The shape of a foil undergoing a combined pitching heaving motion is critical to its design in applications that demand high efficiency and thrust. This study focuses on understanding of how the shape of a foil affects its propulsive performance. We perform two-dimensional numerical simulations of fluid flows around a flapping foil for different governing parameters in the range of biological swimmers and bio-inspired underwater vehicles. By varying the foil shape using a class-shape transformation method, we investigate a broad range of foil-like shapes. In the study, we also show consistent results with previous studies that a thicker leading-edge and sharper trailing-edge makes for a more efficient foil shape undergoing a flapping motion. In addition, we explain that the performance of the foil is highly sensitive to its shape, specifically the thickness of the foil between the 18th and 50th percent along the chord of the foil. Moreover, we elucidate the flow mechanisms behind variations in performance metrics, particularly focused on constructive interference between the vortices generated at the leading-edge with the trailing-edge vortex, as well as the pressure field differences that lead to higher power consumption in less efficient foil shapes.
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WAKE STRUCTURES AND EFFECT OF HYDROFOIL SHAPES IN EFFICIENT FLAPPING PROPULSION
In this work, direct numerical simulation (DNS) is used to
investigate how airfoil shape affects wake structure and
performance during a pitching-heaving motion. First, a classshape transformation (CST) method is used to generate airfoil
shapes. CST coefficients are then varied in a parametric study to
create geometries that are simulated in a pitching and heaving
motion via an immersed boundary method-based numerical
solver. The results show that most coefficients have little effect
on the propulsive efficiency, but the second coefficient does have
a very large effect. Looking at the CST basis functions shows that
the effect of this coefficient is concentrated near the 25% mark
of the foils chord length. By observing the thrust force and
hydrodynamic power through a period of motion it is shown that
the effect of the foil shape change is realized near the middle of
each flapping motion. Through further inspection of the wake
structures, we conclude that this is due to the leading-edge vortex
attaching better to the foil shapes with a larger thickness around
25% of the chord length. This is verified by the pressure
contours, which show a lower pressure along the leading edge of
the better performing foils. The more favorable pressure gradient
generated allows for higher efficiency motion.
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- Award ID(s):
- 1931929
- NSF-PAR ID:
- 10473190
- Publisher / Repository:
- ASME Fluids Engineering Division Summer Meeting
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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