Numerical study of flexible flapping wings with an immersed boundary method: Fluid–structure–acoustics interaction

Abstract

The fluid–structure–acoustics interaction of flexible flapping wings is numerically studied by using an immersed boundary method at a Mach number of 0.1. In this study, a wing flapping in a uniform flow with prescribed leading edge motion is considered. Apart from rigid wings, flexible wings of various bending rigidities and mass ratios are also examined. By using a direct numerical simulation technique, the sound generation mechanism is identified. The numerical results show that the wing with combined translational and rotational motion generates smaller sounds than the translating wing, and larger rotational angles transform the dipole sound to a monopole one. Similar sound fields are observed among all these wings, except that the direction of the sound shifts to the downstream for large flexible wings. Frequency analysis shows that the sound is dominated by the flapping frequency and two times of the flapping frequency in the vertical and horizontal directions, respectively. The results also show that the thrust increases first then decreases with bending rigidity for all three mass ratios considered, and the lighter wing suffers more decrease. However, the fluctuating pressure for the three mass ratios varies significantly. Specifically, it increases significantly with the decreasing bending rigidity when the structural inertia is dominating at m∗=5.0; but it experiences a reduction contrarily when the aerodynamics becomes dominant at m∗=0.5; for the medium mass ratio at m∗=1.0, no significant effects are observed. The comparisons indicate that the flexible wing with a lower mass ratio (e.g. m∗=0.5) and a medium flexibility (e.g. ω∗=0.3) achieves lower sound generation without significant thrust decrease. In addition, the sound on the windward side is pronounced significantly when the wing is flapping with a stroke plane angle less than 90o. The present results can expand the currently limited database of fluid–structure–acoustics interaction, and also provide an insight for the optimization of flight vehicle using flapping wing.