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232 results on '"Ji, Chunning"'

1. Revisit the intrinsic features of flip-flopping flow behind side-by-side circular cylinders

Author

Chen, Wailing, Yan, Yuhao, Ji, Chunning, Alam, Md. Mahbub, Srinil, Narakron, Noack, Bernd R., and Deng, Nan

Subjects
Physics - Fluid Dynamics
Abstract

As one of the most intriguing wake patterns of two side-by-side circular cylinders at an intermediate gap spacing, the flip-flopping (FF) flow has attracted great attention of fundamental research interest. This FF flow is featured by the intermittently and randomly switching gap flow with correspondingly changing forces of the two cylinders. In this paper, we first present a partition map of the wake patterns behind two side-by-side circular cylinders and briefly introduce intrinsic features of each flow pattern. We focus on the FF flow aiming to explain: (i) the origin of the FF flow between laminar and turbulent regimes, (ii) their connections in different flow regimes, and (iii) mechanisms of the significantly varying flip-over time scale of the FF flows. In the laminar regime, we further divide the FF flow into the sub-classed I (FF1) and II (FF2), based on their different origins from the in-phase and anti-phase synchronized vortex shedding instabilities, respectively. By exploring the vortex interactions, we show that the FF flow in the turbulent regime has the same origin and similar vortex dynamics as the FF2 wake in the laminar regime, despite some minor disparities. Thus, a connection is established between the FF2 pattern in the laminar flow and the FF pattern in the turbulent flow. For the FF flow in the laminar regime (Re < 150-200), the mildly decreasing switching time, is several vortex shedding periods. However, for the FF flow in the weak turbulence regime (150-200 < Re < 1000-1700), the switching time scale increases significantly with Re owing to the increased vortex formation length. The FF in the strong turbulence regime (Re > 1000-1700) has a switching time scale of several orders of magnitude longer than the vortex shedding period, where the switching scale decreases gradually with Re due to the stronger Kelvin-Helmholtz vortices.

Published
2023

18. Shape Classification Using a Single Seal-Whisker-Style Sensor Based on the Neural Network Method.

Author

Mao, Yitian, Lv, Yingxue, Wang, Yaohong, Yuan, Dekui, Liu, Luyao, Song, Ziyu, and Ji, Chunning

Abstract

Seals, sea lions, and other aquatic animals rely on their whiskers to identify and track underwater targets, offering valuable inspiration for the development of low-power, portable, and environmentally friendly sensors. Here, we design a single seal-whisker-like cylinder and conduct experiments to measure the forces acting on it with nine different upstream targets. Using sample sets constructed from these force signals, a convolutional neural network (CNN) is trained and tested. The results demonstrate that combining the seal-whisker-style sensor with a CNN enables the identification of objects in the water in most cases, although there may be some confusion for certain targets. Increasing the length of the signal samples can enhance the results but may not eliminate these confusions. Our study reveals that high frequencies (greater than 5 Hz) are irrelevant in our model. Lift signals present more distinct and distinguishable features than drag signals, serving as the primary basis for the model to differentiate between various targets. Fourier analysis indicates that the model's efficacy in recognizing different targets relies heavily on the discrepancies in the spectral features of the lift signals. [ABSTRACT FROM AUTHOR]

Published
2024
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19. Dynamics of harbor seal whiskers at different angles of attack in wake flow.

Author

Zhao, Hanghao, Zhang, Zhimeng, Ji, Chunning, Zhao, Yawei, Li, Xianghe, and Du, Muyuan

Abstract

Harbor seals can exquisitely perceive and distinguish different hydrodynamic signals through their distinctive undulatory whiskers. In this study, an elastically supported Harbor seal whisker model undergoing one degree-of-freedom vibration in the cross-flow direction was positioned right downstream of a fixed cylinder and a flapping caudal fin model, respectively, as wake generators. The vibration responses, spectral frequencies, and fluid forces of the whisker model at different angles of attack (0° ≤ α ≤ 90°) are experimentally investigated. The reduced velocity, Ur, ranges from 4.0 to 26.0, and the Reynolds number, Re, varies within the range of 730–4770. The whisker model exhibits a combined vortex-induced vibration (VIV)—wake-induced vibration (WIV) response in the cylinder wake flow,whereas in the caudal fin wake flow, it displays a combination of VIV—wake-induced galloping response at α ≤ 45° and VIV–WIV response at α ≥ 60°. The frequency features of the wake generated by the fixed cylinder and the flapping caudal fin model, along with the increased VIV influence due to the angle of attack, cause the whisker model to exhibit different vibration characteristics. Understanding the vibration responses and fluid forces of the whisker model at various angles of attack in different wake flows is pivotal for future studies on developing whisker-inspired sensors. [ABSTRACT FROM AUTHOR]

Published
2024
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33. On Monami modes and scales of a flexible vegetation array in a laminar boundary layer

Author

Ni, Jinyuan, Ji, Chunning, Xu, Dong, Zhang, Xing, Liang, Dongfang, Ni, Jinyuan [0009-0003-8127-9671], Ji, Chunning [0000-0003-0376-8309], Xu, Dong [0000-0003-1394-5364], Zhang, Xing [0000-0002-4960-0221], Liang, Dongfang [0000-0001-5639-7375], and Apollo - University of Cambridge Repository

Abstract

Flexible aquatic vegetation exists widely in nature and serves multiple hydro-environmental functions mainly through fluid–structure interactions. The waving motion of vegetation arrays, known as Monami, is predominantly governed by Kelvin–Helmholtz (KH) instability, and its characteristic scales, such as wave height and wavelength, are still being explored. In this paper, the interactions between a large array of flexible vegetations and a laminar boundary-layer flow are investigated using direct numerical simulation. The parameters used are the Reynolds number Re = 400, mass ratio β = 1.0, bending rigidity γ = 0.04–0.22, and gap distance d = 0.4–1.6. A low frequency in Monami is found to be related to the fluctuation frequency of the onset position of the KH instability, which leads to the identification of four different Monami modes: regular Monami, quasi-regular Monami A, quasi-regular Monami B, and irregular Monami. The influences of the bending rigidity and gap distance on the Monami modes, KH instability onset position, and Monami characteristic scales are discussed. It was found that the causes of spatial and temporal variations in the characteristic scales of Monami vary depending on the mode. In the regular Monami mode, these variations result from the evolution of the KH vortex. In the quasi-regular Monami A mode, they are strongly affected by the shifting of the onset position of the KH instability. In the other two modes, these variations are caused by a combination of the fluctuation in the KH instability onset position and the complex interaction between vortices.

Published
2023
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43. Fluid–structure interaction of flexible submerged vegetation stems and kinetic turbine blades

Author

Wang, Mingyang, Avital, Eldad J., Bai, Xin, Ji, Chunning, Xu, Dong, Williams, John J. R., and Munjiza, Antonio

Abstract

A fluid–structure interaction (FSI) methodology is presented for simulating elastic bodies embedded and/or encapsulating viscous incompressible fluid. The fluid solver is based on finite volume and the large eddy simulation approach to account for turbulent flow. The structural dynamic solver is based on the combined finite element method–discrete element method (FEM-DEM). The two solvers are tied up using an immersed boundary method (IBM) iterative algorithm to improve information transfer between the two solvers. The FSI solver is applied to submerged vegetation stems and blades of small-scale horizontal axis kinetic turbines. Both bodies are slender and of cylinder-like shape. While the stem mostly experiences a dominant drag force, the blade experiences a dominant lift force. Following verification cases of a single-stem deformation and a spinning Magnus blade in laminar flows, vegetation flexible stems and flexible rotor blades are analysed, while they are embedded in turbulent flow. It is shown that the single stem’s flexibility has higher effect on the flow as compared to the rigid stem than when in a dense vegetation patch. Making a marine kinetic turbine rotor flexible has the potential of significantly reducing the power production due to undesired twisting and bending of the blades. These studies point to the importance of FSI in flow problems where there is a noticeable deflection of a cylinder-shaped body and the capability of coupling FEM-DEM with flow solver through IBM.

Published
2024
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44. Numerical investigation on the incipient motion of non-spherical sediment particles in bedload regime of open channel flows

Author

Zhang, Boxi, Xu, Dong, Zhang, Bingchang, Ji, Chunning, Munjiza, Antonio, and Williams, John

Abstract

Theoretical analysis and nowadays advanced numerical simulations for bedload particle transport widely share a popular assumption: All sediments are spherical and can be simplified as spheres, which apparently differs from reality. In order to explore the shape effects of sediment particles on their transport and to find out the possible consequence originated from the spherical assumption, a particle resolved computational model is established by combining the large eddy simulation for fluid and the discrete element method for the solid particles using an immersed boundary (IB) technique. Using this model, simulations are conducted for spherical and non-spherical bedload particles in both laminar and turbulent open channel flows. Simulation results show that shape of sediment particles has direct influence on the incipient motion in both laminar and turbulent flows. In laminar flows, the widely adopted spherical particles tend to exhibit over 5 times higher traveling speed compared with the non-spherical particles, and over 4 times longer time lag during incipient motion. In turbulent flows, the disk-shaped non-spherical particle yields up to 60% higher travel speed, 50% larger saltation height and 126% longer saltation length compared with spherical particles. The travel modes of non-spherical sediment particles during incipient motion are also analyzed.

Published
2024
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45. Numerical investigations on the drag-thrust transition of an asymmetric sinusoidal pitching foil.

Author

Chao, Li-Ming, Alam, Md. Mahbub, and Ji, Chunning

Subjects
HYDRODYNAMICS, OSCILLATIONS
Abstract

We numerically investigated the hydrodynamics of a foil undergoing the asymmetric sinusoidal pitching motion when the thickness-based Strouhal number StD and amplitude ratio AD vary from 0.1 to 0.3 and 0.5 to 2.0, respectively. The asymmetric oscillation is achieved by making the downstroke slower than the upstroke, and vice versa. The result uncovers that the drag-thrust boundary can be advanced via enhancing the asymmetry in oscillation. The P+S wake emerged at the asymmetric pitching is found that produces both negative and positive propulsive force, hence appearing around the drag-thrust boundary. [ABSTRACT FROM AUTHOR]

Published
2023
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