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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Propulsive performance and vortex wakes of multiple tandem foils pitching in-line
Numerical studies are presented on the propulsive performance and vortex dynamics
of multiple hydrofoils pitching in an in-line configuration. The study is motivated by
the quest to understand the hydrodynamics of multiple fin–fin interactions in fish
swimming. Using the flow conditions (Strouhal and Reynolds numbers) obtained from
a solitary pitching foil of zero net thrust, the effect of phase differences between
neighboring foils on the hydrodynamic performance is examined both in position-fixed
two- and three-foil systems at Reynolds number Re = 500. It is found that the threefoil
system achieves a thrust enhancement up to 118% and an efficiency enhancement
up to 115% compared to the two-foil system. Correspondingly, the leading-edge vortex
(LEV) and the trailing-edge vortex (TEV) of the hindmost foil combine to form a ‘2P’
wake structure behind the three-foil system with the optimal phase differences instead
of a ‘2S’ wake, a coherent wake pattern observed behind the optimal two-foil system.
The finding suggests that a position-fixed three-foil system can generate a ‘2P’ wake
to achieve the maximum thrust production and propulsive efficiency simultaneously
by deliberately choosing the undulatory phase for each foil. When increasing Reynolds
number to 1000, though the maximum thrust and propulsive efficiency are not achieved
simultaneously, the most efficient case still produces more thrust than most of the other
cases. Besides, the study on the effects of three-dimensionality shows that when the foils
have a larger aspect ratio, the three-foil system has a better hydrodynamic performance,
and it follows a similar trend as the two-dimensional (2D) foil system. This work aids
in the future design of high-performance underwater vehicles with multiple controlled
propulsion elements.
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- Award ID(s):
- 1931929
- NSF-PAR ID:
- 10473206
- Publisher / Repository:
- ELSEVIER
- Date Published:
- Journal Name:
- Journal of Fluids and Structures
- Volume:
- 108
- Issue:
- C
- ISSN:
- 0889-9746
- Page Range / eLocation ID:
- 103422
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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In this study, we numerically investigate the effects of the tail-beat phase differences between the trailing fish and its neighboring fish on the hydrodynamic performance and wake dynamics in a two-dimensional high-density school. Foils undulating with a wavy-like motion are employed to mimic swimming fish. The phase difference varies from 0° to 360°. A sharp-interface immersed boundary method is used to simulate flows over the fish-like bodies and provide quantitative analysis of the hydrodynamic performance and wakes of the school. It is found that the highest net thrust and swimming efficiency can be reached at the same time in the fish school with a phase difference of 180°. In particular, when the phase difference is 90°, the trailing fish achieves the highest efficiency, 58% enhancement compared with a single fish, while it has the highest thrust production, increased by 108% over a single fish, at a phase difference of 0°. The performance and flow visualization results suggest that the phase of the trailing fish in the dense school can be controlled to improve thrust and propulsive efficiency, and these improvements occur through the hydrodynamic interactions with the vortices shed by the neighboring fish and the channel formed by the side fish. In addition, the investigation of the phase difference effects on the wake dynamics of schools performed in this work represents the first study in which the wake patterns for systems consisting of multiple undulating bodies are categorized. In particular, a reversed Bénard–von Kármán vortex wake is generated by the trailing fish in the school with a phase difference of 90°, while a Bénard–von Kármán vortex wake is produced when the phase difference is 0°. Results have revealed that the wake patterns are critical to predicting the hydrodynamic performance of a fish school and are highly dependent on the phase difference.
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null (Ed.)Scaling laws for the thrust production and power consumption of a purely pitching hydrofoil in ground effect are presented. For the first time, ground-effect scaling laws based on physical insights capture the propulsive performance over a wide range of biologically relevant Strouhal numbers, dimensionless amplitudes and dimensionless ground distances. This is achieved by advancing previous scaling laws (Moored & Quinn ( AIAA J. , 2018, pp. 1–15)) with physics-driven modifications to the added mass and circulatory forces to account for ground distance variations. The key physics introduced are the increase in the added mass of a foil near the ground and the reduction in the influence of a wake-vortex system due to the influence of its image system. The scaling laws are found to be in good agreement with new inviscid simulations and viscous experiments, and can be used to accelerate the design of bio-inspired hydrofoils that oscillate near a ground plane or two out-of-phase foils in a side-by-side arrangement.more » « less
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Gorb, S. (Ed.)Through computational fluid dynamics (CFD) simulations of a model manta ray body, the hydrodynamic role of manta-like bioinspired flapping is investigated. The manta ray model motion is reconstructed from synchronized high-resolution videos of manta ray swimming. Rotation angles of the model skeletal joints are altered to scale the pitching and bending, resulting in eight models with different pectoral fin pitching and bending ratios. Simulations are performed using an in-house developed immersed boundary method-based numerical solver. Pectoral fin pitching ratio (PR) is found to have significant implications in the thrust and efficiency of the manta model. This occurs due to more optimal vortex formation and shedding caused by the lower pitching ratio. Leading edge vortexes (LEVs) formed on the bottom of the fin, a characteristic of the higher PR cases, produced parasitic low pressure that hinders thrust force. Lowering the PR reduces the influence of this vortex while another LEV that forms on the top surface of the fin strengthens it. A moderately high bending ratio (BR) can slightly reduce power consumption. Finally, by combining a moderately high BR = 0.83 with PR = 0.67, further performance improvements can be made. This enhanced understanding of manta-inspired propulsive mechanics fills a gap in our understanding of the manta-like mobuliform locomotion. This motivates a new generation of manta-inspired robots that can mimic the high speed and efficiency of their biological counterpart.more » « less
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MDPI (Ed.)Through computational fluid dynamics (CFD) simulations of a model manta ray body, the hydrodynamic role of manta-like bioinspired flapping is investigated. The manta ray model motion is reconstructed from synchronized high-resolution videos of manta ray swimming. Rotation angles of the model skeletal joints are altered to scale the pitching and bending, resulting in eight models with different pectoral fin pitching and bending ratios. Simulations are performed using an in-house developed immersed boundary method-based numerical solver. Pectoral fin pitching ratio (PR) is found to have significant implications in the thrust and efficiency of the manta model. This occurs due to more optimal vortex formation and shedding caused by the lower pitching ratio. Leading edge vortexes (LEVs) formed on the bottom of the fin, a characteristic of the higher PR cases, produced parasitic low pressure that hinders thrust force. Lowering the PR reduces the influence of this vortex while another LEV that forms on the top surface of the fin strengthens it. A moderately high bending ratio (BR) can slightly reduce power consumption. Finally, by combining a moderately high BR = 0.83 with PR = 0.67, further performance improvements can be made. This enhanced understanding of manta-inspired propulsive mechanics fills a gap in our understanding of the manta-like mobuliform locomotion. This motivates a new generation of manta-inspired robots that can mimic the high speed and efficiency of their biological counterpartmore » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jfluidstructs.2021.103422
