Abstract
Aerodynamic functions of the avian tail have been studied previously using observations of bird flight, physical models in wind tunnels, theoretical modelling and flow visualization. However, none of these approaches has provided rigorous, quantitative evidence concerning tail functions because (i) appropriate manipulation and controls cannot be achieved using live animals and (ii) the aerodynamic interplay between the wings and body challenges reductive theoretical or physical modelling approaches. Here, we have developed a comprehensive analytical drag model, calibrated by high-fidelity computational fluid dynamics (CFD), and used it to investigate the aerodynamic action of the tail by virtually manipulating its posture. The bird geometry used for CFD was reconstructed previously using stereo-photogrammetry of a freely gliding barn owl (Tyto alba) and we validated the CFD simulations against wake measurements. Using this CFD-calibrated drag model, we predicted the drag production for 16 gliding flights with a range of tail postures. These observed postures are set in the context of a wider parameter sweep of theoretical postures, where the tail spread and elevation angles were manipulated independently. The observed postures of our gliding bird corresponded to near minimal total drag.
Highlights
Avian tails have many aerodynamic functions during steady gliding
We measured the wake behind the same gliding barn owl using large-volume particle-tracking velocimetry (PTV), which tracked over 20 000 neutrally buoyant soap bubbles
We developed a comprehensive analytical drag model, calibrated by high-fidelity computational fluid dynamics (CFD), to investigate the aerodynamic action of the tail by virtually manipulating its posture
Summary
Avian tails have many aerodynamic functions during steady gliding. At slower speeds, tails spread and pitch downward, producing lift, which supplements that from the wings to support body weight and can adjust the pitching moment [4–7]. Tails can reduce drag and increase glide efficiency [8–12] in multiple ways. Tails might alter the flow over the body and prevent flow separation, effectively streamlining the bird, which reduces body drag [8]. The lift produced by the tail can alter the flow over the wings, modifying lift-induced (inviscid) drag [9]. The lift provided by the tail may allow the wing to operate at more efficient angles of attack, improving viscous efficiency [10]. We examine whether the correlation between tail spread and pitch with glide speed is consistent with enhancing efficient gliding through modulating this wide array of drag-reducing mechanisms
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