Straight-line climbing flight aerodynamics of a fruit bat

Abstract

From flight data obtained on a fruit bat, Cynopterus brachyotis, a kinematic model for straight-line flapping motion is extracted and analyzed in a computational fluid dynamics (CFD) framework to gain insight into the complexity of bat flight. The intricate functional mechanics and architecture of the bat wings set it apart from other vertebrate flight. The extracted kinematic model is simulated for a range of Reynolds numbers, to observe the effect these phenomena have on the unsteady transient mechanisms of the flow produced by the flapping wings. The Strouhal number calculated from the data is high indicating that the oscillatory motion dominates the flow physics. From the obtained data, the bat exhibits fine control of its mechanics by actively varying wing camber, wing area, torsional rotation of the wing, forward and backward translational sweep of the wing, and wing conformation to dictate the fluid dynamics. As is common in flapping flight, the primary force generation is through the attached unsteady vortices on the wing surface. The bat through varying the wing camber and the wing area modulates this force output. The power requirement for the kinematics is analyzed and correlated with the aerodynamic performance.

Bat flight: aerodynamics, kinematics and flight morphology

“…Interestingly, flight speed of the minimum angular velocity of the wing, which is directly related to the flight muscle contraction speed and thus the efficiency of the muscles, coincides with the flight speed of maximum L/D ( Fig. 7; von Busse et al, 2012). This would suggest that the bats are both aerodynamically and physiologically optimized for the same flight speed.…”

“…Many of the changes are found in a wide range of species and conform to general expectations for flapping flight. Among variables related to the velocity of the air meeting the wing (U eff , Eqn 1), we find that wing beat frequency decreases (Schnitzler, 1971;Norberg, 1976a;Aldridge, 1986;Lindhe Norberg and Winter, 2006;Riskin et al, 2010;Wolf et al, 2010;Hubel et al, 2012), stroke plane angle increases (becomes more vertical) Lindhe Norberg and Winter, 2006;Hubel et al, 2010;Aldridge, 1986;Riskin et al, 2010; but see Hubel et al, 2012 for contrasting results) and Strouhal number (St=fA/U 1 , where f is wingbeat frequency, A is peak-peak amplitude of the wing stroke and U 1 is forward airspeed) decreases with increasing flight speed in both micro-and megachiropterans von Busse et al, 2012;Lindhe Norberg and Winter, 2006). The flapping frequency is also expected to decrease with increasing size (Pennycuick, 2008;Bullen and McKenzie, 2002), which is indeed found in comparative data (Bullen and McKenzie, 2002;Riskin et al, 2010;Lindhe-Norberg and Norberg, 2012).…”

“…This has been suggested to allow for high levels of bending without failure of these bones when subjected to aerodynamic forces (Papadimitriou et al, 1996). However, kinematic analysis suggests relatively little bending of these bones during flight (von Busse et al, 2012). Unlike the other phalanges, which are laterally compressed, these bones are dorso-ventrally flattened or circular in cross section (Vaughan, 1959;Norberg, 1970Norberg, , 1972b.…”

“…At the lift to drag ratio (L/D) typical of bats, this may in fact be the case, suggesting that the observed upstroke vortices indicate bats are performing as well as they can considering their force demands (Muijres et al, 2012b). Below a critical speed (∼2.5 m s −1 in the species studied thus far) the backward speed of the outer wing of the inclined upstroke becomes higher than the forward flight speed (often referred to as a backward 'flick') (Norberg, 1976a,b;Aldridge, 1986Aldridge, , 1987von Helversen, 1986;Lindhe Norberg and Winter, 2006;Iriarte-Diaz et al, 2011;Wolf et al, 2010;von Busse et al, 2012), with the result that the outer wing generates aerodynamic force mainly perpendicular to the wing path (Hedenström et al, 2007). The hand wing works in an upside-down fashion, and the net force is directed upwards, but mainly forwards (Fig.…”

The Lift Problem in Flapping Forward Flight at Low Reynolds Numbers

“…Figure 18 shows the time histories of the lift coefficients of the flying bat in one period. The time history of Cl is similar to that for a fruit bat at a higher Reynolds number (Viswanath et al 2014 …”