Effects of flapping wing kinematics on the aeroacoustics of hovering flight

Despite the recent interest in flapping wing aerodynamics, the aerodynamic sound generation mechanism of a flapping wing is inadequately understood. In this paper, the interplay between the wing motion, resulting unsteady aerodynamics, and aeroacoustics of a flapping wing flyer is investigated. The wing motion is varied in terms of the flapping amplitude, pitching amplitude, and the phase difference between flap and pitch. The unsteady flow around the flapping wing is numerically calculated using a well-validated Navier-Stokes equation solver. Acoustic pressures are computed using the FfowcsWilliams-Hawkings equation in a three-dimensional space at varying distances from the wing. Two main sound generation mechanisms are found. The flapping motion induces the highest sound pressure level (SPL) in the stroke plane. Furthermore, the SPL peaks under the wing due to the wake induced by lift generation. Effective motions, generating the highest lift and lowest SPL, are found when the flap and pitch amplitudes are high and when the pitch rotation is delayed with respect to flap. These results suggest that the delayed rotations observed for small hovering insects may aim to minimize sound production rather than maximize lift generation. (C) 2018 Elsevier Ltd. All rights reserved.