In this paper, the interplay between the wing kinematics, resulting aerodynamics and structural dynamics, and sound generation of a flapping wing is investigated using a numerical method. A chordwise flexible two-dimensional wing in hover at the fruit fly scale is considered. The plunge amplitude and Young's modulus of the wing are varied to assess the influences of the resulting coupled unsteady aerodynamics and structural dynamics on the sound generation. The flow field around the wing is numerically calculated using a well-validated Navier-Stokes equation solver, fully coupled to a linear beam Euler-Bernoulli solver. Adapted Ffowcs-William-Hawking's equation is used to calculate the acoustic pressure based on computed flow field. Our results indicate that the resultant motion with the most flexible wing among the considered cases produces the highest mean lift coefficient C_L=3.3 with the highest maximum sound pressure level SPL_(max)=85 dB. The motion with the highest efficiency of 56% is produced by a moderately flexible wing, resulting in a relatively large C_L of 1 and a low SPL_(max) of 80.5 dB. The resulting lift, passive pitch, and sound generation of this optimal efficiency motion resembles the values reported in the literature for fruit flies. This implies that the natural flyers prefer efficiency and lower sound production over generating larger lift.
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