Abstract
The importance of the leading-edge sweep angle of propulsive surfaces used by unsteady swimming and flying animals has been an issue of debate for many years, spurring studies in biology, engineering, and robotics with mixed conclusions. In this work, we provide results from three-dimensional simulations on single-planform finite foils undergoing tail-like (pitch-heave) and flipper-like (twist-roll) kinematics for a range of sweep angles covering a substantial portion of animals while carefully controlling all other parameters. Our primary finding is the negligible 0.043 maximum correlation between the sweep angle and the propulsive force and power for both tail-like and flipper-like motions. This indicates that fish tails and mammal flukes with similar range and size can have a large range of potential sweep angles without significant negative propulsive impact. Although there is a slight benefit to avoiding large sweep angles, this is easily compensated by adjusting the fin’s motion parameters such as flapping frequency, amplitude and maximum angle of attack to gain higher thrust and efficiency.
Highlights
A large fraction of swimming and flying animals use propulsive flapping to move, and there are significant potential applications for this biologically inspired form of propulsion in engineering and robotics
Significant research effort has been devoted towards understanding and optimizing both the kinematics and morphology of the propulsion surface itself, with a particular emphasis on explaining propulsive efficiency
The leading edge sweep back angle is a key geometric feature observed in fish tails, mammal flukes, aquatic-animal flippers, and bird and insect wings
Summary
A large fraction of swimming and flying animals use propulsive flapping to move, and there are significant potential applications for this biologically inspired form of propulsion in engineering and robotics. Collection of biological data for sweep angles L and aspect ratio for fish caudal fins and mammal flukes (in red) and wings, flippers, pectoral, and dorsal fins (in blue) showing the extremely wide scatter across this evolutionary parameter space for different successful animals. The tail-like kinematics are inspired by biological data for fish caudal fins and mammal flukes and made up coupled heave and pitch motions (figure 2b). The majority of the simulations use a fairly low-frequency k* = 0.3 and high amplitude 2A = l* condition to imitate the average mammal-fluke kinematics as reported in biological data [1], and the Strouhal number St = 0.3 is fixed in the middle of the optimal propulsive efficiency range [27].
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