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Coherent structures over riblets in turbulent boundary layer studied by combining time-resolved particle image velocimetry(TRPIV),proper orthogonal decomposition(POD),and finite-time lyapunov exponent(FTLE)

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Chinese Physics B 2018年第10期27卷 395-404页

作者： Shan Li Nan Jiang Shaoqiong Yang Yongxiang Huang Yanhua Wu School of Mechanical Engineering Tianjin University Tianjin 300072 China Xiamen University Xiamen 361102 China Nanyang Technological University Singapore 639798 Republic of Singapore

time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures（CSs） over a smooth surface and riblets, the proper orthogonal decomposition（POD） and finite-time lyapunov exponent（FTLE） are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.

关键词： turbulent boundary layer riblets proper orthogonal decomposition finite-time lyapunov exponent

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Lagrangian coherent structures and their heat-transport mechanism in the turbulent Rayleigh-Bénard convection

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Science China(Technological Sciences) 2022年第4期65卷 966-976页

作者： CHENG Hang SHEN Jie ZHANG YiZhao ZHOU Quan CHONG Kai Leong LIU YuLu LU ZhiMing Shanghai Institute of Applied Mathematics and Mechanics Shanghai Key Laboratory of Mechanics in Energy Engineering and School of Mechanics and Engineering ScienceShanghai UniversityShanghai 200072China The School of Sciences Shanghai Institute of TechnologyShanghai 200072China

In this paper,we investigate the Lagrangian coherent structures(LCSs)and their heat-transport mechanism in turbulent Rayleigh-Bénard(RB)*** numerical simulations(DNS)are performed in a closed square cell with Rayleigh numbers(Ra)ranging from 10^(6) to 10^(9) and Prandtl(Pr)number fixed at Pr=***,our results show the power-law relationship between Nusselt number(Nu)and Ra,Nu=0.99Ra^(0.30±0.02),confirming the results from previous *** gain insights into the material transport,LCSs are extracted using the finite-time lyapunov exponent(FTLE)***,lobe structures are widely present,and we elucidate their role in transporting heat from the corner rolls to large-scale ***,the relationships between LCSs and thermal plumes are examined,and we identify two behaviors of thermal plumes:first,most plumes transport along the LCSs;second,few plumes are exposed to the bulk and subsequently mix with the turbulent ***,we quantify the heat flux along the LCSs,which contributes to about 85%of the total flux regardless of *** suggests that LCSs play a significant role in heat ***,the viscous(thermal)dissipation rate along the LCSs is quantified,which is larger than 80%(60%)of the total value,suggesting that LCSs are responsible for the large viscous and thermal dissipations.

关键词： Rayleigh-Bénard convection finite-time lyapunov exponent Lagrangian coherent structures

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Lagrangian coherent structures analysis of gas-liquid flow in a bubble column

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Science China(Physics,Mechanics & Astronomy) 2014年第6期57卷 1169-1177页

作者： WU Qin WANG GuoYu HUANG Biao Bai ZeYu School of Mechanical and Vehicular Engineering Beijing Institute of Technology

The objective of this paper is to apply a new identifying method to investigating the gas-liquid two-phase flow behaviors in a bubble column with air injected into water. In the numerical simulations, the standard k-c turbulence model is employed to de- scribe the turbulence phenomenon occurring in the continuous fluid. The finite-time lyapunov exponent （FTLE） and Lagrangian Coherent Structures （LCS） are applied to analyze the vortex structures in multiphase flow. Reasonable agreements are obtained between the numerical and experimental data. The numerical results show that the evolution of gas-liquid in the column includes initial and periodical developing stages. During the initial stage, the bubble hose is forming and extending along the vertical direction with the vortex structures formed symmetrically. During the periodical developing stage, the bub- ble hose starts to oscillate periodically, and the vortexes move along the bubble hose to the bottom of column alternately. Compared to the Euler-system-based identification criterion of a vortex, the FTLE field presents the boundary of a vortex without any threshold defined and the LCS represents the divergence extent of infinite neighboring particles. During the initial stage, the interfaces between the forward and backward flows are highlighted by the LCS. As for the periodical developing stage, the LCS curls near the vortex centers, providing a method of analyzing a flow field from a dynamical system perspec- tive.

关键词： bubble columns multiphase flow finite-time lyapunov exponent Lagrangian coherent structures

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AN OPTICAL FLOW APPROACH TO ANALYZING SPECIES DENSITY DYNAMICS AND TRANSPORT

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Journal of Computational Mathematics 2012年第3期30卷 249-261页

作者： Aaron Luttman Erik Bollt Jason Holloway Department of Mathematics Clarkson UniversityPotsdamNYUSA Department of Electrical Engineering Rice UniversityHoustonTXUSA

Classical optical flow techniques were developed for computing virtual motion fields between two images of the same scene, assuming conservation of intensity and a smooth- ness of the flow field. If these assumptions are dropped, such techniques can be adapted to compute apparent fows between time snapshots of data that do not come from images, even if these flows are turbulent and divergent, as in the case of flows representing complex spatiotemporal dynamics. While imaging methods have been used to analyze dynamics in experimental applications, they are only beginning to be applied to dynamics computa- tions in settings outside the laboratory, for example in the analysis of species population dynamics from satellite data. In this work we present a variational optical flow approach based on the continuity equation and total variation regularization for computing the flow fields between population densities generated from a two-species predator-prey model for phyto- and zooplankton interactions. Given the time-varying vector fields produced from the optical flow~ computational methods from dynamical systems can be employed to study pseudo-barriers present in the species interaction. This method allows to measure the mix- ing of the species, as well as the transport of the populations throughout the domain.

关键词： Optical flow finite-time lyapunov exponent Mass transport Data-drivendynamical systems.

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