Flow-mediated interaction between a forced-oscillating cylinder and an elastically mounted cylinder in less regular regimes

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 λ=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 the wavelength and the 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 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 λ=1.2, where St is consistent with that observed in the linear-accelerating natural swimmers, e.g., 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.

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How wavelength affects hydrodynamic performance of two accelerating mirror-symmetric undulating hydrofoils

Lin

Liang

Bhalla

et al. 2023

Physics of Fluids

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Flow regime identification and flow instability analysis of oscillatory flows over twin circular cylinders

Lu,

Zhou,

Zhang

2023

Physics of Fluids

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Oscillatory flows past two identical circular cylinders are investigated by two-dimensional direct numerical simulations in the parameter space of gap ratio (0.5 ≤ G ≤ 4.0), angle of flow incidence (0° ≤ α ≤ 90°) and Keulegan–Carpenter number (4 ≤ KC ≤ 12) with a constant Reynolds number Re = 150, where G = L/D, KC = UmT/D and Re = UmD/υ with D being the dimeter of the identical cylinders, L the shortest surface-to-surface distance between the two cylinders, Um and T being the velocity amplitude and period of the sinusoidal oscillatory flow, respectively, and α is defined as the angle between the flow direction to the line connecting the centers of the two cylinders. Comparing with the tandem or side-by-side arrangements of twin circular cylinders in oscillatory flows, the staggered twin cylinders (0° < α < 90°) involve more diverse flow regimes, including the periodic, quasi-periodic and chaotic flow states, due to the inherent asymmetric flow interference around the cylinder pair. In addition to introducing four flow regimes for the tandem and side-by-side arrangements, this study newly identifies 11 flow regimes for the staggered twin cylinders. The newly reported flow regimes in this work are collaboratively identified through the flow visualizations, steady streaming, frequency spectra of fluid forces and Lissajous phase diagrams, as well as the temporal-spatial symmetry features of the wake flows. Connecting with the previous work by Zhao and Cheng [“Two-dimensional numerical study of vortex shedding regimes of oscillatory flow past two circular cylinders in side-by-side and tandem arrangements at low Reynolds numbers,” J. Fluid Mech. 751, 1–37 (2014)], this study presents overall regime maps in the KC-α plane for varied gap ratios. It is found that the flow regimes previously and presently identified for the tandem and side-by-side arrangements may also appear for the staggered twin cylinders. The present numerical results suggest the sensitive dependence of the flow regimes on the parameters of KC, α, and G. It is also found that a specific flow regime with narrow parameter bands may appear within another flow regime, forming the abnormal regime hole in the regime map. To understand the profound influence of the control parameters on the flow regime transition, and the relevant temporal-spatial symmetry breaking, the linear Floquet stability analysis is conducted in this work. It was confirmed that the variation of the KC number may result in the Ky symmetry breaking over several periodic flow regimes, while the change of the angle of flow incidence may account for the H2 symmetry covering various periodic and quasi-periodic flow regimes. The stability analysis also explains the temporal flow transition and the abnormal occurrence of the regime holes within either quasi-periodic or chaotic flow regimes.

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Effects of oscillation frequency, phase difference, and arrangement of two triangular cylinders on the hydrothermal performance of a channel

Abbasinezhad Fallah,

Rezazadeh,

Dadvand

et al. 2024

Physics of Fluids

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This study aims to numerically explore the effects of two cylinders with triangular cross sections, undergoing transverse oscillation on the heat and fluid flow within a channel. The governing equations along with the pertinent initial and boundary conditions are solved using the finite element method. The results are validated against the available data in the literature, demonstrating admissible agreement. Different oscillation frequencies, phase differences, and arrangement of the cylinders are considered. The results are presented in terms of temperature, pressure, and vorticity contours, alongside the Nusselt (Nu) number and thermal performance index (TPI). The Reynolds number based on the triangle's side length (Reb=200) is deliberately chosen to induce the vortex shedding phenomenon in the wake of the cylinders. For a single oscillatory cylinder, an optimal oscillation frequency fr=0.5 was found resulting in a remarkable increase in 22.3% and 33.1% in the Nu number and TPI, respectively, when compared to the fixed single cylinder. Using the obtained optimal oscillation frequency fr=0.5, the side-by-side arrangement of two cylinders gives rise to a higher TPI than the tandem one. The maximum TPI in side-by-side arrangement corresponds to the oscillation phase of φ=π/6, which is about 1.69. In tandem arrangement, it corresponds to φ=π/3, which is approximately 1.3. Furthermore, the oscillation phase φ=π exhibits the lowest TPI for both arrangements.

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Flow-mediated interaction between a forced-oscillating cylinder and an elastically mounted cylinder in less regular regimes

Cited by 3 publications

References 49 publications

How wavelength affects hydrodynamic performance of two accelerating mirror-symmetric undulating hydrofoils

How wavelength affects hydrodynamic performance of two accelerating mirror-symmetric undulating hydrofoils

Flow regime identification and flow instability analysis of oscillatory flows over twin circular cylinders

Effects of oscillation frequency, phase difference, and arrangement of two triangular cylinders on the hydrothermal performance of a channel

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