  Novel ultrasonic bat deterrents based on aerodynamic whistles are
investigated experimentally and numerically. The baseline deterrent, a
single-whistle design inspired by Beeken [1], is examined first. It consists of
two resonating cavities/chambers. The whistle is ``powered'' by a regulated
high-pressure air supply and the performance of the whistle is examined for a
range of supply pressures. Farfield acoustic measurements in the $20$ Hz - $50$
kHz frequency range are made in an anechoic chamber. The noise measurements are
supplemented with two- and three-dimensional unsteady Reynolds-averaged
Navier-Stokes (uRANS) simulations to investigate the mechanisms of ultrasound
generation. The Ffowcs Williams-Hawkings acoustic analogy is used with the 3-D
uRANS results to predict the far field radiation. The farfield acoustic
predictions are in good agreement with the measurements in the anechoic
chamber. The peak frequency (fundamental) of the radiated ultrasound of the
baseline deterrent is approximately $23$ kHz. Harmonics and a sub-harmonic of
the fundamental tone are also observed. The numerical simulations show that the
two resonating chambers of the baseline deterrent operate out-of-phase and
Helmholtz resonance determines the whistling frequency over the range of supply
air pressure considered.
  A six-whistle deterrent targeting a broad spectral coverage in the $20-50$
kHz frequency range is designed, fabricated and tested in the anechoic chamber.
Each whistle in the deterrent is obtained by geometrically scaling the baseline
whistle. Measurements show that the six-whistle ultrasound deterrent generates
ultrasound with six dominant peaks in the designed frequency range when the
supply air pressure exceeds $5$ psig. The proposed ultrasound devices can serve
as effective bat deterrents for a variety of purposes including reducing bat
mortality at wind turbines.
