arXiv subjects

Zhonglu Lin

Publications and source records attributed to Zhonglu Lin.

6 recordsLinked to original sources

arXiv2024

Numerical Study of Flow Past a Wall-Mounted Dolphin Dorsal Fin at Low Reynolds Numbers

Dolphin swimming has been a captivating area of study, yet the hydrodynamics of the dorsal fin remain underexplored. In this study, we present three-dimensional simulations of flow around a wall-mounted dolphin dorsal fin, derived from a real dolphin scan. The NEK5000 (spectral element method) is employed with a second-order hex20 mesh to ensure high accuracy and computational efficiency in the simulations. A total of 13 cases were simulated, covering angles of attack (AoA) ranging from $0^\circ$ to $60^\circ$ and Reynolds numbers ($\text{Re}$) between 691 and 2000. Our results show that both drag and lift increase significantly with the AoA. Almost no vortex is observed at $\text{AoA} = 0^\circ$, whereas complex vortex structures emerge for $\text{AoA} \geq 30^\circ$, including half-horseshoe, hairpin, arch, and wake vortices. This study offers insights that could inform the design of next-generation underwater robots, heat exchangers, and submarine sails.

physics.flu-dyn↗

arXiv2024

Morphing median fin enhances untethered bionic robotic tuna's linear acceleration and turning maneuverability

Median fins of fish-like swimmers play a crucial role in linear acceleration and maneuvering processes. However, few research focused on untethered robotic fish experiments. Imitating the behaviour of real tuna, we developed a free-swimming bionic tuna with a foldable dorsal fin. The erection of dorsal fin, at proper conditions, can reduce head heave by 50%, enhance linear acceleration by 15.7%, increase turning angular velocity by 32.78%, and turning radius decreasing by 33.13%. Conversely, erecting the dorsal fin increases the wetted surface area, resulting in decreased maximum speed and efficiency during steady swimming phase. This finding partially explains why tuna erect their median fins during maneuvers or acceleration and fold them afterward to reduce drag. In addition, we verified that folding the median fins after acceleration does not significantly affect locomotion efficiency. This study supports the application of morphing median fins in undulating underwater robots and helps to further understand the impact of median fins on fish locomotion.

cs.RO↗

arXiv2023

In-phase schooling hinders linear acceleration in wavy hydrofoils paired in parallel across various wavelengths

This study examines the impact of in-phase schooling on the hydrodynamic efficiency during linear acceleration in a simplified model using two undulating NACA0012 hydrofoils arranged in phalanx formation as a minimal representation of a fish school. The research focuses on key parameters, namely Strouhal (0.2-0.7) and Reynolds (1000-2000) numbers, and explores the effect of varying fish-body wavelengths (0.5-2), reflecting natural changes during actual fish linear acceleration. Contrary to expectations, in-phase schooling did not enhance acceleration performance. Both propulsive efficiency and net thrust were found to be lower compared to solitary swimming and anti-phase schooling conditions. The study also identifies and categorizes five distinct flow structure patterns within the parameters investigated, providing insight into the fluid dynamics of schooling fish during acceleration.

physics.flu-dyn↗

arXiv2023

How Biomimetic Morphing Dorsal Fin Affects the Swimming Performance of a Free-swimming Tuna Robot

It is well known that tuna fish in the ocean can dynamically morph their median fins to achieve optimal hydrodynamic performance, e.g. linear acceleration and maneuverability. In this study, based on the previous studies about the median fin's hydrodynamic effects focusing on tethered conditions, we continue to explore the hydrodynamic function of tuna morphing dorsal fin in free swimming conditions for better approaching real-life situations.Here, we developed a tuna-inspired robotic fish platform that can swim independently in three dimensions, equipped with a biomimetic morphing dorsal fin magnetically attached to the robotic fish. Based on the free-swimming robotic fish platform, we investigated how the erected dorsal fin affects the speed, cost of transport (COT), and robotic fish's yaw angle at different frequencies and amplitudes. The erected dorsal fin plays a positive role in improving the yaw stability of robotic fish. However, it shows little influence on the speed and COT in our test. This remains to be further investigated in the future.

cs.RO↗

arXiv2022

How wavelength affects the hydrodynamic performance of two accelerating mirror-symmetric slender swimmers

Fish schools are capable of simultaneous linear acceleration. To reveal the underlying hydrodynamic mechanism, we numerically investigate how Reynolds number $ Re = 1000 - 2000 $, Strouhal number $ St = 0.2 - 0.7 $ and wavelength $ \lambda = 0.5 - 2 $ affect the mean net thrust and net propulsive efficiency of two side-by-side hydrofoils undulating in anti-phase. In total, $ 550 $ cases are simulated using immersed boundary method. The thrust increases significantly with wavelength and Strouhal number, yet only slightly with the Reynolds number. We apply a symbolic regression algorithm to formulate this relationship. Furthermore, we find that mirror-symmetric schooling can achieve a \textit{net} thrust more than ten times that of a single swimmer, especially at low Reynolds numbers. The highest efficiency is obtained at $ St = 0.5 $ and $ \lambda = 1.2 $, where $ St $ is consistent with that observed in the linear-accelerating natural swimmers, \eg Crevalle jack. Six distinct flow structures are identified. The highest thrust corresponds to an asymmetric flow pattern, whereas the highest efficiency occurs when the flow is symmetric with converging vortex streets.

physics.flu-dyn↗

arXiv2022

How swimming style affects schooling of two fish-like wavy hydrofoils

Fish swimming style and schooling behaviour are two important aspects of underwater locomotion. How swimming style affects fish schooling is investigated by a representative problem setup consisting of two tethered NACA0012 hydrofoils undulating in a free stream flow at various wavelengths $ \lambda^* = 0.8 - 8 $, front-back distance $ D = 0,\ 0.25,\ 0.5,\ 0.75 $, phase difference $ \phi/\pi = 0,\ 0.5,\ 1,\ 1.5 $, and lateral gap distance $ G = 0.25,\ 0.3,\ 0.35 $ with fixed Reynolds number $ Re = 5000 $, Strouhal number $ St = 0.4 $, and maximum amplitude $ A_{max} = 0.1 $. In total, 336 cases were simulated by open-source software IBAMR based on immersed boundary method. The increase in wavelength generally leads to higher thrust and lateral force, consistent with a previous study regarding a single swimmer. The highest Froude efficiency is obtained at intermediate wavelength $ \lambda^* = 2 $ for either individual or two foils as a group. At side-by-side arrangement $ D = 0 $, the thrust force upon the two foils can be equivalent, indicating a stable formation. At staggered arrangement, $ D > 0 $, the follower can take significant advantage of the leader in locomotion performance by tuning phase difference. The follower's benefit decreases with relative distance. Various combinations of wavelength and relative distance can lead to distinct flow structures, indicating a tunable stealth capacity of the schooling swimmers.

physics.flu-dyn↗