Bio-inspired rotor designs: enhancing thrust, energy efficiency, and noise reduction

Urban noise pollution is an increasingly pressing concern, driven by rapid infrastructural development and evolving environmental regulations. Among its most significant sources is the aeroacoustic emission from mechanical ventilation systems, where fan noise, comprising both tonal and broadband components, can be particularly disruptive. Inspired by the silent flight of owls, this study investigates the potential of trailing-edge serrations as a passive noise-reduction strategy for fan blades. A combined numerical and experimental approach is adopted. The acoustic performance is predicted using a hybrid methodology that couples large eddy simulations (LES) of the incompressible Navier-Stokes equations in their vorticity formulation with acoustic analogy models to capture far-field noise characteristics. A sensitivity study examines the influence of key geometrical parameters, specifically the number of serrations and the sawtooth ratio, defined in terms of pitch and depth. Results show that adjustments to these parameters allow for noticeable noise reductions, with improvements reaching up to 5 decibels. Although analyses are conducted at constant rotational speed, only marginal reductions in thrust and drag are observed, with aerodynamic efficiency remaining essentially unchanged. Flow analysis reveals that serrations enhance spanwise flow coherence, contributing to the passive stabilisation of turbulence near the trailing edge and blade tip. Experimental tests at varying rotational speeds support and extend the numerical findings, enabling a broader assessment across operating conditions. A multi-criteria evaluation framework is proposed to identify optimal serration configurations. These results provide valuable insights into bioinspired noise-control strategies and offer a foundation for the development of predictive tools for the design of next-generation low-noise fan systems.