In the present review, recent progress on the vortex identification methods are introduced with a focus on the newly proposed omega method（ Ω method）. The advantages of Ω method are summarized with many illustrati...In the present review, recent progress on the vortex identification methods are introduced with a focus on the newly proposed omega method（ Ω method）. The advantages of Ω method are summarized with many illustrating examples. Furthermore, comparing with other existing methods（e.g., Q criterion and λ2 criterion）, one of the characteristics of Ω method is its independence on the chosen threshold values for vortex identifications. The important parameters involved for the practical applications of Ω method are further discussed in detail together with the physical interpretation of the Ω and some suggestions of the future work. Other emerging topics（e.g., Lagrangian coherent structure and Rortex） are also introduced with comments.展开更多
Vortex double layers (VDLs) and vortex projectiles (VPs) are the essential coherent structures which emerge in the shock excited (s/f/s) planar parallel "curtain" simulations of a 2D shock tube with PPM. The...Vortex double layers (VDLs) and vortex projectiles (VPs) are the essential coherent structures which emerge in the shock excited (s/f/s) planar parallel "curtain" simulations of a 2D shock tube with PPM. These opposite signed layers, formed by shock induced baroclinic deposition of vorticity, "ind" and are strongly affected by secondary reflected shocks and vortex interactions. In our visiometric mode of working, we quantify several of these processes and introduce time epochs to discuss the emerging phenomena and normalizations to scale (collapse) the data at M =1.5 and 2.0. This versatile configuration, easily obtained in the laboratory, allows us to study the formation, evolution and reacceleration of VPs and stratified turbulence and mixing.展开更多
The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics （CFD） turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under ze...The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics （CFD） turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation （DNS） data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Reo, skin-friction coefficient, Cf, approximate entropy generation rates, S, dissipation coefficient, Cd , and the intermittency, y, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes （RANS） turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition SST k - ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other RANS models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.展开更多
Vortices have been regarded as the building blocks and muscles of turbulence for a long time. To better describe and analyze vortices or vortical structures, recently a new physical quantity called Liutex (previously ...Vortices have been regarded as the building blocks and muscles of turbulence for a long time. To better describe and analyze vortices or vortical structures, recently a new physical quantity called Liutex (previously named Rortex) is introduced to present the rigid rotation part of fluid motion (Liu et al. 2018). Since turbulence is closely related to the vortex, it can be postulated that there exists no turbulence without Liutex. According to direct numerical simulations (DNS) and experiments, forest of hairpin vortices has been found in transitional and low Reynolds number turbulent flows, while one-leg vortices are predominant in full developed turbulent flows. This paper demonstrates that the hairpin vortex is unstable. The hairpin vortex will be weakened or lose one leg by the shear and Liutex interaction, based on the Liutex definition and mathematical analysis without any physical assumptions. The asymmetry of the vortex is caused by the interaction of symmetric shear and symmetric Liutex since the smaller element of a pair of vorticity elements determines the rotational strength. For a 2-D fluid rotation, if a disturbance shear effects the larger element, the rotation strength will not be changed, but if the disturbance shear effects the smaller element, the rotation strength will be immediately changed due to the definition of the Liutex strength. For a rigid rotation, if the shearing part of the vorticity and Liutex present the same directions, e.g., clockwise, the Liutex strength will not be changed. If the shearing part of the vorticity and Liutex present different directions, e.g., one clockwise and another counterclockwise, the Liutex strength will be weakened.Consequently, the hairpin vortex could lose the symmetry and even deform to a one-leg vortex. The one-leg vortex cannot keep balance, and the chaotic motion and flow fluctuation are doomed. This is considered as the mathematical foundation of turbulence formation. The DNS results of boundary layer transition are used to justify this theory.展开更多
为探究湍流状态下涡发生器对矩形截面螺旋细通道传热与熵产的影响,课题组采用RNGκ-ε湍流模型对内置5种不同涡发生器的螺旋细通道的传热和熵产进行了数值研究。选取的涡发生器结构为具有相同长宽高的矩形、棱形、椭圆形及2种放置方式...为探究湍流状态下涡发生器对矩形截面螺旋细通道传热与熵产的影响,课题组采用RNGκ-ε湍流模型对内置5种不同涡发生器的螺旋细通道的传热和熵产进行了数值研究。选取的涡发生器结构为具有相同长宽高的矩形、棱形、椭圆形及2种放置方式不同的三角形。在热流密度300 k W/m^2和雷诺数Re 4500~12000的条件下,对内置不同涡发生器的螺旋细通道与光滑螺旋细通道的摩阻系数、努塞尔数、热阻和总熵产进行分析。结果表明:在研究的雷诺数范围内,5种加入涡发生器结构的通道内流体努塞尔数、摩阻系数均大于光滑通道,热阻均低于光滑通道;当Re<7500时总熵产率均低于光滑通道,而7500<Re<12000时反之。几种涡发生器结构中矩形涡发生器结构能源利用率最佳。展开更多
A new strategy is presented to explain the creation and persistence of zonal flows widely observed in plasma edge turbulence.The core physics in the edge regime of the magnetic-fusion tokamaks can be described qualita...A new strategy is presented to explain the creation and persistence of zonal flows widely observed in plasma edge turbulence.The core physics in the edge regime of the magnetic-fusion tokamaks can be described qualitatively by the one-state modified Hasegawa-Mima(MHM for short)model,which creates enhanced zonal flows and more physically relevant features in comparison with the familiar Charney-Hasegawa-Mima(CHM for short)model for both plasma and geophysical flows.The generation mechanism of zonal jets is displayed from the secondary instability analysis via nonlinear interactions with a background base state.Strong exponential growth in the zonal modes is induced due to a non-zonal drift wave base state in the MHM model,while stabilizing damping effect is shown with a zonal flow base state.Together with the selective decay effect from the dissipation,the secondary instability offers a complete characterization of the convergence process to the purely zonal structure.Direct numerical simulations with and without dissipation are carried out to confirm the instability theory.It shows clearly the emergence of a dominant zonal flow from pure non-zonal drift waves with small perturbation in the initial configuration.In comparison,the CHM model does not create instability in the zonal modes and usually converges to homogeneous turbulence.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51506051,51606221)
文摘In the present review, recent progress on the vortex identification methods are introduced with a focus on the newly proposed omega method（ Ω method）. The advantages of Ω method are summarized with many illustrating examples. Furthermore, comparing with other existing methods（e.g., Q criterion and λ2 criterion）, one of the characteristics of Ω method is its independence on the chosen threshold values for vortex identifications. The important parameters involved for the practical applications of Ω method are further discussed in detail together with the physical interpretation of the Ω and some suggestions of the future work. Other emerging topics（e.g., Lagrangian coherent structure and Rortex） are also introduced with comments.
文摘Vortex double layers (VDLs) and vortex projectiles (VPs) are the essential coherent structures which emerge in the shock excited (s/f/s) planar parallel "curtain" simulations of a 2D shock tube with PPM. These opposite signed layers, formed by shock induced baroclinic deposition of vorticity, "ind" and are strongly affected by secondary reflected shocks and vortex interactions. In our visiometric mode of working, we quantify several of these processes and introduce time epochs to discuss the emerging phenomena and normalizations to scale (collapse) the data at M =1.5 and 2.0. This versatile configuration, easily obtained in the laboratory, allows us to study the formation, evolution and reacceleration of VPs and stratified turbulence and mixing.
基金supported by the U.S. Department of Energy,Office of Science,Basic Energy Sciences,under Award # DE-SC0004751
文摘The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics （CFD） turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation （DNS） data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Reo, skin-friction coefficient, Cf, approximate entropy generation rates, S, dissipation coefficient, Cd , and the intermittency, y, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes （RANS） turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition SST k - ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other RANS models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.
基金the National Nature Science Foundation of China (Grant No. 91530325).
文摘Vortices have been regarded as the building blocks and muscles of turbulence for a long time. To better describe and analyze vortices or vortical structures, recently a new physical quantity called Liutex (previously named Rortex) is introduced to present the rigid rotation part of fluid motion (Liu et al. 2018). Since turbulence is closely related to the vortex, it can be postulated that there exists no turbulence without Liutex. According to direct numerical simulations (DNS) and experiments, forest of hairpin vortices has been found in transitional and low Reynolds number turbulent flows, while one-leg vortices are predominant in full developed turbulent flows. This paper demonstrates that the hairpin vortex is unstable. The hairpin vortex will be weakened or lose one leg by the shear and Liutex interaction, based on the Liutex definition and mathematical analysis without any physical assumptions. The asymmetry of the vortex is caused by the interaction of symmetric shear and symmetric Liutex since the smaller element of a pair of vorticity elements determines the rotational strength. For a 2-D fluid rotation, if a disturbance shear effects the larger element, the rotation strength will not be changed, but if the disturbance shear effects the smaller element, the rotation strength will be immediately changed due to the definition of the Liutex strength. For a rigid rotation, if the shearing part of the vorticity and Liutex present the same directions, e.g., clockwise, the Liutex strength will not be changed. If the shearing part of the vorticity and Liutex present different directions, e.g., one clockwise and another counterclockwise, the Liutex strength will be weakened.Consequently, the hairpin vortex could lose the symmetry and even deform to a one-leg vortex. The one-leg vortex cannot keep balance, and the chaotic motion and flow fluctuation are doomed. This is considered as the mathematical foundation of turbulence formation. The DNS results of boundary layer transition are used to justify this theory.
文摘为探究湍流状态下涡发生器对矩形截面螺旋细通道传热与熵产的影响,课题组采用RNGκ-ε湍流模型对内置5种不同涡发生器的螺旋细通道的传热和熵产进行了数值研究。选取的涡发生器结构为具有相同长宽高的矩形、棱形、椭圆形及2种放置方式不同的三角形。在热流密度300 k W/m^2和雷诺数Re 4500~12000的条件下,对内置不同涡发生器的螺旋细通道与光滑螺旋细通道的摩阻系数、努塞尔数、热阻和总熵产进行分析。结果表明:在研究的雷诺数范围内,5种加入涡发生器结构的通道内流体努塞尔数、摩阻系数均大于光滑通道,热阻均低于光滑通道;当Re<7500时总熵产率均低于光滑通道,而7500<Re<12000时反之。几种涡发生器结构中矩形涡发生器结构能源利用率最佳。
基金supported by the Office of Naval Research through MURI(No.N00014-16-1-2161)
文摘A new strategy is presented to explain the creation and persistence of zonal flows widely observed in plasma edge turbulence.The core physics in the edge regime of the magnetic-fusion tokamaks can be described qualitatively by the one-state modified Hasegawa-Mima(MHM for short)model,which creates enhanced zonal flows and more physically relevant features in comparison with the familiar Charney-Hasegawa-Mima(CHM for short)model for both plasma and geophysical flows.The generation mechanism of zonal jets is displayed from the secondary instability analysis via nonlinear interactions with a background base state.Strong exponential growth in the zonal modes is induced due to a non-zonal drift wave base state in the MHM model,while stabilizing damping effect is shown with a zonal flow base state.Together with the selective decay effect from the dissipation,the secondary instability offers a complete characterization of the convergence process to the purely zonal structure.Direct numerical simulations with and without dissipation are carried out to confirm the instability theory.It shows clearly the emergence of a dominant zonal flow from pure non-zonal drift waves with small perturbation in the initial configuration.In comparison,the CHM model does not create instability in the zonal modes and usually converges to homogeneous turbulence.
