检索结果-南通市图书馆

Lagrangian coherent structures and their heat-transport mechanism in the turbulent rayleigh-bénard convection

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

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

Scaling of maximum probability density function of velocity increments in turbulent rayleigh-bénard convection

在线全文

学校读者我要写书评

暂无评论

Journal of Hydrodynamics 2014年第3期26卷 351-362页

作者：邱翔黄永祥周全孙超 School of Science Shanghai Institute of Technology Shanghai Institute of Applied Mathematics and Mechanics Shanghai University Physics of Fluids Group University of Twente

In this paper, we apply a scaling analysis of the maximum of the probability density function（pdf） of velocity increments, i.e., max（） = max（）up p u, for a velocity field of turbulent rayleigh-bénard convection obtained at the Taylor-microscale Reynolds number Re60. The scaling exponent is comparable with that of the first-order velocity structure function, （1）, in which the large-scale effect might be constrained, showing the background fluctuations of the velocity field. It is found that the integral time T（x/ D） scales as T（x/ D）（x/ D）, with a scaling exponent =0.25 0.01, suggesting the large-scale inhomogeneity of the flow. Moreover, the pdf scaling exponent （x, z） is strongly inhomogeneous in the x（horizontal） direction. The vertical-direction-averaged pdf scaling exponent （x） obeys a logarithm law with respect to x, the distance from the cell sidewall, with a scaling exponent 0.22 within the velocity boundary layer and 0.28 near the cell sidewall. In the cell＇s central region, （x, z） fluctuates around 0.37, which agrees well with （1） obtained in high-Reynolds-number turbulent flows, implying the same intermittent correction. Moreover, the length of the inertial range represented in decade（）IT x is found to be linearly increasing with the wall distance x with an exponent 0.65 0.05.

关键词： rayleigh-bénard convection scaling probability density function(pdf)

EXTENDED SELF SIMILARITY OF PASSIVE SCALAR IN rayleigh-bnard convection FLOW bASED ON WAVELET TRANSFORM

在线全文

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2002年第7期23卷 804-810页

作者：傅强夏克青 College of Physics Meterology Institute of Science and Engineering University of PLA Nanjing P R China Department of Physics Chinese University of Hong Kong Shatian Hong KongP R China

Wavetet transform was used to analyze the scaling law of temperature data (passive scalar) in rayleigh-bénard convection flow from two aspects. The first one was to utilize the method of extended self similarity, presented first by benzi et al., to study the scaling exponent of temperature data. The obtained results show that the inertial range is much wider than that one determined directly from the conventional structure function, and find the obtained scaling exponent agrees well with the one obtained from the temperature data in an experiment of wind tunnel. The second one was that, by extending the formula which was proposed by A. Arneodo et al. for extracting the scaling exponent ζ(q) of velocity data to temperature data, a newly defined formula which is also based on wavelet transform, and can determine the scaling exponent ξ(q) of temperature data was proposed. The obtained results demonstrate that by using the method which is named as WTMM (wavelet transform maximum modulus) ξ(q) correctly can be extracted.

关键词： rayleigh-bénard convection wavelet transform extended self similarity scaling law temperature data

Hybrid LES/URANS Simulation of rayleigh-bénard convection Using bEM

在线全文

学校读者我要写书评

暂无评论

Computer Modeling in Engineering & Sciences 2020年第4期123卷 1-22页

作者： Primož Kocutar Jure Ravnik Leopold Škerget Faculty of Mechanical Engineering University of MariborSmetanova 17SI-2000 MariborSlovenia

In this paper,we develop and test a unified hybrid LES/URANS turbulence model with two different Large Eddy Simulation(LES)turbulence *** numerical algorithm is based on the boundary Element *** the existing hybrid LES/URANS turbulence model we implemented a new Smagorinsky LES turbulence *** hybrid LES/URANS turbulence model is unified,which means that the LES/URANS interface is changed dynamically during simulation using a physical *** order to define the interface between LES and unsteady Reynolds Averaged Navier Stokes(URANS)zones during the simulation,we use the Reynolds number based on turbulent kinetic energy as a switching *** means that the flow characteristics define where the sub-grid scale or URANS effective viscosity and thermal conductivity are used in the governing equations in the next time *** unified hybrid turbulence models,only one set of governing equations is used for LES and URANS *** developed hybrid LES/URANS model was tested on non-isothermal,unsteady and turbulent rayleigh-bénard convection and compared with an existing model,where LES is based on turbulent kinetic *** hybrid turbulence model was implemented within a numerical algorithm based on the boundary-Domain Integral Method,where a single domain and sub-domain approaches were *** numerical algorithm uses governing equations written in a velocity-vorticity *** false transient time scheme is used for the kinematics equation.

关键词： boundary-domain integral method hybrid LES-RANS turbulent fluid flow heat transfer rayleigh-bénard convection

A model for universal spatial variations of temperature fluctuations inturbulent rayleigh-bénard convection

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Theoretical & Applied Mechanics Letters 2021年第2期11卷 102-104页

作者： Xiaozhou He Eberhard bodenschatz Guenter Ahlers School of Mechanical Engineering and Automation Harbin Institute of TechnologyShenzhen 518055China Max Planck Institute for Dynamics and Self Organization D-37073 GöttingenGermany International Collaboration for Turbulence Research Institute for Nonlinear Dynamics University of GöttingenD-37073 GöttingenGermany Laboratory of Atomic and Solid-State Physics and Sibley School of Mechanical and Aerospace Engineering Cornell UniversityIthacaNY 14853USA Department of Physics University of CaliforniaSanta BarbaraCA 93106USA

We propose a theoretical model for spatial variations of the temperature varianceσ2(z,r)(z is the dis-tance from the sample bottom and r the radial coordinate)in turbulent rayleigh-bénard convection(RbC).Adapting the“attached-eddy”modelofshearflowtothe plumesofRbC,wederivedanequationforσ2 which is based on the universal scaling of the normalized RbC temperature *** equation in-cludes both logarithmic and power-law dependences on z/λth,whereλth is the thermal boundary layer *** equation parameters depend on r and the Prandtl number Pr,but have only an extremelyweak dependence on the rayleigh number Ra Thus our model provides a near-universal equation for thetemperature variance profile in turbulent RbC.

关键词： rayleigh-bénard convection Temperature variance profile Attached-eddy Law of wall

Heat transport in horizontally periodic and confined rayleigh-bénard convection with no-slip and free-slip plates

在线全文

学校读者我要写书评

暂无评论

Theoretical & Applied Mechanics Letters 2022年第2期12卷 104-110页

作者： Maojing Huang Xiaozhou He School of Mechanical Engineering and Automation Harbin Institute of Technology(Shenzhen)Shenzhen 518055China

We report the results of the direct numerical simulations of two-dimensional rayleigh-bénard convection(RbC)in order to study the influence of the periodic(PD)and confined(CF)samples on the heat transport *** numerical study is conducted with the rayleigh number(Ra)varied in the range 10^(6)≤Ra≤10^(9) at a fixed Prandtl number Pr=4.3 and aspect ratio Γ=2 with the no-slip(NS)and freeslip(FS)*** exists a zonal flow for Ra≥3×10^(6) with the free-slip plates in the periodic *** all the other cases,the flow is the closed large-scale circulation(closed LSC).The striking features are that the heat transport Nu is influenced and the temperature profiles do not be influenced when the flow pattern is zonal flow.

关键词： rayleigh-bénard convection Zonal flow Temperature profile Turbulence

POD Analysis and Low-Dimensional Model based on POD-Galerkin for Two-Dimensional rayleigh-bénard convection

在线全文

学校读者我要写书评

暂无评论

Journal of Harbin Institute of Technology(New Series) 2018年第1期25卷 67-78页

作者： Hanghang Ma Xin Zheng Yue Wang Jianping Chen Hongna Zhang Weihua Cai School of Energy Science and Engineering Harbin Institute of Technology Harbin 150001 China School of Municipal and Environmental Engineering Harbin Institute of Technology Harbin 150001 China Universite de Lyon CNRS INSA-Lyon Universite Claude Bernard Lyon 1 CETHIL UMR5008 F-69621 Villeurbanne France

Direct numerical simulation based on OpenFOAM is carried out for two-dimensional rayleighbénard( Rb) convection in a square domain at high rayleigh number of 107 and Pr = 0.71. Proper orthogonal decomposition( POD) is used to analyze the flow and temperature characteristics from POD energy spectrum and eigenmodes. The results show that the energy spectrum converges fast and the scale of vortex structures captured by eigenmodes becomes smaller as the eigenmode order increases. Meanwhile,a low-dimensional model( LDM) for Rb convection is derived based on POD eigenmodes used as a basis of Galerkin project of Navier-Stokes-boussinesq equations. LDM is built based on different number of eigenmodes and through the analysis of phase portraits,streamline and isothermal predicted by LDM,it is suggested that the error between LDM and DNS is still large.

关键词： rayleigh-bénard convection proper orthogonal decomposition(POD) low-dimensional model POD-Galerkin direct numerical simulation(DNS)

Large Eddy Simulations of rayleigh-bénard convection in a Cubic Cavity

在线全文

学校读者我要写书评

暂无评论

Open Journal of Fluid Dynamics 2023年第3期13卷 177-190页

作者： Tian Qin Yingying Luo Jiemin Zhan Wenqing Hu Yutian Li Ziwei Huang Department of Applied Mechanics and Engineering School of Aeronautics and Astronautics Shenzhen Campus of Sun Yat-sen University Shenzhen China

This paper conducts a Large Eddy Simulation (LES) of rayleigh bénard convection in a cubic cavity based on the WMLES S-Omega subgrid-scale model. For a cubic cavity with a vertical temperature difference of 6.7°C and 20°C, the velocity pulsation profiles and the mean velocity profiles of the vertical section in the middle of the cubic cavity were simulated, respectively. And they are consistent with the experiment results. Furthermore, the mean velocity field of the vertical cross-section in the middle of the cavity was calculated. Structures of the mean velocity field in the two cases are similar. A counterclockwise large vortex is found to occupy the cavity, and there are two small clockwise vortices in the lower left and upper right corners, and the mean velocity fields at two different temperature differences are consistent with the experimental results. The two-dimensional instantaneous temperature field and mean temperature field with different cross-sections in the z-direction, as well as the three-dimensional instantaneous isothermal surface structure, indicate that the large-scale circulation motion within the cubic cavity is moving diagonally. In addition, the structure of the mean streamline also illustrates this viewpoint. For the reverse vortex formed at two corners in the mean streamline structure, we used the Q criterion to identify and obtain two vortex structures similar to boomerangs. The basic turbulent structure in Rb thermal convection includes the rising and falling plumes generated by buoyancy effects.

关键词： Large Eddy Simulation rayleigh-bénard convection Cubic Cavity Plume Flow

On the Features of Thermal convection in a Compressible Gas

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Fluid Dynamics & Materials Processing 2024年第5期20卷 957-974页

作者： Igor b.Palymskiy Physics Department Siberian State University of Telecommunications and Information SciencesNovosibirsk City630102Russia Physics Department Siberian State University of Geosystems and TechnologiesNovosibirsk City630108Russia

The fully nonlinear equations of gas dynamics are solved in the framework of a numerical approach in order to study the stability of the steady mode of rayleigh-bénard convection in compressible,viscous and heat-conducting gases encapsulated in containers with no-slip boundaries and isothermal top and bottom *** initial linear temperature profile is assumed.A map of the possible convective modes is presented assuming the height of the region and the value of the temperature gradient as influential *** a relatively small height,isobaric convection is found to take place,which is taken over by an adiabatic mode when the height exceeds the critical value,or by a super-adiabatic mode in case of a relatively high temperature *** the adiabatic mode,convective flow develops due to adiabatic processes given a stable initial *** analytic formula for the critical height of the region is derived taking into account and neglecting the dependence of the gas viscosity on the ***,an analytic formula is obtained for the upper boundary of the region of applicability of the boussinesq approximation for incompressible *** models for compressible gases are relevant to practical situations such as the study of convective flows in spatially extended gas mixtures when dealing with safety issues related to hydrocarbons stored in gas stations.A dangerous situation arises when the tank is almost empty but some hydrocarbon is left at the bottom of the *** the presence of convective flows,the vaporized fuel is mixed with the oxidizer(air)forming a gas-vapor ***,if the volumetric concentration of fuel vapor(hydrocarbon)is in the interval between the lower and upper concentration limits of ignition,then the gas-vapor mixture becomes explosive and any accidental spark is sufficient to cause an emergency.

关键词： rayleigh-bénard convection gas stable stratification unstable stratification temperature gradient