Fly low: The ground effect of a barn owl ( Tyto alba ) in gliding flight

Birds take advantage of the ground effect to improve their flight performance by flying low over ground. In this paper, we created a high-fidelity computational fluid dynamics model of a barn owl (Tyto alba) to study its ground effect in gliding flight. A computational fluid dynamics simulation shows that the ground effect leads to increases in the lift/drag ratio and span efficiency. Interestingly, the span efficiency exceeds one when the bird is below a certain ground clearance (h/c = 0.8). Such an estimation is under the condition of different weight supports; hence, there is no fair comparison for many parameters. Therefore, we used a vortex induction model validated by computational fluid dynamics to estimate the aerodynamics at different ground clearances under a constant weight support. As the ground blocks the downwash of the bird, an image wake system can equivalently replace the ground, forming a vortex system with four components: the wake vortex, bound vortex, image wake vortex and image bound vortex. The vortex induction model shows the vertical flows induced by these four vortex components. Such vertical flow can be used to estimate the drag production on the bird. As the ground clearance decreases, the drag due to the wake vortex and its image counterpart as a whole decreases, while that due to the bound vortex and its image counterpart increases slightly and then decreases. The remaining drag, namely, the zero-lift drag, undergoes a shallow “U” shape as a function of the ground clearance. We also analyzed the streamwise flow induction using this model and showed that the streamwise flow is reduced due to the ground effect, which might cause insufficient weight support at low speeds.