In gas-liquid mass transfer processes,interfacial turbulence may occur due to the surface tension gradient and the density gradient produced by mass transfer near the interface.The interfacial turbulence can enhance t...In gas-liquid mass transfer processes,interfacial turbulence may occur due to the surface tension gradient and the density gradient produced by mass transfer near the interface.The interfacial turbulence can enhance the mass transfer since it intensifies the movement of interfacial fluid.By means of the shadowgraph optical method,the interfacial turbulence patterns vertical to the interface were observed directly in the volatilization process of binary systems.The images of the amplified interfacial turbulence showed the variation of concentration and the fluid movement under the interface.Two patterns of interfacial turbulence were observed in the experiments:plume and vortex.With the plume,the interfacial fluid moved slowly and penetrated the liquid deeply.With the vortex,the interfacial turbulence occurred in the vicinity of the liquid interface and the fluid moves quite fast.A qualitative analysis was carried out based on the mechanism of Rayleigh-Bénard convection induced by density gradient and Marangoni convection induced by surface tension gradient.展开更多
Turbulence is one of the most common nature phenomena in everyday experience, but that is not adequately understood yet. This article reviews the history and present state of development of the turbulence theory and i...Turbulence is one of the most common nature phenomena in everyday experience, but that is not adequately understood yet. This article reviews the history and present state of development of the turbulence theory and indicates the necessity to probe into the turbulent features and mechanism with the different methods at different levels. Therefore this article proves a theorem of turbulent transpor- tation and a theorem of turbulent intensity by using the theory of the nonequilibrium thermodynamics, and that the Reynolds turbulence and the Rayleigh-Bénard turbulence are united in the theorems of the turbulent intensity and the turbulent transportation. The macroscopic cause of the development of fluid turbulence is a result from shearing effect of the velocity together with the temperature, which is also the macroscopic cause of the stretch and fold of trajectory in the phase space of turbulent field. And it is proved by the observed data of atmosphere that the phenomenological coefficient of turbulent in- tensity is not only a function of the velocity shear but also a function of temperature shear, viz the sta- bility of temperature stratification, in the atmosphere. Accordingly, authenticity of the theorem, which is proved by the theory of nonequilibrium thermodynamics, of turbulent intensity is testified by the facts of observational experiment.展开更多
本文采用湍流热对流的并行直接数值模拟(Parallel Direct Method of Direct Numerical Simulation,PDM-DNS),计算了系列Ra数的二维方腔Rayleigh-Bénard热对流.选取典型Ra=10^(10),Pr=4.3,讨论了流动平均场的近底板区域温度分布.温...本文采用湍流热对流的并行直接数值模拟(Parallel Direct Method of Direct Numerical Simulation,PDM-DNS),计算了系列Ra数的二维方腔Rayleigh-Bénard热对流.选取典型Ra=10^(10),Pr=4.3,讨论了流动平均场的近底板区域温度分布.温度边界层沿横向可分为三个区域:羽流发射区、冲击区和大尺度环流剪切区.在羽流发射区温度剖面存在对数律,冲击区和大尺度环流剪切区没有明显的对数律特征.横向平均得到的温度剖面也存在对数律.研究系列Ra数的横向平均温度剖面,得到温度变化满足A×log(y)+B的关系,温度剖面对数律振幅–A和Ra数呈现标度关系–A^Ra^(–0.145),与实验中发现的关系基本一致.展开更多
提出二维湍流热对流DNS模拟的并行直接求解方法(Parallel Direct Method of DNS,PDMDNS),在"天河二号"超级计算机上实现高Ra和极高Ra湍流热对流大规模DNS计算。高分辨率的湍流热对流计算结果表明不同Ra(10~8≤Ra≤10^(13))的...提出二维湍流热对流DNS模拟的并行直接求解方法(Parallel Direct Method of DNS,PDMDNS),在"天河二号"超级计算机上实现高Ra和极高Ra湍流热对流大规模DNS计算。高分辨率的湍流热对流计算结果表明不同Ra(10~8≤Ra≤10^(13))的瞬时温度场的流场特性完全不同。较低Ra流场中有明显的大尺度环流和角涡;较高Ra流场中羽流运动充满随机性;更高Ra流场出现小尺寸漩涡并不断从上下底板产生,这些涡相互影响作用,随大尺度环流一起作绕行运动。二维湍流热对流的Nu与Ra存在标度关系,标度律约为0.3。展开更多
Logarithmic boundary layers have been observed in different regions in turbulence. However, how thermal plumes correlate to the log law of temperature and how the velocity profile changes with pressure gradient are no...Logarithmic boundary layers have been observed in different regions in turbulence. However, how thermal plumes correlate to the log law of temperature and how the velocity profile changes with pressure gradient are not fully understood. Here, we perform three-dimensional simulations of turbulence in a slim-box without the front and back walls with aspect ratio, width:depth:height=L:D:H=1:1/6:1width:depth:height=L:D:H=1:1/6:1 (respectively corresponding to xx, yy and zz coordinates), in the Rayleigh number Ra=[1×10^8,1×10^10]Ra=[1×10^8,1×10^10] for Prandtl number Pr=0.7Pr=0.7. To investigate the structures of the viscous and thermal boundary layers, we examine the velocity profiles in the streamwise and vertical directions (i.e. UU and WW) along with the mean temperature profile throughout the plume-impacting, plume-ejecting, and wind-shearing regions. The velocity profile is successfully quantified by a two-layer function of a stress length, e^+u=e^+0(z^+)3/2[1+(z^+/z^+sub)4]^1/4eu+=e^+0(z+)3/2[1+(z+/zsub+)4]1/4, as proposed by She et al.(J Fluid Mech, 2017), though it is neither \pb type nor logarithmic. In contrast, the temperature profile in the plume-ejecting region is logarithmic for all simulated cases, being attributed to the emission of thermal plumes. The coefficient of the temperature log-law, AA, can be described by composition of the thermal stress length ■■θ0■θ0■ and the thicknesses of thermal boundary layer z■subzsub■ and z?bufzbuf■, i.e. A■z?sub/(■■θ0z■buf3/2)A■zsub?/(■θ0■zbuf^3/2). The adverse pressure gradient responsible for turning the wind direction contributes to intensively emitting plumes and the logarithmic temperature profile at the plume-ejecting region. The Nusselt number scaling and the local heat flux in the slim box are consistent with previous results of the confined cells. Therefore, the slim-box RBC is a preferable system for investigating in-box kinetic and thermal structures of turbulent convection with the large-scale circulation in a fixed p展开更多
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 Nu.The ...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 Nu.The 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)plates.There exists a zonal flow for Ra≥3×10^(6) with the free-slip plates in the periodic sample.In 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.展开更多
We present an experimental study of flow structures in turbulent Rayleigh-Bénard convection in annular cells of aspect ratiosΓ=1,2 and 4,and radius ratio 0.5.The convecting fluid is water with Prandtl number Pr=...We present an experimental study of flow structures in turbulent Rayleigh-Bénard convection in annular cells of aspect ratiosΓ=1,2 and 4,and radius ratio 0.5.The convecting fluid is water with Prandtl number Pr=4.3 and 5.3.Rayleigh number Ra ranges 4.8×10^(7)≤Ra≤4.5×10^(10).The dipole state(two-roll flow structure)forΓ=1 and the quadrupole state(fourroll flow structure)forΓ=2 and 4 are found by multi-temperature-probe measurement.Nusselt number Nu is described by a power-law scaling Nu=0.11Ra^(0.31),which is insensitive to the change of flow structures.However,the Reynolds number Re is influenced by increasing aspect ratios,where Re is found to scale with Ra andΓas Re~Ra^(0.46)Γ^(-0.52).The normalized amplitudes of two flow structures as a function of Ra exist difference.Based on relative weights of the first four modes using the Fourier analysis,we find that the first mode dominates inΓ=1 cell,but the second mode contains the most energy inΓ=2 and 4 cells.With increasingΓ,the flow structures exhibit different characteristics.展开更多
The tuning of turbulent Rayleigh-Bénard(RB)convection in a box is realized numerically by designed rough element arrangement.Considering the nonlinear dynamics of the thermal turbulence system,five models with ro...The tuning of turbulent Rayleigh-Bénard(RB)convection in a box is realized numerically by designed rough element arrangement.Considering the nonlinear dynamics of the thermal turbulence system,five models with rough elements of different widths and the same height are proposed to tune the fluid flow heat-transport capacity.Numerical simulations are performed using spectral element method for Rayleigh number in the range 10^(6)≤Ra≤10^(9) and a fixed Prandtl number Pr=0.7.It is found that heat transport is enhanced for large roughness widths as the interaction between the large-scale circulation and secondary flows inside the cavity regions between the rough elements promotes the eruptions of thermal plumes,but is suppressed for small ones as more heat are trapped inside the cavities.In all the rough models studied,different scaling exponents for the heat transport are identified and the influences of roughness arrangement on flow structure are studied.展开更多
文摘In gas-liquid mass transfer processes,interfacial turbulence may occur due to the surface tension gradient and the density gradient produced by mass transfer near the interface.The interfacial turbulence can enhance the mass transfer since it intensifies the movement of interfacial fluid.By means of the shadowgraph optical method,the interfacial turbulence patterns vertical to the interface were observed directly in the volatilization process of binary systems.The images of the amplified interfacial turbulence showed the variation of concentration and the fluid movement under the interface.Two patterns of interfacial turbulence were observed in the experiments:plume and vortex.With the plume,the interfacial fluid moved slowly and penetrated the liquid deeply.With the vortex,the interfacial turbulence occurred in the vicinity of the liquid interface and the fluid moves quite fast.A qualitative analysis was carried out based on the mechanism of Rayleigh-Bénard convection induced by density gradient and Marangoni convection induced by surface tension gradient.
基金Supported by the National Natural Science Foundation of China (Grant Nos. 40633014 and 40233035)
文摘Turbulence is one of the most common nature phenomena in everyday experience, but that is not adequately understood yet. This article reviews the history and present state of development of the turbulence theory and indicates the necessity to probe into the turbulent features and mechanism with the different methods at different levels. Therefore this article proves a theorem of turbulent transpor- tation and a theorem of turbulent intensity by using the theory of the nonequilibrium thermodynamics, and that the Reynolds turbulence and the Rayleigh-Bénard turbulence are united in the theorems of the turbulent intensity and the turbulent transportation. The macroscopic cause of the development of fluid turbulence is a result from shearing effect of the velocity together with the temperature, which is also the macroscopic cause of the stretch and fold of trajectory in the phase space of turbulent field. And it is proved by the observed data of atmosphere that the phenomenological coefficient of turbulent in- tensity is not only a function of the velocity shear but also a function of temperature shear, viz the sta- bility of temperature stratification, in the atmosphere. Accordingly, authenticity of the theorem, which is proved by the theory of nonequilibrium thermodynamics, of turbulent intensity is testified by the facts of observational experiment.
文摘本文采用湍流热对流的并行直接数值模拟(Parallel Direct Method of Direct Numerical Simulation,PDM-DNS),计算了系列Ra数的二维方腔Rayleigh-Bénard热对流.选取典型Ra=10^(10),Pr=4.3,讨论了流动平均场的近底板区域温度分布.温度边界层沿横向可分为三个区域:羽流发射区、冲击区和大尺度环流剪切区.在羽流发射区温度剖面存在对数律,冲击区和大尺度环流剪切区没有明显的对数律特征.横向平均得到的温度剖面也存在对数律.研究系列Ra数的横向平均温度剖面,得到温度变化满足A×log(y)+B的关系,温度剖面对数律振幅–A和Ra数呈现标度关系–A^Ra^(–0.145),与实验中发现的关系基本一致.
文摘提出二维湍流热对流DNS模拟的并行直接求解方法(Parallel Direct Method of DNS,PDMDNS),在"天河二号"超级计算机上实现高Ra和极高Ra湍流热对流大规模DNS计算。高分辨率的湍流热对流计算结果表明不同Ra(10~8≤Ra≤10^(13))的瞬时温度场的流场特性完全不同。较低Ra流场中有明显的大尺度环流和角涡;较高Ra流场中羽流运动充满随机性;更高Ra流场出现小尺寸漩涡并不断从上下底板产生,这些涡相互影响作用,随大尺度环流一起作绕行运动。二维湍流热对流的Nu与Ra存在标度关系,标度律约为0.3。
基金The Project was supported by the National Natural Science Foundation of China (Grants 11452002, 11521091, and 11372362)MOST (China) 973 Project (Grant 2009CB724100).
文摘Logarithmic boundary layers have been observed in different regions in turbulence. However, how thermal plumes correlate to the log law of temperature and how the velocity profile changes with pressure gradient are not fully understood. Here, we perform three-dimensional simulations of turbulence in a slim-box without the front and back walls with aspect ratio, width:depth:height=L:D:H=1:1/6:1width:depth:height=L:D:H=1:1/6:1 (respectively corresponding to xx, yy and zz coordinates), in the Rayleigh number Ra=[1×10^8,1×10^10]Ra=[1×10^8,1×10^10] for Prandtl number Pr=0.7Pr=0.7. To investigate the structures of the viscous and thermal boundary layers, we examine the velocity profiles in the streamwise and vertical directions (i.e. UU and WW) along with the mean temperature profile throughout the plume-impacting, plume-ejecting, and wind-shearing regions. The velocity profile is successfully quantified by a two-layer function of a stress length, e^+u=e^+0(z^+)3/2[1+(z^+/z^+sub)4]^1/4eu+=e^+0(z+)3/2[1+(z+/zsub+)4]1/4, as proposed by She et al.(J Fluid Mech, 2017), though it is neither \pb type nor logarithmic. In contrast, the temperature profile in the plume-ejecting region is logarithmic for all simulated cases, being attributed to the emission of thermal plumes. The coefficient of the temperature log-law, AA, can be described by composition of the thermal stress length ■■θ0■θ0■ and the thicknesses of thermal boundary layer z■subzsub■ and z?bufzbuf■, i.e. A■z?sub/(■■θ0z■buf3/2)A■zsub?/(■θ0■zbuf^3/2). The adverse pressure gradient responsible for turning the wind direction contributes to intensively emitting plumes and the logarithmic temperature profile at the plume-ejecting region. The Nusselt number scaling and the local heat flux in the slim box are consistent with previous results of the confined cells. Therefore, the slim-box RBC is a preferable system for investigating in-box kinetic and thermal structures of turbulent convection with the large-scale circulation in a fixed p
基金supported by the Natural Science Foundation of Guangdong Province(Grant No.2020A1515011094)the Science,Technology and Innovation Commission of Shenzhen Municipality(Grant No.KQJSCX20180328165817522).
文摘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 Nu.The 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)plates.There exists a zonal flow for Ra≥3×10^(6) with the free-slip plates in the periodic sample.In 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.
基金supported by the National Natural Science Foundation of China(Grants 11988102,11825204,92052201,91852202)the Program of Shanghai Academic Research Leader(Grant 19XD1421400)Science and Technology Innovation Plan Of Shanghai Science and Technology Commission(STCSM)(Project 19JC1412802).
文摘We present an experimental study of flow structures in turbulent Rayleigh-Bénard convection in annular cells of aspect ratiosΓ=1,2 and 4,and radius ratio 0.5.The convecting fluid is water with Prandtl number Pr=4.3 and 5.3.Rayleigh number Ra ranges 4.8×10^(7)≤Ra≤4.5×10^(10).The dipole state(two-roll flow structure)forΓ=1 and the quadrupole state(fourroll flow structure)forΓ=2 and 4 are found by multi-temperature-probe measurement.Nusselt number Nu is described by a power-law scaling Nu=0.11Ra^(0.31),which is insensitive to the change of flow structures.However,the Reynolds number Re is influenced by increasing aspect ratios,where Re is found to scale with Ra andΓas Re~Ra^(0.46)Γ^(-0.52).The normalized amplitudes of two flow structures as a function of Ra exist difference.Based on relative weights of the first four modes using the Fourier analysis,we find that the first mode dominates inΓ=1 cell,but the second mode contains the most energy inΓ=2 and 4 cells.With increasingΓ,the flow structures exhibit different characteristics.
基金supported by the Natural Science Foundationof China(Grant Nos.11988102,92052201,91852202,11825204,12102246 and 11972220).
文摘The tuning of turbulent Rayleigh-Bénard(RB)convection in a box is realized numerically by designed rough element arrangement.Considering the nonlinear dynamics of the thermal turbulence system,five models with rough elements of different widths and the same height are proposed to tune the fluid flow heat-transport capacity.Numerical simulations are performed using spectral element method for Rayleigh number in the range 10^(6)≤Ra≤10^(9) and a fixed Prandtl number Pr=0.7.It is found that heat transport is enhanced for large roughness widths as the interaction between the large-scale circulation and secondary flows inside the cavity regions between the rough elements promotes the eruptions of thermal plumes,but is suppressed for small ones as more heat are trapped inside the cavities.In all the rough models studied,different scaling exponents for the heat transport are identified and the influences of roughness arrangement on flow structure are studied.
