Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation （DNS）. For an uncontrolled cavity, as the thickness of the boundary layer declines, transition ...Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation （DNS）. For an uncontrolled cavity, as the thickness of the boundary layer declines, transition of the dominant mode from the steady mode to the Rossiter Ⅱ mode and then to the Rossiter III mode is observed due to the change of vortex-corner interactions. Meanwhile, a low frequency mode appears. However, the wake mode observed in a subsonic cavity flow is absent in the current simulation. The oscillation frequencies obtained from a global dynamic mode decomposition （DMD） approach are consistent with the local power spectral density （PSD） analysis. The dominant mode transition is clearly shown by the dynamic modes obtained from the DMD. A passive control technique of substituting the cavity trailing edge with a quarter-circle is studied. As the effective cavity length increases, the dominant mode transition from the Rossiter Ⅱ mode to the Rossiter Ⅲ mode occurs. With the control, the pressure oscillations are reduced significantly. The interaction of the shear layer and the recirculation zone is greatly weakened, combined with weaker shear layer instability, responsible for the suppression of pressure oscillations. Moreover, active control using steady subsonic mass injection upstream of a cavity leading edge can stabilize the flow.展开更多
Complex phenomena such as prolongedly undamped ultra-low frequency oscillation(ULFO)and eigenmode re-excitation are observed in the simulations of hydroelectric power systems.Emphases are put on nonlinearities and mod...Complex phenomena such as prolongedly undamped ultra-low frequency oscillation(ULFO)and eigenmode re-excitation are observed in the simulations of hydroelectric power systems.Emphases are put on nonlinearities and mode interactions,which cannot be analyzed by traditional eigen-analysis methods.In order to investigate the mechanism of the evolvement of the nonlinear dynamic process in ULFO,this paper proposes a method to analyze the mode interactions quantificationally.First,a disturbed trajectory is decoupled into a set of time-varying components.Second,transfer matrices of eigenmodes are extracted along the trajectory.Third,consecutive sequences of eigenvalues and trajectories of components are formed by a proposed technique.Based on the decoupled components and transfer matrices,the mechanisms of mode interactions and inheritance relationships between eigenmodes are analyzed.The causes and developments of the above complex phenomena are revealed by the proposed method in a test two-machine system.Meanwhile,the accuracy of the eigenmode matching technique is verified in the New England system.展开更多
A linear and nonlinear analysis of crossed-field gyrotron operating on whispering-gallery mode TE<sub>m11</sub> is presented. The detailed discussions are given for the starting oscillationcondition, the e...A linear and nonlinear analysis of crossed-field gyrotron operating on whispering-gallery mode TE<sub>m11</sub> is presented. The detailed discussions are given for the starting oscillationcondition, the effect of the internal conductor in coaxial cavity on the cut-off frequency of modeand the spacing between adjacent modes as well as the effect of the applied voltage on thecyclotron frequency of electron, the starting oscillation beam power and the electron efficiency.It is shown that the efficiency of 41% can be attained for m=2. A comparison of these resultswith those in cylindrical cavity is made. The analyses are helpful for the engineering design.展开更多
为研究分网接入方式下电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)系统的交互振荡模式及阻尼特征,基于系统的状态空间模型及系列文章(一)建立的运动方程模型,提取了表征逆变侧...为研究分网接入方式下电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)系统的交互振荡模式及阻尼特征,基于系统的状态空间模型及系列文章(一)建立的运动方程模型,提取了表征逆变侧电气与控制环节强耦合特性的多个弱阻尼交互振荡模式,研究了不同短路比工况下交互振荡模式的变化特征。在此基础上,通过复转矩系数法量化评估了整流侧/逆变侧内部自稳性路径及双极交互作用致稳性路径对主导交互振荡模式阻尼特性的贡献度。结果表明:1)不同短路比工况下交互振荡模式的阻尼比会进行重新分配;2)当逆变侧正负极短路比相差较大时,双极交互作用较弱,正负极系统的稳定性由2个交互振荡模式各自主导,且稳定性特征有所差异;3)当逆变侧正负极短路比相近时,双极间动态交互加强,交互振荡模式会同时主导参与系统两极的稳定性,正负极稳定性特征相似。展开更多
该文主要研究了由开环模式耦合引起的统一潮流控制器(UPFC)与系统的强动态交互作用对系统机电振荡模式的影响。建立了含UPFC的多机电力系统闭环互联动态模型,将UPFC与系统分为两个互联开环子系统。当UPFC开环控制模式与开环机电模式在...该文主要研究了由开环模式耦合引起的统一潮流控制器(UPFC)与系统的强动态交互作用对系统机电振荡模式的影响。建立了含UPFC的多机电力系统闭环互联动态模型,将UPFC与系统分为两个互联开环子系统。当UPFC开环控制模式与开环机电模式在复平面上位置靠近时,相应的两闭环模式彼此相斥并趋向反方向移动,该模式耦合现象可造成闭环机电模式的阻尼严重恶化。依据阻尼转矩分析理论,该文从UPFC向系统中同步机提供阻尼贡献这一角度清晰阐释了开环模式耦合时UPFC对机电模式的动态影响。另外,提出一个量化指标,能够近似估计开环模式耦合情况下两个相关的闭环模式在复平面的相对位移。利用新英格兰10机系统与我国华东某500 k V等值电网算例详细说明和验证了所得结论。该文还比较了UPFC串、并联电压源换流器(VSC)对系统影响的强弱,并分析了UPFC潮流调节与安装地点两个因素对模式耦合的影响。展开更多
基金supported by the National Natural Science Foundation of China(Nos.11232011 and11402262)the 111 Project of China(No.B07033)+1 种基金the China Postdoctoral Science Foundation(No.2014M561833)the Fundamental Research Funds for the Central Universities
文摘Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation （DNS）. For an uncontrolled cavity, as the thickness of the boundary layer declines, transition of the dominant mode from the steady mode to the Rossiter Ⅱ mode and then to the Rossiter III mode is observed due to the change of vortex-corner interactions. Meanwhile, a low frequency mode appears. However, the wake mode observed in a subsonic cavity flow is absent in the current simulation. The oscillation frequencies obtained from a global dynamic mode decomposition （DMD） approach are consistent with the local power spectral density （PSD） analysis. The dominant mode transition is clearly shown by the dynamic modes obtained from the DMD. A passive control technique of substituting the cavity trailing edge with a quarter-circle is studied. As the effective cavity length increases, the dominant mode transition from the Rossiter Ⅱ mode to the Rossiter Ⅲ mode occurs. With the control, the pressure oscillations are reduced significantly. The interaction of the shear layer and the recirculation zone is greatly weakened, combined with weaker shear layer instability, responsible for the suppression of pressure oscillations. Moreover, active control using steady subsonic mass injection upstream of a cavity leading edge can stabilize the flow.
基金supported by the Program of State Grid Corporation of China“Theoretical BasisAlgorithmand Application of Trajectory Eigenvalue Method”
文摘Complex phenomena such as prolongedly undamped ultra-low frequency oscillation(ULFO)and eigenmode re-excitation are observed in the simulations of hydroelectric power systems.Emphases are put on nonlinearities and mode interactions,which cannot be analyzed by traditional eigen-analysis methods.In order to investigate the mechanism of the evolvement of the nonlinear dynamic process in ULFO,this paper proposes a method to analyze the mode interactions quantificationally.First,a disturbed trajectory is decoupled into a set of time-varying components.Second,transfer matrices of eigenmodes are extracted along the trajectory.Third,consecutive sequences of eigenvalues and trajectories of components are formed by a proposed technique.Based on the decoupled components and transfer matrices,the mechanisms of mode interactions and inheritance relationships between eigenmodes are analyzed.The causes and developments of the above complex phenomena are revealed by the proposed method in a test two-machine system.Meanwhile,the accuracy of the eigenmode matching technique is verified in the New England system.
文摘A linear and nonlinear analysis of crossed-field gyrotron operating on whispering-gallery mode TE<sub>m11</sub> is presented. The detailed discussions are given for the starting oscillationcondition, the effect of the internal conductor in coaxial cavity on the cut-off frequency of modeand the spacing between adjacent modes as well as the effect of the applied voltage on thecyclotron frequency of electron, the starting oscillation beam power and the electron efficiency.It is shown that the efficiency of 41% can be attained for m=2. A comparison of these resultswith those in cylindrical cavity is made. The analyses are helpful for the engineering design.
文摘为研究分网接入方式下电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)系统的交互振荡模式及阻尼特征,基于系统的状态空间模型及系列文章(一)建立的运动方程模型,提取了表征逆变侧电气与控制环节强耦合特性的多个弱阻尼交互振荡模式,研究了不同短路比工况下交互振荡模式的变化特征。在此基础上,通过复转矩系数法量化评估了整流侧/逆变侧内部自稳性路径及双极交互作用致稳性路径对主导交互振荡模式阻尼特性的贡献度。结果表明:1)不同短路比工况下交互振荡模式的阻尼比会进行重新分配;2)当逆变侧正负极短路比相差较大时,双极交互作用较弱,正负极系统的稳定性由2个交互振荡模式各自主导,且稳定性特征有所差异;3)当逆变侧正负极短路比相近时,双极间动态交互加强,交互振荡模式会同时主导参与系统两极的稳定性,正负极稳定性特征相似。
文摘该文主要研究了由开环模式耦合引起的统一潮流控制器(UPFC)与系统的强动态交互作用对系统机电振荡模式的影响。建立了含UPFC的多机电力系统闭环互联动态模型,将UPFC与系统分为两个互联开环子系统。当UPFC开环控制模式与开环机电模式在复平面上位置靠近时,相应的两闭环模式彼此相斥并趋向反方向移动,该模式耦合现象可造成闭环机电模式的阻尼严重恶化。依据阻尼转矩分析理论,该文从UPFC向系统中同步机提供阻尼贡献这一角度清晰阐释了开环模式耦合时UPFC对机电模式的动态影响。另外,提出一个量化指标,能够近似估计开环模式耦合情况下两个相关的闭环模式在复平面的相对位移。利用新英格兰10机系统与我国华东某500 k V等值电网算例详细说明和验证了所得结论。该文还比较了UPFC串、并联电压源换流器(VSC)对系统影响的强弱,并分析了UPFC潮流调节与安装地点两个因素对模式耦合的影响。
