A computational investigation of lift generation and power expenditure of Pratt’s roundleaf bat (Hipposideros pratti) in forward flight

Abstract

The aerodynamic mechanisms of bat flight have been studied using a numerical approach. Kinematic data acquired using a high resolution motion capture system was employed to simulate the unsteady air flow around a bat’s wings. A flapping bat wing contains many degrees of freedom, which make 3D motion tracking challenging. In order to overcome this challenge, an optical motion capture system of 21 cameras was used to reduce wing self-occlusion. Over the course of a meter-long flight, 108 discrete marker points on the bat’s wings (Pratt’s roundleaf bat, Hipposideros pratti) were tracked. The time evolution of the surface of each wing was computationally reconstructed in 3D space. The resulting kinematic model was interfaced with an unsteady incompressible flow solver using the immersed boundary method (IBM) and large eddy simulation (LES). Verification and validation of the flow simulation were conducted to establish accuracy. The aerodynamic forces calculated from the simulation compared well to the forces theoretically needed to sustain the observed flight trajectory. The transient flow field generated by the simulation allowed for the direct calculation of lift, drag, and power output of the bat during flight. The mean lift coefficient was found to be 3.21, and the flap cycle averaged aerodynamic power output was 1.05 W. Throughout the flap cycle, the planform area of the wings varied up to 46% between the largest and smallest values. During the upstroke, wing rotation was found to mitigate negative lift thereby improving overall flight efficiency. The high resolution motion capture and flow simulation framework presented here has the potential to facilitate the understanding of complex bat flight aerodynamics for both straight and maneuvering flight modes.

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.…”

“…During solution of Navier-Stoke, a no-slip boundary condition is imposed at the location of the embedded wing mesh on both the top and bottom surface of the mesh. Further details on the implementation of the method are given in Windes et al [ 17 , 18 ].…”

“…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.…”

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

“…Raw wing kinematic data was obtained from a prior study [43,52], in which a 3D optical motion capture system was used to record an adult male H . pratti in straight flight.…”

“…The experimental setup consisted of an array of 21 cameras inside a flight tunnel with a cross section of 1.2 m × 1.2 m, and an approximate length of 4 m. The Svoboda multi-camera self-calibration open source MATLAB package was used for the calibration of the camera array [53]. Additional details related to the experimental facility, motion capture system and the kinematic dataset pre-processing set-up are described in the authors prior work [43,52].…”