In the flow on a mobile bed in an open channel, sand ripple often appears after the sediment begins to move. Different scholars have different views on the formation of sand ripples. This paper holds that as the rippl...In the flow on a mobile bed in an open channel, sand ripple often appears after the sediment begins to move. Different scholars have different views on the formation of sand ripples. This paper holds that as the ripple in general is very small, its formation is due to the instability of the laminar flow or the evolution of the small-scale coherent structures in the sublayer adjacent to the wall of the open channel. When the shear stresses caused by the disturbing waves or the coherent structure near the bed surface boundary and the water flow itself are greater than the shields stresses, responses on the bed surface appear and the sand ripple forms. If the frequency of the shear stress caused by the disturbance is close to the natural frequency of the sand grains that produced resonance, such a phenomenon is called the 'detection property' of the sediment. It is at this point that the maximum resonance appears and the sand ripple develops rapidly.展开更多
Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the linear stability beh...Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the linear stability behaviour of parallel shear flow, presents unstable results of plane Couette flow and pipe Poiseuille flow to two-dimensional or axisymmetric disturbances for the first time, and obtains neutral curves of these two motions under certain definition.展开更多
Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the nonlinear stability ...Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the nonlinear stability behaviour of parallel shear flow, carries on stability calculation taking account of the perturbations of background turbulence noise under certain assumption, and obtains some results in accordance qualitatively with those of experiment for plane Poiseuille flow and pipe Poiseuille flow.展开更多
This paper suggests a hydrodynamic stability theory of distorted laminar flow, and presents a kind of distortion profile of mean velocity in parallel shear flow. With such distortion profiles, the new theory can be us...This paper suggests a hydrodynamic stability theory of distorted laminar flow, and presents a kind of distortion profile of mean velocity in parallel shear flow. With such distortion profiles, the new theory can be used to investigate the stability behaviour of parallel shear flow, and thus suggests a new possible approach to instability.展开更多
This paper provides a review of the recent results on the stability of vortex sheets in compressible flows.Vortex sheets are contact discontinuities of the underlying flows.The vortex sheet problem is a free boundary ...This paper provides a review of the recent results on the stability of vortex sheets in compressible flows.Vortex sheets are contact discontinuities of the underlying flows.The vortex sheet problem is a free boundary problem with a characteristic boundary and is challenging in analysis.The formulation of the vortex sheet problem will be introduced.The linear stability and nonlinear stability for both the two-dimensional two-phase compressible flows and the two-dimensional elastic flows are summarized.The linear stability of vortex sheets for the three-dimensional elastic flows is also presented.The difficulties of the vortex sheet problems and the ideas of proofs are discussed.展开更多
Load flow studies play a critical role in the analysis of power systems. They enable the computation of voltage, current, and power flows in a power system. They provide valuable insights into the steady-state perform...Load flow studies play a critical role in the analysis of power systems. They enable the computation of voltage, current, and power flows in a power system. They provide valuable insights into the steady-state performance of the power system under different operating conditions. Choosing a slack bus is a vital step in conducting load flow simulations. A slack bus is a PV bus that includes a generator and is used to balance real and reactive power during load flow studies. Many studies have been conducted on the selection of slack buses in load flow analysis. However, varied conclusions regarding the impact on system losses and power flows were obtained during these studies. Therefore, using the IEEE-14 bus test system, this study investigated the effects of slack bus selection in strong and weak grids by alternating slack buses among PV buses and observing the effects on bus voltage magnitude, bus voltage phase angle, total power flows, and active and reactive power losses. The study noted that the effect of slack bus selection on these system quantities is contingent upon the voltage stability of the grid. Whereas in a robust grid, system losses and power flows remained constant irrespective of the choice of slack bus, a weak grid experienced some variations in these system quantities under similar circumstances. The simulation results led to the conclusion that, to a large extent, the voltage stability of the grid plays a significant role in determining the degree to which slack bus selection affects system losses and other quantities in load flow studies.展开更多
Flow transition from laminar to turbulent is prerequisite to decide whereabouts to apply surface flow control techniques. This appears missing in a number of works in which the control effects were merely investigated...Flow transition from laminar to turbulent is prerequisite to decide whereabouts to apply surface flow control techniques. This appears missing in a number of works in which the control effects were merely investigated without getting insight into alteration of transition position. The aim of this study is to capture the correct position of transition over NACA0012 aerofoil at different angles of attack. Firstly, an implicit, time marching, high resolution total variation diminishing (TVD) scheme was developed to solve the governing Navier-Stokes equations for compressible fluid flows around aerofoil sections to obtain velocity profiles around the aerofoil surfaces. Secondly, the linear instability solver based on the Orr-Sommerfeld equations and the eg methods were developed to calculate the onset of transition over the aerofoil surfaces. For the low subsonic Mach number of 0.16, the accuracy of the compressible solutions was assessed by some available experimental results of low speed incompressible flows. In all cases, transition positions were accurately predicted which shows applicability and superiority of the present work to be extended for higher Mach number compressible flows. Here, transition prediction methodology is described and the results of this analysis without active flow control or separation are presented.展开更多
In the present paper the variational solution of velocity profile for an incompressible laminar and fully developed flow in isosceles triangular ducts is derived by applying the Kantovorich method. The theoretical and...In the present paper the variational solution of velocity profile for an incompressible laminar and fully developed flow in isosceles triangular ducts is derived by applying the Kantovorich method. The theoretical and experimental results of pressure loss are also given. The velocity distribution model, additional pressure loss coefficient and calculating method of inlet length in the entrance region of isosceles triangular ducts are also derived, which are suitable for various kinds of vertex angles. The calculations and experiments are also performed for two models: the isosceles triangular channels with vertex angles 2a=45.1 and 2a= 60 . Comparisons are made between the theoretical analysis in this paper and those of the other authors. It can be seen that the present analytical result is of high. accuracy and wide practicability, and agrees well with the authors' experiment.展开更多
A linear stability analysis is performed for a plume flow inside a cylinder of aspect ratio 1. The configu- ration is identical to that used by Lopez and Marques (2013) for their direct numerical simulation study, I...A linear stability analysis is performed for a plume flow inside a cylinder of aspect ratio 1. The configu- ration is identical to that used by Lopez and Marques (2013) for their direct numerical simulation study, It is found that the first bifurcation, which leads to a periodic axisymmetric flow state, is accurately pre- dicted by linear analysis: both the critical Rayleigh number and the global frequency are consistent with the reported DNS results. It is further shown that pressure feedback drives the global mode, rather than absolute instability.展开更多
It is of great practical and theoretical importance to study the properties and flow behavior of laminar jet,because it tightly links the physical phenomena of nonlinerity such as flow stability and transition and so ...It is of great practical and theoretical importance to study the properties and flow behavior of laminar jet,because it tightly links the physical phenomena of nonlinerity such as flow stability and transition and so forth.However,many problems and differences exist in ba- sic concepts and in understanding of stable and transitional properties and flow patterns of ax- isymmetric laminar jet.Therefore,a further study on this subject is needed to evaluate the exist- ing understandings and to find out the mechanism governing the behavior of actual axisymmetric laminar jet flow. In this thesis,the flow behavior of axisymmetric laminar jet issuing from nozzles of several diameters in the direction of gravity was studied experimentally using flow visualization tech- nique in the range of Reynolds number from tens to thousands.The unstable and transitional processes in the laminar jet were observed.The influence on the jet flow of different geometry contours of jet chamber boundary was investigated.Some new flow phenomena were discovered which gave deeper understanding of flow patterns and their dependent factors in low Reynolds number axisymmetric laminar jet.A theoretical model to describe the variation of the axisym- metric laminar jet length influenced by the weak density difference is proposed based on the ex- perimental observation,which will be helpful in recognizing the flow behavior of axisymmetric laminar jet of low Reynolds numbers.展开更多
This paper concerns the dynamic characteristics of incompressible laminar flow hydrostatic bearings. An improved method based on the mathematical perturbation technique for calculating the stiffness and damping coeffi...This paper concerns the dynamic characteristics of incompressible laminar flow hydrostatic bearings. An improved method based on the mathematical perturbation technique for calculating the stiffness and damping coefficients of hydrostatic bearings is presented;it raises the calculation efficiency by uncoupling the recess flow continuity equations from the solving of the Reynolds equation. The dynamic coefficients of a water-lubricated four-recess hydrostatic bearing are calculated with both the improved method and the conventional method. The comparisons between these two methods validate the effectiveness and correctness of the improved method. Furthermore, the dynamic response and stability characteristics of a rotor supported by the hydrostatic bearing are calculated and compared with experimental results. First, the dynamic responses calculated with the linear and nonlinear bearing dynamic coefficients models show that the linear model is inaccurate if the rotor whirls in a large eccentricity ratio range, owing to the nonlinearity of stiffness and damping. Second, according to the numerical and experimental results, the nonlinearity of bearing stiffness and damping coefficients could induce double rotational frequency component in the unbalance response frequency spectrum. Finally, the numerical results indicate that the stability threshold speed of the bearing increases with eccentricity ratio. The improved method proposed can be used to evaluate the dynamic performance of hydrostatic bearings efficiently, and the bearing characteristics presented could contribute to a better understanding of the performance of waterlubricated hydrostatic bearings.展开更多
森林火灾后因火烧迹地土壤斥水性,导致坡面径流和土壤可蚀性增强,提高了火后泥石流易发性,而土壤团聚体稳定性是影响土壤入渗能力和侵蚀敏感性的关键指标。目前常用于火烧迹地土壤团聚体稳定性测定的水滴冲击测定方法(counting the numb...森林火灾后因火烧迹地土壤斥水性,导致坡面径流和土壤可蚀性增强,提高了火后泥石流易发性,而土壤团聚体稳定性是影响土壤入渗能力和侵蚀敏感性的关键指标。目前常用于火烧迹地土壤团聚体稳定性测定的水滴冲击测定方法(counting the number of water drop impacts,CND),不适用于原位测定且耗时较长(滴定一组团聚体需要数小时)。因此文章提出一种基于冲击振荡破坏效应的团聚体稳定性测定方法(shock and vibration damage method,SVD)。充分考虑容重、有机质含量和斥水性对土壤团聚体稳定性的影响,通过室内火烧模拟试验,制备了13种类型的土壤团聚体。采用自制的试验仪器进行SVD法正交试验测定土壤团聚体质量损失率,并与传统CND法测得的破坏团聚体的水滴数量进行对比。结果表明:SVD法的测定MT-6方案(冲击高度1 m、容器容水量40%、冲击5次、测定团聚体20颗)与CND法的测定结果具有很强的一致性(Kendall系数=0.797)和相关性(R2=0.634),测定时间较短(测定一组团聚体约5 min),且测定结果区分度较好(约62%的团聚体MLR位于区分度良好的40%~60%区间),将其作为SVD法的最优测定方案。此外,SVD法试验装置结构简单、便携易拆卸,可用于原位快速且定量地区分火烧迹地不同火烈度下土壤团聚体稳定性水平,对火烧迹地土壤侵蚀、水土流失治理以及火后泥石流起动机理研究具有重要指导意义。展开更多
文摘In the flow on a mobile bed in an open channel, sand ripple often appears after the sediment begins to move. Different scholars have different views on the formation of sand ripples. This paper holds that as the ripple in general is very small, its formation is due to the instability of the laminar flow or the evolution of the small-scale coherent structures in the sublayer adjacent to the wall of the open channel. When the shear stresses caused by the disturbing waves or the coherent structure near the bed surface boundary and the water flow itself are greater than the shields stresses, responses on the bed surface appear and the sand ripple forms. If the frequency of the shear stress caused by the disturbance is close to the natural frequency of the sand grains that produced resonance, such a phenomenon is called the 'detection property' of the sediment. It is at this point that the maximum resonance appears and the sand ripple develops rapidly.
文摘Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the linear stability behaviour of parallel shear flow, presents unstable results of plane Couette flow and pipe Poiseuille flow to two-dimensional or axisymmetric disturbances for the first time, and obtains neutral curves of these two motions under certain definition.
文摘Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the nonlinear stability behaviour of parallel shear flow, carries on stability calculation taking account of the perturbations of background turbulence noise under certain assumption, and obtains some results in accordance qualitatively with those of experiment for plane Poiseuille flow and pipe Poiseuille flow.
基金Project Supported by National Natural Science Foundation of China
文摘This paper suggests a hydrodynamic stability theory of distorted laminar flow, and presents a kind of distortion profile of mean velocity in parallel shear flow. With such distortion profiles, the new theory can be used to investigate the stability behaviour of parallel shear flow, and thus suggests a new possible approach to instability.
基金R.M.Chen is supported in part by the NSF grant DMS-1907584F.Huang was supported in part by the National Center for Mathematics and Interdisciplinary Sciences,Academy of Mathematics and Systems Science,Chinese Academy of Sciences and the National Natural Sciences Foundation of China under Grant Nos.11371349 and 11688101+1 种基金D.Wang was supported in part by the NSF under grant DMS-1907519D.Yuan was supported in part by the National Natural Sciences Foundation of China under Grant No.12001045 and the China Postdoctoral Science Foundation under Grant Nos.2020M680428 and 2021T140063.
文摘This paper provides a review of the recent results on the stability of vortex sheets in compressible flows.Vortex sheets are contact discontinuities of the underlying flows.The vortex sheet problem is a free boundary problem with a characteristic boundary and is challenging in analysis.The formulation of the vortex sheet problem will be introduced.The linear stability and nonlinear stability for both the two-dimensional two-phase compressible flows and the two-dimensional elastic flows are summarized.The linear stability of vortex sheets for the three-dimensional elastic flows is also presented.The difficulties of the vortex sheet problems and the ideas of proofs are discussed.
文摘Load flow studies play a critical role in the analysis of power systems. They enable the computation of voltage, current, and power flows in a power system. They provide valuable insights into the steady-state performance of the power system under different operating conditions. Choosing a slack bus is a vital step in conducting load flow simulations. A slack bus is a PV bus that includes a generator and is used to balance real and reactive power during load flow studies. Many studies have been conducted on the selection of slack buses in load flow analysis. However, varied conclusions regarding the impact on system losses and power flows were obtained during these studies. Therefore, using the IEEE-14 bus test system, this study investigated the effects of slack bus selection in strong and weak grids by alternating slack buses among PV buses and observing the effects on bus voltage magnitude, bus voltage phase angle, total power flows, and active and reactive power losses. The study noted that the effect of slack bus selection on these system quantities is contingent upon the voltage stability of the grid. Whereas in a robust grid, system losses and power flows remained constant irrespective of the choice of slack bus, a weak grid experienced some variations in these system quantities under similar circumstances. The simulation results led to the conclusion that, to a large extent, the voltage stability of the grid plays a significant role in determining the degree to which slack bus selection affects system losses and other quantities in load flow studies.
文摘Flow transition from laminar to turbulent is prerequisite to decide whereabouts to apply surface flow control techniques. This appears missing in a number of works in which the control effects were merely investigated without getting insight into alteration of transition position. The aim of this study is to capture the correct position of transition over NACA0012 aerofoil at different angles of attack. Firstly, an implicit, time marching, high resolution total variation diminishing (TVD) scheme was developed to solve the governing Navier-Stokes equations for compressible fluid flows around aerofoil sections to obtain velocity profiles around the aerofoil surfaces. Secondly, the linear instability solver based on the Orr-Sommerfeld equations and the eg methods were developed to calculate the onset of transition over the aerofoil surfaces. For the low subsonic Mach number of 0.16, the accuracy of the compressible solutions was assessed by some available experimental results of low speed incompressible flows. In all cases, transition positions were accurately predicted which shows applicability and superiority of the present work to be extended for higher Mach number compressible flows. Here, transition prediction methodology is described and the results of this analysis without active flow control or separation are presented.
文摘In the present paper the variational solution of velocity profile for an incompressible laminar and fully developed flow in isosceles triangular ducts is derived by applying the Kantovorich method. The theoretical and experimental results of pressure loss are also given. The velocity distribution model, additional pressure loss coefficient and calculating method of inlet length in the entrance region of isosceles triangular ducts are also derived, which are suitable for various kinds of vertex angles. The calculations and experiments are also performed for two models: the isosceles triangular channels with vertex angles 2a=45.1 and 2a= 60 . Comparisons are made between the theoretical analysis in this paper and those of the other authors. It can be seen that the present analytical result is of high. accuracy and wide practicability, and agrees well with the authors' experiment.
基金provided by the Agence Nationale de la Recherche under the "Cool Jazz" project
文摘A linear stability analysis is performed for a plume flow inside a cylinder of aspect ratio 1. The configu- ration is identical to that used by Lopez and Marques (2013) for their direct numerical simulation study, It is found that the first bifurcation, which leads to a periodic axisymmetric flow state, is accurately pre- dicted by linear analysis: both the critical Rayleigh number and the global frequency are consistent with the reported DNS results. It is further shown that pressure feedback drives the global mode, rather than absolute instability.
文摘It is of great practical and theoretical importance to study the properties and flow behavior of laminar jet,because it tightly links the physical phenomena of nonlinerity such as flow stability and transition and so forth.However,many problems and differences exist in ba- sic concepts and in understanding of stable and transitional properties and flow patterns of ax- isymmetric laminar jet.Therefore,a further study on this subject is needed to evaluate the exist- ing understandings and to find out the mechanism governing the behavior of actual axisymmetric laminar jet flow. In this thesis,the flow behavior of axisymmetric laminar jet issuing from nozzles of several diameters in the direction of gravity was studied experimentally using flow visualization tech- nique in the range of Reynolds number from tens to thousands.The unstable and transitional processes in the laminar jet were observed.The influence on the jet flow of different geometry contours of jet chamber boundary was investigated.Some new flow phenomena were discovered which gave deeper understanding of flow patterns and their dependent factors in low Reynolds number axisymmetric laminar jet.A theoretical model to describe the variation of the axisym- metric laminar jet length influenced by the weak density difference is proposed based on the ex- perimental observation,which will be helpful in recognizing the flow behavior of axisymmetric laminar jet of low Reynolds numbers.
文摘This paper concerns the dynamic characteristics of incompressible laminar flow hydrostatic bearings. An improved method based on the mathematical perturbation technique for calculating the stiffness and damping coefficients of hydrostatic bearings is presented;it raises the calculation efficiency by uncoupling the recess flow continuity equations from the solving of the Reynolds equation. The dynamic coefficients of a water-lubricated four-recess hydrostatic bearing are calculated with both the improved method and the conventional method. The comparisons between these two methods validate the effectiveness and correctness of the improved method. Furthermore, the dynamic response and stability characteristics of a rotor supported by the hydrostatic bearing are calculated and compared with experimental results. First, the dynamic responses calculated with the linear and nonlinear bearing dynamic coefficients models show that the linear model is inaccurate if the rotor whirls in a large eccentricity ratio range, owing to the nonlinearity of stiffness and damping. Second, according to the numerical and experimental results, the nonlinearity of bearing stiffness and damping coefficients could induce double rotational frequency component in the unbalance response frequency spectrum. Finally, the numerical results indicate that the stability threshold speed of the bearing increases with eccentricity ratio. The improved method proposed can be used to evaluate the dynamic performance of hydrostatic bearings efficiently, and the bearing characteristics presented could contribute to a better understanding of the performance of waterlubricated hydrostatic bearings.
文摘森林火灾后因火烧迹地土壤斥水性,导致坡面径流和土壤可蚀性增强,提高了火后泥石流易发性,而土壤团聚体稳定性是影响土壤入渗能力和侵蚀敏感性的关键指标。目前常用于火烧迹地土壤团聚体稳定性测定的水滴冲击测定方法(counting the number of water drop impacts,CND),不适用于原位测定且耗时较长(滴定一组团聚体需要数小时)。因此文章提出一种基于冲击振荡破坏效应的团聚体稳定性测定方法(shock and vibration damage method,SVD)。充分考虑容重、有机质含量和斥水性对土壤团聚体稳定性的影响,通过室内火烧模拟试验,制备了13种类型的土壤团聚体。采用自制的试验仪器进行SVD法正交试验测定土壤团聚体质量损失率,并与传统CND法测得的破坏团聚体的水滴数量进行对比。结果表明:SVD法的测定MT-6方案(冲击高度1 m、容器容水量40%、冲击5次、测定团聚体20颗)与CND法的测定结果具有很强的一致性(Kendall系数=0.797)和相关性(R2=0.634),测定时间较短(测定一组团聚体约5 min),且测定结果区分度较好(约62%的团聚体MLR位于区分度良好的40%~60%区间),将其作为SVD法的最优测定方案。此外,SVD法试验装置结构简单、便携易拆卸,可用于原位快速且定量地区分火烧迹地不同火烈度下土壤团聚体稳定性水平,对火烧迹地土壤侵蚀、水土流失治理以及火后泥石流起动机理研究具有重要指导意义。