We start with a recently introduced spherically symmetric geodesic fluid model (arXiv: 1601.07030) whose energy-momentum tensor (EMT) in the comoving frame is dust-like with nontrivial energy flux. In the non-comoving...We start with a recently introduced spherically symmetric geodesic fluid model (arXiv: 1601.07030) whose energy-momentum tensor (EMT) in the comoving frame is dust-like with nontrivial energy flux. In the non-comoving energy frame (vanishing energy flux), the same EMT contains besides dust only radial pressure. We present Einstein’s equations together with the matter equations in static spherically symmetric coordinates. These equations are self-contained (four equations for four unknowns). We solve them analytically except for a resulting nonlinear ordinary differential equation (ODE) for the gravitational potential. This ODE can be rewritten as a Lienard differential equation which, however, may be transformed into a rational Abel differential equation of the first kind. Finally, we list some open mathematical problems and outline possible physical applications (galactic halos, dark energy stars) and related open problems.展开更多
A non-stationary solution of the field equation in 1+1 dimension gravity is given with the advanced Eddington-Finkelstein coordinate.There exists a 1+1 dimensional non-stationary black hole in the space-time.The locat...A non-stationary solution of the field equation in 1+1 dimension gravity is given with the advanced Eddington-Finkelstein coordinate.There exists a 1+1 dimensional non-stationary black hole in the space-time.The location of event horizons,the Hawking temperature and the spectrum of thermal radiation are derived.展开更多
Hawking radiation of the stationary Kerr–de Sitter black hole is investigated using the relativistic Hamilton–Jacobi method. Meanwhile, extending this work to a non-stationary black hole using Dirac equations and ge...Hawking radiation of the stationary Kerr–de Sitter black hole is investigated using the relativistic Hamilton–Jacobi method. Meanwhile, extending this work to a non-stationary black hole using Dirac equations and generalized tortoise coordinate transformation, we derived the locations, the temperature of the thermal radiation as well as the maximum energy of the non-thermal radiation. It is found that the surface gravity and the Hawking temperature depend on both time and different angles. An extra coupling effect is obtained in the thermal radiation spectrum of Dirac particles which is absent from thermal radiation of scalar particles. Further, the chemical potential derived from the thermal radiation spectrum of scalar particle has been found to be equal to the highest energy of the negative energy state of the scalar particle in the non-thermal radiation for the Kerr–de Sitter black hole. It is also shown that for stationary black hole space time, these two different methods give the same Hawking radiation temperature.展开更多
According to the stationary principle of potential energy and the generalized coordinate method, a stiffness matrix of a beam element considering distortion effects is derived. Using the stiffness matrix of the beam e...According to the stationary principle of potential energy and the generalized coordinate method, a stiffness matrix of a beam element considering distortion effects is derived. Using the stiffness matrix of the beam element, a finite element program for computing thin-walled box steel beams is developed. And the program can take the section distortion and warping effects into account. The influences of diaphragm spacing on the mechanical behavior of thin-walled box beams are analyzed by the program. The numerical analysis shows that setting diaphragms have the greatest influence on the distortion normal stress, while there is very little influence on the bending normal stress. Only when the distance of adjacent diaphragms decreases to a certain value, will the distortion normal stress in the thin-walled box beam obviously reduce under the distortion load. Finally, a distortion-warping coefficient γ is introduced for simplifying the calculation of the longitudinal normal stress of thin-walled box beams. When the ratio of diaphragms adjacent space L to the maximum section dimension H is less than 2, the distortion-warping coefficient γ tends to one, which means that the distortion normal stress of the thin-walled box beam tends to zero, and the effect of the section distortion can be ignored.展开更多
控制策略对保证基于模块化多电平换流器的高压直流输电系统(modular multilevel converter-high voltage direct current transmission,MMC-HVDC)正常工作起着至关重要的作用。经典双闭环矢量控制具有控制精度高、响应速度快等特点,但...控制策略对保证基于模块化多电平换流器的高压直流输电系统(modular multilevel converter-high voltage direct current transmission,MMC-HVDC)正常工作起着至关重要的作用。经典双闭环矢量控制具有控制精度高、响应速度快等特点,但需要控制器设计和控制参数调节;模型预测控制具有建模直观、无需控制器优点,但预测指令的求解过程稍显复杂。该文在两相静止坐标系下,综合2种控制策略的优点,基于功能等效思想,将双闭环矢量控制等效划分为2个模型预测环节,构成改进模型预测控制。研究2种不同的控制指令获取方法,避免传统预测控制运算量大、权重因子调节复杂等问题。通过对模块化多电平换流器技术(modular multi-level converter,MMC)数学模型离散化,建立MMC阀侧电压控制指令输出预测模型,实现内环控制器等同功能。分别针对电网电压平衡和不平衡工况,从功率跟踪、负序电流抑制及功率波动抑制等方面验证所设计控制策略的有效性。展开更多
文摘We start with a recently introduced spherically symmetric geodesic fluid model (arXiv: 1601.07030) whose energy-momentum tensor (EMT) in the comoving frame is dust-like with nontrivial energy flux. In the non-comoving energy frame (vanishing energy flux), the same EMT contains besides dust only radial pressure. We present Einstein’s equations together with the matter equations in static spherically symmetric coordinates. These equations are self-contained (four equations for four unknowns). We solve them analytically except for a resulting nonlinear ordinary differential equation (ODE) for the gravitational potential. This ODE can be rewritten as a Lienard differential equation which, however, may be transformed into a rational Abel differential equation of the first kind. Finally, we list some open mathematical problems and outline possible physical applications (galactic halos, dark energy stars) and related open problems.
文摘A non-stationary solution of the field equation in 1+1 dimension gravity is given with the advanced Eddington-Finkelstein coordinate.There exists a 1+1 dimensional non-stationary black hole in the space-time.The location of event horizons,the Hawking temperature and the spectrum of thermal radiation are derived.
文摘Hawking radiation of the stationary Kerr–de Sitter black hole is investigated using the relativistic Hamilton–Jacobi method. Meanwhile, extending this work to a non-stationary black hole using Dirac equations and generalized tortoise coordinate transformation, we derived the locations, the temperature of the thermal radiation as well as the maximum energy of the non-thermal radiation. It is found that the surface gravity and the Hawking temperature depend on both time and different angles. An extra coupling effect is obtained in the thermal radiation spectrum of Dirac particles which is absent from thermal radiation of scalar particles. Further, the chemical potential derived from the thermal radiation spectrum of scalar particle has been found to be equal to the highest energy of the negative energy state of the scalar particle in the non-thermal radiation for the Kerr–de Sitter black hole. It is also shown that for stationary black hole space time, these two different methods give the same Hawking radiation temperature.
基金Specialized Research Fund for the Doctoral Program of Higher Education (No.20070247002)
文摘According to the stationary principle of potential energy and the generalized coordinate method, a stiffness matrix of a beam element considering distortion effects is derived. Using the stiffness matrix of the beam element, a finite element program for computing thin-walled box steel beams is developed. And the program can take the section distortion and warping effects into account. The influences of diaphragm spacing on the mechanical behavior of thin-walled box beams are analyzed by the program. The numerical analysis shows that setting diaphragms have the greatest influence on the distortion normal stress, while there is very little influence on the bending normal stress. Only when the distance of adjacent diaphragms decreases to a certain value, will the distortion normal stress in the thin-walled box beam obviously reduce under the distortion load. Finally, a distortion-warping coefficient γ is introduced for simplifying the calculation of the longitudinal normal stress of thin-walled box beams. When the ratio of diaphragms adjacent space L to the maximum section dimension H is less than 2, the distortion-warping coefficient γ tends to one, which means that the distortion normal stress of the thin-walled box beam tends to zero, and the effect of the section distortion can be ignored.
文摘控制策略对保证基于模块化多电平换流器的高压直流输电系统(modular multilevel converter-high voltage direct current transmission,MMC-HVDC)正常工作起着至关重要的作用。经典双闭环矢量控制具有控制精度高、响应速度快等特点,但需要控制器设计和控制参数调节;模型预测控制具有建模直观、无需控制器优点,但预测指令的求解过程稍显复杂。该文在两相静止坐标系下,综合2种控制策略的优点,基于功能等效思想,将双闭环矢量控制等效划分为2个模型预测环节,构成改进模型预测控制。研究2种不同的控制指令获取方法,避免传统预测控制运算量大、权重因子调节复杂等问题。通过对模块化多电平换流器技术(modular multi-level converter,MMC)数学模型离散化,建立MMC阀侧电压控制指令输出预测模型,实现内环控制器等同功能。分别针对电网电压平衡和不平衡工况,从功率跟踪、负序电流抑制及功率波动抑制等方面验证所设计控制策略的有效性。
