Impact of inter-wing spacing on aerodynamic noise in multi-flapping wing configurations

Investigation into noise reduction techniques for flapping-wing micro aerial vehicles is crucial for enhancing their application potential. The phenomenon of clustering among flying animals has garnered considerable research interest, as it provides valuable insights for the development of aircraft. While extensive research has concentrated on optimal aerodynamic performance and propulsion mechanisms, the acoustic characteristics of multi-flapping wings remain underexplored. This study examined the impact of inter-wing spacing on the aerodynamic noise of three-dimensional multi-wing configurations. The wings, subjected to a uniform flow of 10 m/s at a Reynolds number of 5300, underwent heaving motion. We employed the large eddy simulation (LES) method and FW-H acoustic analogy to numerically simulate the flow and sound fields. The results indicate that a single flapping wing generates an up-and-down symmetric sound field, with peak noise levels occurring directly above and below the wing's path. In multi-wing configurations with smaller individual spacings, significant aerodynamic interference leads to increased sound pressure levels (SPL). Conversely, increasing the spacing between wings reduces both aerodynamic interference and overall noise. For tandem configurations, the SPL peaks at a horizontal spacing of 1.5 chord lengths (c) and reaches its minimum at 3c, with a reduction of nearly 4 dB. In the diamond configuration with four wings, the SPL is minimized at a horizontal spacing of 3.5c, showing a 3.6 dB reduction from the maximum value at 1.5c. The vertical individual spacing has a negligible effect on the overall noise in the diamond configuration. These findings suggest an optimal inter-wing spacing that minimizes overall noise, offering critical insights for the design of quiet multi-wing aircraft and swarm flight.