Experimental Investigation of Aerodynamic Characteristics of Bat Carcasses after Collision with a Wind Turbine

Abstract

Abstract. Large number of bat fatalities have been reported in wind energy facilities in different parts of the world. The wind farm regulators are required to monitor the bat fatalities by conducting carcass survey in the wind farms. Previous studies have implemented ballistic model to characterize the carcass fall zone after strike with turbine blades. Ballistic model contains the aerodynamic drag force term which is dependent upon carcass drag coefficient. The bat carcass drag coefficient is highly uncertain and of which no measurement is available. This manuscript introduces a new methodology for bat carcass drag coefficient estimation. Field investigation at Macksburg wind farm resulted in the discovery of three bat species: Eastern Red bat (Lasiurus borealis), Hoary bat (Lasiurus cinereus) and Evening bat (Nycticeius humeralis). Carcass drop experiments were performed from a dropping platform at finite height and carcass position time series data was recorded using a high-speed camera. Falling carcasses were subjected to aerodynamic drag and gravitational force. Carcasses were observed to undergo rotation; often rotating around multiple axes simultaneously and lateral translation. The carcass complex fall dynamics along with drop from limited height prohibits it from attaining the terminal velocity. Under this limitation, drag coefficient can be estimated by fitting ballistic model to the measured data. A new multivariable optimization algorithm was performed to find the best-fit of the ballistic model to the measured data resulting in an optimized drag coefficient estimate. Sensitivity analysis demonstrated significant variation in drag coefficient with small a change in initial position highlighting the chaotic nature of carcass fall dynamics. Based on the limited sampling, the bat carcass drag coefficient range was found to be between 0.70–1.23.