The pre-gate suction vortex,gate-bottom-edge transverse vortex,gate-slot vertical vortex,and downstream-of-gate return vortex are important factors affecting the flow instability of flat gates,which may lead to fatigu...The pre-gate suction vortex,gate-bottom-edge transverse vortex,gate-slot vertical vortex,and downstream-of-gate return vortex are important factors affecting the flow instability of flat gates,which may lead to fatigue failure in severe cases.This study used the volume of fluid(VOF)model and large eddy simulation(LES)method to accurately capture the transient turbulence characteristics of flow under different water flow conditions and reveal the flow field and vortex structure.The Q—criterion,Omega(Ω)method,and latest third-generation Liutex vortex identification method were used to analyze and compare the pre-gate suction vortex,gate-slot vertical vortex,and downstream-of-gate return vortex,focusing on the ability of each vortex identification method to capture the flow field information and vortex characteristics.The results reveal that theΩmethod and Liutex method are less dependent on the threshold value,and the Liutex method captures a wide range of pre-gate vortices.Different flow conditions cause changes in the vortex structure of over-gate flow.When the relative opening of the gate is smaller,the intensity of the vortices in the flow field around the gate is greater,the return vortices downstream of the gate are more disordered,and the vortex changes are more violent,which in turn affects the efficient and stable operation of the gate.展开更多
本文回顾了涡定义和涡识别方法的发展历史,着重介绍了作者UTA(University of Texas at Arlington)团队及其合作者在涡科学和湍流研究的一些最新学术创新成果。UTA团队发现了可以定量描述流体刚性转动部分的物理量——Liutex向量,其主要...本文回顾了涡定义和涡识别方法的发展历史,着重介绍了作者UTA(University of Texas at Arlington)团队及其合作者在涡科学和湍流研究的一些最新学术创新成果。UTA团队发现了可以定量描述流体刚性转动部分的物理量——Liutex向量,其主要思想是把流体刚性转动从流体运动中提取出来,进而用Liutex来定义和识别涡结构,并已在广泛应用中证明了其作为涡识别方法的优越性。基于Liutex向量可以进一步研究涡量分解、速度梯度张量分解、流体运动分解、湍流结构、湍流生成机理以及旋涡的科学识别,为流体运动学的发展开辟了广阔的研究空间。区别于第一代涡识别方法和第二代涡识别方法,Liutex是一个向量,其方向代表当地转轴,大小代表当地流体刚性旋转角速度的二倍。本文详细介绍了基于Liutex向量的第三代涡的定义和识别方法,包括Liutex等值面、Liutex-Omega等值面、Liutex向量线、Liutex涡核线、以及最新发现的中低雷诺数湍流边界层中的Liutex-5/3幂次相似律,其发现很大程度上扩大了传统湍流能谱幂次律的适用范围,对建立湍流模型具有重要意义。展开更多
In the new vortex identification method(Liu et al.2016)to represent the rotation level and capture and visualize the vortices,proposed in our previous study,the independence of the reference frame and the Galilean inv...In the new vortex identification method(Liu et al.2016)to represent the rotation level and capture and visualize the vortices,proposed in our previous study,the independence of the reference frame and the Galilean invariant were not proved.In the present study,the Galilean invariance of the omega vortex identification method is proved and several examples are presented to verify the conclusion.展开更多
In this paper,the cavitating flow over a flexible NACA66 hydrofoil is studied numerically by a modified fluid-structure interaction strategy with particular emphasis on understanding the flow-induced vibration and the...In this paper,the cavitating flow over a flexible NACA66 hydrofoil is studied numerically by a modified fluid-structure interaction strategy with particular emphasis on understanding the flow-induced vibration and the cavitating vortical flow structures.The modified coupling approaches include(1)the hydrodynamic solution obtained by the large eddy simulation(LES)together with a homogenous cavitation model,(2)the structural deformation solved with a cantilever beam equation,(3)fluid-structural interpolation and volume mesh motion based on the radial basis functions and greedy algorithm.For the flexible hydrofoil,the dominant flow-induced vibration frequency is twice of the cavity shedding frequency.The cavity shedding frequency is same for the rigid and flexible hydrofoils,demonstrating that the structure vibration is not large enough to affect the cavitation evolution.The predicted cavitating behaviors are strongly three-dimensional,that is,the cavity is(a)of a triangular shape near the hydrofoil tip,(b)of a rectangular shape near the hydrofoil root,and(c)with a strong unsteadiness in the middle of the span,including the attached cavity growth,oscillation and shrinkage,break-off and collapse downstream.The unsteady hydroelastic response would strongly affect the cavitation shedding process with small-scale fragments at the cavity rear part.Furthermore,three vortex identification methods(i.e.,the vorticity,the Q-criteria and the Ω method)are adopted to investigate the cavitating vortex structures around the flexible hydrofoil.It is indicated that the cavity variation trend is consistent with the vortex evolution.The vortex structures are distributed near the foil trailing edge and in the cavitation region,especially at the cavity-liquid interface.With the transporting downstream the shedding cavities,the vortices gradually increase in the wake flows.展开更多
Based on the chemical cross-linking method,this paper uses polydimethylsiloxane with various viscosities of 10 cSt,20 cSt,50 cSt,and 100 cSt to synthesize mesoporous and macroporous SiOC ceramics.Their thermal conduct...Based on the chemical cross-linking method,this paper uses polydimethylsiloxane with various viscosities of 10 cSt,20 cSt,50 cSt,and 100 cSt to synthesize mesoporous and macroporous SiOC ceramics.Their thermal conductivities are measured by using 3ωmethod with high accuracy.Three typical models for their thermal conductivities,i.e.,series model(SM),maxwell-Eucken 1 model(ME1),and effective medium theory(EMT)model,are utilized to derive the empirical formula through the multi-parameter linear optimization algorithm,which agrees well with the experimental results.The effects of pore size and specific surface area on the overall thermal conductivity of the porous structure are explored.Interestingly,it is found that the thermal conductivities of both gas phase and solid phase inside the porous structure increase with the increasing pore size at the nanometer scale,but the overall thermal conductivity of the porous structure decreases with the increasing pore size.Scanning electron microscopy graphs corroborate that the extension of the heat transfer route and the barrier of more pores between the solid phases together cause the reduction of the gas-solid coupling thermal conductivity of SiOC ceramics with larger pore size.On the contrary,the miniaturization of individual particles through modulating the synthesis parameters can increase the number of small pores in the sample itself to meet the pseudo-lattice vibration conditions,which results in the increment of the gas-solid coupling thermal conductivity and the overall thermal conductivity of the porous structure.These findings would provide meaningful guidance for designing SiOC porous ceramic super-insulation materials with extremely low thermal conductivity.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.52069009,51369013).
文摘The pre-gate suction vortex,gate-bottom-edge transverse vortex,gate-slot vertical vortex,and downstream-of-gate return vortex are important factors affecting the flow instability of flat gates,which may lead to fatigue failure in severe cases.This study used the volume of fluid(VOF)model and large eddy simulation(LES)method to accurately capture the transient turbulence characteristics of flow under different water flow conditions and reveal the flow field and vortex structure.The Q—criterion,Omega(Ω)method,and latest third-generation Liutex vortex identification method were used to analyze and compare the pre-gate suction vortex,gate-slot vertical vortex,and downstream-of-gate return vortex,focusing on the ability of each vortex identification method to capture the flow field information and vortex characteristics.The results reveal that theΩmethod and Liutex method are less dependent on the threshold value,and the Liutex method captures a wide range of pre-gate vortices.Different flow conditions cause changes in the vortex structure of over-gate flow.When the relative opening of the gate is smaller,the intensity of the vortices in the flow field around the gate is greater,the return vortices downstream of the gate are more disordered,and the vortex changes are more violent,which in turn affects the efficient and stable operation of the gate.
文摘本文回顾了涡定义和涡识别方法的发展历史,着重介绍了作者UTA(University of Texas at Arlington)团队及其合作者在涡科学和湍流研究的一些最新学术创新成果。UTA团队发现了可以定量描述流体刚性转动部分的物理量——Liutex向量,其主要思想是把流体刚性转动从流体运动中提取出来,进而用Liutex来定义和识别涡结构,并已在广泛应用中证明了其作为涡识别方法的优越性。基于Liutex向量可以进一步研究涡量分解、速度梯度张量分解、流体运动分解、湍流结构、湍流生成机理以及旋涡的科学识别,为流体运动学的发展开辟了广阔的研究空间。区别于第一代涡识别方法和第二代涡识别方法,Liutex是一个向量,其方向代表当地转轴,大小代表当地流体刚性旋转角速度的二倍。本文详细介绍了基于Liutex向量的第三代涡的定义和识别方法,包括Liutex等值面、Liutex-Omega等值面、Liutex向量线、Liutex涡核线、以及最新发现的中低雷诺数湍流边界层中的Liutex-5/3幂次相似律,其发现很大程度上扩大了传统湍流能谱幂次律的适用范围,对建立湍流模型具有重要意义。
基金the National Natural Science Foundation of China(Grant Nos.91530325,11702159).
文摘In the new vortex identification method(Liu et al.2016)to represent the rotation level and capture and visualize the vortices,proposed in our previous study,the independence of the reference frame and the Galilean invariant were not proved.In the present study,the Galilean invariance of the omega vortex identification method is proved and several examples are presented to verify the conclusion.
基金supported by the National Natural Science Foundation of China(Grant Nos.11772340,11872065).
文摘In this paper,the cavitating flow over a flexible NACA66 hydrofoil is studied numerically by a modified fluid-structure interaction strategy with particular emphasis on understanding the flow-induced vibration and the cavitating vortical flow structures.The modified coupling approaches include(1)the hydrodynamic solution obtained by the large eddy simulation(LES)together with a homogenous cavitation model,(2)the structural deformation solved with a cantilever beam equation,(3)fluid-structural interpolation and volume mesh motion based on the radial basis functions and greedy algorithm.For the flexible hydrofoil,the dominant flow-induced vibration frequency is twice of the cavity shedding frequency.The cavity shedding frequency is same for the rigid and flexible hydrofoils,demonstrating that the structure vibration is not large enough to affect the cavitation evolution.The predicted cavitating behaviors are strongly three-dimensional,that is,the cavity is(a)of a triangular shape near the hydrofoil tip,(b)of a rectangular shape near the hydrofoil root,and(c)with a strong unsteadiness in the middle of the span,including the attached cavity growth,oscillation and shrinkage,break-off and collapse downstream.The unsteady hydroelastic response would strongly affect the cavitation shedding process with small-scale fragments at the cavity rear part.Furthermore,three vortex identification methods(i.e.,the vorticity,the Q-criteria and the Ω method)are adopted to investigate the cavitating vortex structures around the flexible hydrofoil.It is indicated that the cavity variation trend is consistent with the vortex evolution.The vortex structures are distributed near the foil trailing edge and in the cavitation region,especially at the cavity-liquid interface.With the transporting downstream the shedding cavities,the vortices gradually increase in the wake flows.
基金This work is financially supported by Beijing Natural Science Foundation(3202020)National Natural Science Foundation of China(No.51876008)+2 种基金Beijing Nova Program(Z201100006820065)Interdisciplinary Research Project for Young Teachers of USTB(Fundamental Research Funds for the Central Universities)(FRF-IDRY-19-004)WU Jin acknowledgesfinancial support from Guangdong Natural Science Funds Grant(2018A030313400).
文摘Based on the chemical cross-linking method,this paper uses polydimethylsiloxane with various viscosities of 10 cSt,20 cSt,50 cSt,and 100 cSt to synthesize mesoporous and macroporous SiOC ceramics.Their thermal conductivities are measured by using 3ωmethod with high accuracy.Three typical models for their thermal conductivities,i.e.,series model(SM),maxwell-Eucken 1 model(ME1),and effective medium theory(EMT)model,are utilized to derive the empirical formula through the multi-parameter linear optimization algorithm,which agrees well with the experimental results.The effects of pore size and specific surface area on the overall thermal conductivity of the porous structure are explored.Interestingly,it is found that the thermal conductivities of both gas phase and solid phase inside the porous structure increase with the increasing pore size at the nanometer scale,but the overall thermal conductivity of the porous structure decreases with the increasing pore size.Scanning electron microscopy graphs corroborate that the extension of the heat transfer route and the barrier of more pores between the solid phases together cause the reduction of the gas-solid coupling thermal conductivity of SiOC ceramics with larger pore size.On the contrary,the miniaturization of individual particles through modulating the synthesis parameters can increase the number of small pores in the sample itself to meet the pseudo-lattice vibration conditions,which results in the increment of the gas-solid coupling thermal conductivity and the overall thermal conductivity of the porous structure.These findings would provide meaningful guidance for designing SiOC porous ceramic super-insulation materials with extremely low thermal conductivity.
