Ear-body lift and a novel thrust generating mechanism revealed by the complex wake of brown long-eared bats (Plecotus auritus)

Abstract

Large ears enhance perception of echolocation and prey generated sounds in bats. However, external ears likely impair aerodynamic performance of bats compared to birds. But large ears may generate lift on their own, mitigating the negative effects. We studied flying brown long-eared bats, using high resolution, time resolved particle image velocimetry, to determine the aerodynamics of flying with large ears. We show that the ears and body generate lift at medium to cruising speeds (3-5 m/s), but at the cost of an interaction with the wing root vortices, likely reducing inner wing performance. We also propose that the bats use a novel wing pitch mechanism at the end of the upstroke generating thrust at low speeds, which should provide effective pitch and yaw control. In addition, the wing tip vortices show a distinct spiraling pattern. The tip vortex of the previous wingbeat remains into the next wingbeat and rotates together with a newly formed tip vortex. Several smaller vortices, related to changes in circulation around the wing also spiral the tip vortex. Our results thus show a new level of complexity in bat wakes and suggest large eared bats are less aerodynamically limited than previous wake studies have suggested.

Figure 1. Iso-surface plots of the Q-criterion (2500) colored by vertical flow speed (red upwards, blue downwards), at speeds U = 1 m/s (a–c), U = 2 m/s (d–f) and U = 4 m/s (g–i) seen obliquely from above and in front (a,d,g), from above (b,e,h) and from the side (c,f,i).

Arrows represent flight direction and all panels are scaled according to the scale bar in panel (g). The color is scaled relative to mean absolute vertical speed + 3*SD of the vertical speed. Maximum red color thus represents 2.8 m/s at U = 1 m/s, 1.3 m/s at U = 2 m/s and 1.1 m/s at U = 4 m/s.

Figure 2. Iso-surface plot of Q-criterion (2500) showing the vortices generated at the transition between upstroke and downstroke at 2 m/s, viewed obliquely from above and behind.

Vortex structures showing upwards and backwards induced flow, formed as the wing performs a pronating, pitch down, motion at the transition are colored purple. Flight direction is indicated by arrow. Rotatable 3D image is available in the SI.

Figure 3. Iso-surface plot of Q-criterion (2500) showing the details of vortices generated at the transition between upstroke and downstroke at 3 m/s, viewed obliquely from above and behind.

The tip vortex (red), reduces in strength during the upstroke resulting in shedding of stop vortices (orange). Towards the end of the upstroke the wing tip sheds a tip vortex (blue) of opposite sense of rotation to the normal tip vortex. At the beginning of the next downstroke, a start vortex is shed (light green). The circulation builds up during the downstroke as indicated by additional start sense vortices (dark green) being shed. The resulting tip vortex constitutes several vortices spiraling around each other. Flight direction indicated by arrow. Rotatable 3D image is available in the SI.

Figure 4. Iso-surface plot of Q-criterion (2000) showing the body/wing root wake at 4 m/s during a fraction of the downstroke seen from above, colored by vertical speed (red upward, blue downward) (A) and streamwise flow relative to free stream (red backward, blue forward) (B).

Multiple vortex ring structures form during a wing beat showing upward and forward induced flow, indicating negative weight support and drag. Flight direction is indicated by the arrow between A and B. Vector field, smoothed once, of the induced flow at mid downstroke of a bat flying at 4 m/s (C). Colors represent streamwise vorticity, with red being counter clockwise and blue clockwise rotation. Note the clear downwash behind the body and the upwash between the body and the wing root vortices.