This study unveils the evolution of two major early signals in the North Pacific atmosphere-ocean system that heralded abnormal high-pressure blockings and cold-vortex activities across Northeast China, based on an an...This study unveils the evolution of two major early signals in the North Pacific atmosphere-ocean system that heralded abnormal high-pressure blockings and cold-vortex activities across Northeast China, based on an analysis of the configurations of major modes including the polar vortex, the North Pacific Oscillation （NPO）, and SST in the preceding winter and spring and atmospheric low-frequency disturbances in Northeast China. We analyzed these aspects to understand the atmosphere ocean physical coupling processes characterized by the two early signals, and here we explain the possible mechanisms through which dipole circulation anomalies affect the summer low-temperature processes in Northeast China. We further analyzed the interdecadal variation background and associated physical processes of the two early signals.展开更多
The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5...The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder;the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.展开更多
Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields ...Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields around the cylinder were measured by Particle Image Velocimetry (PIV). The Keulegan-Carpenter number (KC) varied in the range from 5 to 20 and the viscous parameter beta = Re / KC was set at 500 (Re is Reynolds number). It was found that the strength and frequency of the lift force increased with KC number, the main frequency of the lift force being three times the frequency of the oscillatory flow at KC = 20. The movement and strength of the vortices around the cylinder are discussed for different KC numbers.展开更多
In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-p...In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-perforated conic shroud is numerically simulated at a Reynolds number of 100.Based on previous work by Meiburg and Lasheras (1988),the streamwise and vertical interactions with spanwise vortices are introduced and analyzed.Then vortex-shedding patterns in the near wake for different flow regimes are reinspected and illustrated from the view of these two interactions.Generally,in regime Ⅰ,spanwise vortices are a little distorted due to the weak interaction.Then in regime Ⅱ,spanwise vortices,even though curved obviously,are still shed synchronously with moderate streamwise and vertical interactions.But in regime Ⅲ,violently wavy spanwise vortices in some vortex-shedding patterns,typically an Ω-type vortex,are mainly attributed to the strong vertical interactions,while other cases,such as multiple vortex-shedding patterns in sub-regime Ⅲ-D,are resulted from complex streamwise and vertical interactions.A special phenomenon,spacial distribution of streamwise and vertical components of vorticity with specific signs in the near wake,is analyzed based on two models of streamwise and vertical vortices in explaining physical reasons of top and bottom shear layers wavily varied across the span.Then these two models and above two interactions are unified.Finally two sign laws are summarized:the first sign law for streamwise and vertical components of vorticity is positive in the upper shear layer,but negative in the lower shear layer,while the second sign law for three vorticity components is always negative in the wake.展开更多
A numerical study is presented on roll damping of ships by solving Navier-Stokes equation. Two Dimensional unsteady incompressible viscous flow around the rolling cylinders of various ship-like cross sections are nume...A numerical study is presented on roll damping of ships by solving Navier-Stokes equation. Two Dimensional unsteady incompressible viscous flow around the rolling cylinders of various ship-like cross sections are numerically simulated by use of the computational scheme previously developed by the authors. The numerical results show that the location of the vortices is very similar to the existing experimental result. For comparison of vortex patterns and roll damping on various ship-like cross sections, various distributions of shear stress and pressure on the rolling ship hull surface are presented in this paper. It is found that there are two vortices around the midship-like section and there is one vortex around the fore or stern section. Based on these simulation results, the roll damping of a ship including viscous effects is calculated. The contribution of pressure to the roll moment is larger than the contribution of frictional shear stress.展开更多
As a typical fluid-solid interaction problem,vortex-induced vibration(VIV)is common in engineering,so it is vital to study its control mechanism.Numerical simulations of the active control of VIV of a cylinder are car...As a typical fluid-solid interaction problem,vortex-induced vibration(VIV)is common in engineering,so it is vital to study its control mechanism.Numerical simulations of the active control of VIV of a cylinder are carried out in this study.The splitter plate with harmonic oscillation is used as the control device for the dynamic response of the cylinder.The displacement response,lift and drag coefficient,vibration frequency of the cylinder,energy efficiency of control strategy,and characteristics of the flow field are widely analyzed to reveal the physical mechanism of the control system.The results show that the displacement response of the cylinder can be limited in a small range by the control without feedback in most cases except for high reduced velocity.In addition,the control strategy can be changed through feedback control to keep much superior control effects at the high reduced velocity.The oscillatory splitter plate delays the vortex shedding of shear layers generated on the cylinder,the wake vortices with opposite sense of rotation are paralleled with the streamwise direction,and crosswise distances of them are reduced.Thus,the lift on the cylinder is greatly decreased due to the modification of the flow pattern induced by the oscillatory splitter plate.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 41175083 and 41275096)the Special Fund for Meteorological Scientific Research in the Public Interest (Grant Nos. GYHY201006020,GYHY201106016,and GYHY201106015)
文摘This study unveils the evolution of two major early signals in the North Pacific atmosphere-ocean system that heralded abnormal high-pressure blockings and cold-vortex activities across Northeast China, based on an analysis of the configurations of major modes including the polar vortex, the North Pacific Oscillation （NPO）, and SST in the preceding winter and spring and atmospheric low-frequency disturbances in Northeast China. We analyzed these aspects to understand the atmosphere ocean physical coupling processes characterized by the two early signals, and here we explain the possible mechanisms through which dipole circulation anomalies affect the summer low-temperature processes in Northeast China. We further analyzed the interdecadal variation background and associated physical processes of the two early signals.
文摘The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder;the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.
基金National Science Foundation of China and British Council
文摘Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields around the cylinder were measured by Particle Image Velocimetry (PIV). The Keulegan-Carpenter number (KC) varied in the range from 5 to 20 and the viscous parameter beta = Re / KC was set at 500 (Re is Reynolds number). It was found that the strength and frequency of the lift force increased with KC number, the main frequency of the lift force being three times the frequency of the oscillatory flow at KC = 20. The movement and strength of the vortices around the cylinder are discussed for different KC numbers.
文摘In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-perforated conic shroud is numerically simulated at a Reynolds number of 100.Based on previous work by Meiburg and Lasheras (1988),the streamwise and vertical interactions with spanwise vortices are introduced and analyzed.Then vortex-shedding patterns in the near wake for different flow regimes are reinspected and illustrated from the view of these two interactions.Generally,in regime Ⅰ,spanwise vortices are a little distorted due to the weak interaction.Then in regime Ⅱ,spanwise vortices,even though curved obviously,are still shed synchronously with moderate streamwise and vertical interactions.But in regime Ⅲ,violently wavy spanwise vortices in some vortex-shedding patterns,typically an Ω-type vortex,are mainly attributed to the strong vertical interactions,while other cases,such as multiple vortex-shedding patterns in sub-regime Ⅲ-D,are resulted from complex streamwise and vertical interactions.A special phenomenon,spacial distribution of streamwise and vertical components of vorticity with specific signs in the near wake,is analyzed based on two models of streamwise and vertical vortices in explaining physical reasons of top and bottom shear layers wavily varied across the span.Then these two models and above two interactions are unified.Finally two sign laws are summarized:the first sign law for streamwise and vertical components of vorticity is positive in the upper shear layer,but negative in the lower shear layer,while the second sign law for three vorticity components is always negative in the wake.
文摘A numerical study is presented on roll damping of ships by solving Navier-Stokes equation. Two Dimensional unsteady incompressible viscous flow around the rolling cylinders of various ship-like cross sections are numerically simulated by use of the computational scheme previously developed by the authors. The numerical results show that the location of the vortices is very similar to the existing experimental result. For comparison of vortex patterns and roll damping on various ship-like cross sections, various distributions of shear stress and pressure on the rolling ship hull surface are presented in this paper. It is found that there are two vortices around the midship-like section and there is one vortex around the fore or stern section. Based on these simulation results, the roll damping of a ship including viscous effects is calculated. The contribution of pressure to the roll moment is larger than the contribution of frictional shear stress.
基金supported by the National Natural Science Foundation of China(Grant No.11872174)the Fundamental Research Funds for the Central Universities(Grant No.B200202236)the Key Laboratory of Port,Waterway&Sedimentation Engineering Ministry of Communications,PRC(Grant No.Yk220001-2).
文摘As a typical fluid-solid interaction problem,vortex-induced vibration(VIV)is common in engineering,so it is vital to study its control mechanism.Numerical simulations of the active control of VIV of a cylinder are carried out in this study.The splitter plate with harmonic oscillation is used as the control device for the dynamic response of the cylinder.The displacement response,lift and drag coefficient,vibration frequency of the cylinder,energy efficiency of control strategy,and characteristics of the flow field are widely analyzed to reveal the physical mechanism of the control system.The results show that the displacement response of the cylinder can be limited in a small range by the control without feedback in most cases except for high reduced velocity.In addition,the control strategy can be changed through feedback control to keep much superior control effects at the high reduced velocity.The oscillatory splitter plate delays the vortex shedding of shear layers generated on the cylinder,the wake vortices with opposite sense of rotation are paralleled with the streamwise direction,and crosswise distances of them are reduced.Thus,the lift on the cylinder is greatly decreased due to the modification of the flow pattern induced by the oscillatory splitter plate.
