same reference configuration used in our previous works. The main focus of the present research is on the effects due to test-room confinement, and consequent flow recirculation, on the noise generated by the rotor in hover and in axial flow conditions. This new effort follows our hypothesis of inflow disturbances altering the mechanism of the laminar separation, and thus responsible for the broadband noise prediction discrepancy previously observed in hover conditions in free-field. Simulations carried out in a confined environment that reproduces the A-Tunnel test chamber of Delft University of Technology refute our initial hypothesis. Conversely, simulations with increased mesh resolution along the blade leading edge compare more favorably with the experiments. Therefore, we conclude that, for the considered configuration, the room confinement does not affect the laminar separation bubble mechanism, but causes the emergency, in hover conditions, of harmonics of the blade passing frequency. As a secondary outcome of this work, a source identification technique based on the acoustic analogy approach is presented for the first time.