Evaluation of the Aerodynamic and Acoustic Performance of a Novel Passive Bat Deterrent for Wind Energy Applications

Abstract

The growth of wind energy installations and the continued increase in turbine size over recent decades have contributed to a concerning rise in bat fatalities. Current mitigation strategies suffer from limitations, with curtailment decreasing energy production and active acoustic bat deterrents constrained by atmospheric attenuation, preventing full coverage of the rotor swept area. Passive ultrasonic deterrents integrated along the length of turbine blades offer a potential alternative, but their aerodynamic impact has not been thoroughly investigated. Implementation in the field requires an understanding of these devices to ensure they do not impose a significant penalty on turbine efficiency. This study evaluates the aerodynamic and acoustic performance of a novel passive bat deterrent design that utilizes resonant cavities exposed to airflow over the surface of an airfoil to generate noise. Two wind tunnel experiments were conducted across a range of flow speeds and angles of attack. The first investigated larger cavities designed to generate lower-frequency tones using direct lift, surface pressure, and acoustic measurements. The second tested smaller resonators targeting ultrasonic frequencies using surface pressure, wake pressure deficit, and acoustic recordings. The large cavities generated noise at approximately 10 kHz, though the acoustic performance was irregular at low freestream velocities, with some lower than expected frequencies and occasional lobes rather than tonal peaks appearing in the spectra. These resonators were also more efficient in deflecting the wind tunnel jet, leading to a drop in lift. The smaller cavities produced sound at around 23 kHz, with no risk of human audible noise pollution at lower frequencies. Lift reductions were on the order of 1% across the majority of flow conditions, and the small resonators decreased drag between 2% and 10% only at high angles of attack. Additionally, the aerodynamic impact of the resonators was less significant than that of an upstream trip, and the trip only decreased the acoustic response at high angles of attack.