Canonical description of wing kinematics and dynamics for a straight flying insectivorous bat (Hipposideros pratti)

Abstract

Bats, with highly articulated wings, are some of the most agile flyers in nature. A novel three-dimensional geometric decomposition framework is developed to reduce the complex kinematics of a bat wing into physical movements used to describe flapping flight: namely flapping, stroke plane deviation and pitching, together with cambering and flexion. The decomposition is combined with aerodynamic simulations to investigate the cumulative effect of each motion on force production, and their primary contribution to the unsteady vortex dynamics. For the nearly straight and level flight of Hipposideros pratti , results show that the flapping motion by itself induced a moderate drag and lift. Stroke plane deviation increased lift, and nullified the drag. With the inclusion of the pitching motion into the kinematics, lift production further increased by a factor of more than 2.5, and exhibited a positive net thrust by virtue of the favorable wing orientation during the upstroke. The primary contribution of cambering, which included a maximum chord line displacement of ≈40% standard mean chord, was the stabilization of the leading edge vortex during the downstroke. This increased mean lift by about 35% at the expense of net thrust. Flexion was perhaps the most complex motion with maximum displacements of 75% standard mean chord. This was instrumental in reducing the negative lift during the upstroke by preventing the formation of strong leading edge vortices. The aerodynamic effective angle of attack emerged as a heuristic parameter to describe lift and net thrust production across movements.

Turning-ascending flight of a Hipposideros pratti bat

“…The current bat flies with progressively increasing vertical angles (g v ) for both wings, whereas the previous sweeping turn [49] had somewhat stable values for most of the flight time. The magnitude of g v measured in previous studies by Windes et al [35,49] and Sekhar et al [39] for similar-sized bats and by Iriarte-Díaz & Swartz [44] for a much smaller bat, in straight as well as manoeuvring flight, has ranged between 40°and 60°. Whereas, the current bat shows a clear asymmetry between the right and left wings, with the left wing angle increasing from 55°to 74°and the right wing angle from 35°to 67°.…”

“…[ 35 , 49 ] and Sekhar et al . [ 39 ] for similar-sized bats and by Iriarte-Díaz & Swartz [ 44 ] for a much smaller bat, in straight as well as manoeuvring flight, has ranged between 40° and 60°. Whereas, the current bat shows a clear asymmetry between the right and left wings, with the left wing angle increasing from 55° to 74° and the right wing angle from 35° to 67°.…”

“…Thus, the current study follows a computational approach to deconstruct the detailed kinematic–aerodynamic nuances of a manoeuvring bat flight. This approach has been used in the literature mostly for level flight [ 31 , 35 , 39 ] and recently for manoeuvring flight by Windes et al . [ 49 , 50 ].…”

Analysis of a 180-degree U-turn maneuver executed by a hipposiderid bat

“…Validation of the computational framework for simulating bat flight has been conducted as a part of several of our prior studies (see [ 17 – 19 ]). However, in order to provide additional validation for this specific flight case, a mass dynamics analysis was run.…”

“…In the present study, a structured Cartesian fluid grid containing 69.1 million cells was used for the simulation. In the proximity of the bat, the fluid grid was refined to approximately 44 cells per wing chord length based on our prior grid independence analysis for similar simulations of bat flight [ 17 , 19 ]. Validation of the computational method including selection of the grid refinement level is discussed further in the results section.…”