Geodesic Acoustic Mode in Toroidally Axisymmetric Plasmas with Non-Circular Cross Sections

The geodesic acoustic mode （GAM）, first predicted by Winsor, Johnson and Dawson in an attempt to explain some experimentally observed low frequency oscillations in stellarators, is a special electrostatic fluid mode with low mode number and coupled with the so-called geodesic curvature of a toroidally confined plasma. The recent on this mode are due to the close relevance of the stabi- lization of drift turbulence by both the zonal flow and the GAM. Previously, the GAM was illustrated in very simple geometries,     

Studies of the Geodesic Acoustic Mode Zonal Flow in HL-2A Tokamak Plasmas

Zonal flows are universal phenomena in nature and have attracted interse interest in the magnetic fusion plasma physics community in recent years. The active studies in this field are aimed at an understanding of nonlinear physics processes responsible for coherent structure formation and for anomalous transport in plasmas.     

Numerical Investigation of Finite k Effect on Damping Rate of Geodesic Acoustic Mode in Collisionless Plasmas

Numerical method is applied to the investigation of the GAM damping rate with the finite k effects included. It is found that generally the damping rate given by the analytical method is smaller than that given by the numerical method, and the analytical damping rate has good approximation in the high q region （q 〉 4）. The difference between the analytical and numerical damping rates increases with the increasing kpi. However, for the short-wavelength case （kpi = 0.2）, the analytical methods are only good enough around q = 4 because of the slow convergence of Bessel function with the large variable.     

Collisionless Damping of Geodesic Acoustic Mode in a Multi‐Ion Plasma with Superthermal Ions

The dispersion relation of geodesic acoustic mode is investigated in a multi‐ion toroidal plasma on the basis of linear gyrokinetic equations where ions are assumed to take a nonextensive distribution. It is found that the frequency of GAM becomes larger with the decrease of q. Consequently, GAM will damp more rapidly. The effective charge corresponding to the maximum damping rate is found to move towards unity as q decreases. As indicates that in a plasma with superthermal bulk ions, the influence of the impurity ion on the damping rate is weakened. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pore-Resolving Simulations of Biomass Char Particle Combustion

Biomass char morphology affects combustion behavior at the particle scale for zone II conditions, in which both heterogeneous reaction and intra-particle diffusion govern the overall rate. Furthermore, particle-scale processes affect reactor-scale outputs, and reactor-scale simulations are sensitive to particle-scale models. However, most char particle combustion models employ coarse-grained, effective-continuum approaches, which treat all porosity at the subgrid-scale. Effective-continuum approaches are not valid or accurate in the presence of large, irregular pores which can approach the size of the particle. A 3-D, pore-resolving CFD simulation approach using real biomass char particle geometries obtained from X-ray micro-computed tomography (micro-CT) is therefore used to examine the impact of morphology on zone II combustion for pulverized (∼100 µm) biomass char particles for the first time. In contrast to larger, millimeter to centimeter sized particles, the sub-millimeter, high aspect ratio biomass char particles exhibited localized reactant penetration into the innermost regions of the particles, facilitated by the presence of large pores connected to the external surface. The oxygen mole fraction distributions were governed by the large pore morphology, were non-monotonic with distance from the surface, and achieved minima in thick microporous char regions surrounding the large pores. A comparison between the pore-resolving simulation and an equivalent, spatially resolved, effective-continuum simulation revealed that even in the microporous char, the effective-continuum model underpredicted reactant penetration. A careful comparison was then performed between 30 pore-resolving particle simulations and several effectiveness factor models that employed particle-specific parameters. Commonly used uniform cylinder models significantly underpredicted effectiveness factors for these real pulverized pine char particles, while accessible hollow cylinder models achieved less than 10% relative error when averaged over all 30 particles.     

Non-relativistic Geodesic Behaviors for a Massive Charged Particle Falling in de Sitter Spacetime

In this article, continuing the work done in the previous paper (Fathi et al. in Int. J. Theor. Phys. 51:1938–1945, 2012), we apply a Lagrangian formalism to demonstrate the shape of the geodesic motions for a massive charged particle which is falling freely in a de Sitter spacetime. We will show that a spiral shape of the trajectory is available, due to the logarithmic behavior of time, with respect to the proper time.     

Gyrokinetic Analysis of Influence of Electron Beam Eccentricity on Operation of Higher-Order Mode in a Coaxial Cavity Gyrotron

A gyrokinetic analysis is presented to the influence of the electron beam eccentricity on the power and starting current of a coaxial cavity gyrotron, which operates in a higher-order mode TE_{31, 17, 1} with a frequency of 165 GHz. It is found that the starting current becomes larger because of the existence of the electron-beam eccentricity. Especially, the power will be decreased substantially by the electron beam eccentricity, for example, down to 93% of the power without any eccentricity even if the eccentricity is 1% of the outer conductor radius. The acceptable range of the electron beam voltage and operating magnetic field for the establishment of the electromagnetic oscillation is narrowed by the electron-beam eccentricity.     

Numerical Modeling of Acoustic Processes using Particle Interactions

Russian Physics Journal - A method for numerical modeling of acoustic processes in solids relying on the model of interacting particles in a body-centered cubic crystal lattice is proposed. Every...     

Lagrangian Particle Approach to Large Eddy Simulations of Hydrodynamic Turbulence

The flux of energy from large to small scales in hydrodynamic turbulence controls the dissipation of energy at a given scale in the fluid. An accurate parametrization of this flux is a prerequisite in order to devise reliable methods to simulate turbulent flows without resolving all the scales of motion. This problem is discussed in the context of a particle method based on the Smooth Particles Hydrodynamics algorithm. Motivated by the von Karman–Howarth–Kolmogorov exact relation for the energy flux, and by Lagrangian dynamics considerations we postulate an energy transfer term which is quadratic in the velocity and formally time reversal invariant. The numerical simulation of the model however is observed to spontaneously break the time reversal symmetry, demonstrating that the proposed term acts on average as the desired eddy damping.     

Acoustic modeling of perforated plates with bias flow for Large-Eddy Simulations

The study of the acoustic effect of perforated plates by Large-Eddy Simulations is reported. The ability of compressible Large-Eddy Simulations to provide data on the flow around a perforated plate and the associated acoustic damping is demonstrated. In particular, assumptions of existing models of the acoustic effect of perforated plate are assessed thanks to the Large-Eddy Simulations results. The question of modeling the effect of perforated plates is then addressed in the context of thermo-acoustic instabilities of gas turbine combustion chambers. Details are provided about the implementation, validation and application of a homogeneous boundary condition modeling the acoustic effect of perforated plates for compressible Large-Eddy Simulations of the flow in combustions chambers cooled by full-coverage film cooling.     

Dissipative Particle Dynamics Simulations of Domain Growth and Phase Separation in Binary Immiscible Fluids

用并行耗散粒子动力学方法研究了二维和三维体系中不同淬火深度下二元不相容流体旋节线分离的相区增长过程. 在二维体系中,在浅度淬火条件下得到了动力学标度指数为1/2的融合机制和2/3的惯性流体动力学机制,在深度淬火情况下发现了有限尺寸效应. 在三维体系中,浅度淬火条件下n=1/3的扩散机制和两种不同淬火深度下n=2/3的惯性流体动力学机制. 由于耗散粒子动力学软相互作用势的本质,所以粘性效应无法被清晰地反映,在该时间区间里,不论是浅度还是深度淬火都表现出n=1/2的指数关系.     

Geodesic Motion in Spinning Spaces

In this paper the generalized equations for spinning space are investigated and the constants of motion are derived in terms of the solutions of these equations. We study the geodesic motion of the pseudo-classical spinning particles in the spacetime produced by an idealized cosmic string and the non-extreme stationary axisymmetric black hole spacetime. The bound state orbits in a plane are discussed. We also show, for a conical spacetime and the Kerr spacetime, that the geodesic motion of spinning particles is different.     

利用GEANT4研究轻带电粒子诱发反应出射中子双微分产额

轻带电粒子诱发反应产生次级中子的研究对于加速器屏蔽设计和优化具有重要意义。利用Geant4程序结合INCL、BIC、BERT物理模型分别计算了33 MeV的d核、65 MeV的3He核和4He核轰击厚的碳、铜和铅靶在轻带电粒子诱发反应产生次级中子的研究对于加速器屏蔽设计和优化具有重要意义。利用Geant4程序结合INCL、BIC、BERT物理模型分别计算了33 MeV的d核、65 MeV的3He核和4He核轰击厚的碳、铜和铅靶在$0^{\circ}$,$15^{\circ}$,$45^{\circ}$,$75^{\circ}$和$135^{\circ}$等方向出射中子的双微分产额,并与实验数据进行了比较。研究表明,对于33 MeV的d核诱发的核反应,INCL模型的计算结果基本上再现了碳靶和铜靶的实验数据,但高估了铅靶直接过程产生的中子。BIC模型和BERT模型的计算结果没有重现弹核削裂过程对应的宽峰。对于65 MeV的3He核诱发的核反应,三个模型的计算结果均未能重现前向角弹核削裂过程产生的中子,但在$15^{\circ}$,$45^{\circ}$,$75^{\circ}$和$135^{\circ}$上三个模型的计算结果与实验数据符合较好。对于65 MeV的4He核诱发的核反应,INCL模型的计算结果与碳靶和铜靶的实验数据符合较好,但低估了铅靶的中子产额。BIC模型和BERT模型的计算结果低估了碳靶的实验数据,且在大角度上略微高估了铅靶的实验数据。     

Smart Geodesic Tracing in GRworkbench

We have developed a new tool for numerical work in General Relativity: GRworkbench. We discuss how GRworkbench's implementation of a numerically-amenable analogue to Differential Geometry facilitates the development of robust and chart-independent numerical algorithms. We consider, as an example, geodesic tracing on two charts covering the exterior Schwarzschild space-time.     

Geodesic curvatures in kerr space

The structure of the sources of a gravitational field in Schwarzchild and Kerr spaces is investigated using the method of geodesic curvature. The curvature is calculated in Schwarzchild space for an Isotropie and time-like congruence and in Kerr space for two Isotropie congruences. An analysis of the curvature is made.     

Frequency Response of the Sample Vibration Mode in Scanning Probe Acoustic Microscope

Based on the interaction mechanism between tip and sample in the contact mode of a scanning probe acoustic microscope （SPAM）, an active mass of the sample is introduced in the mass-spring model. The tip motion and frequency response of the sample vibration mode in the SPAM are calculated by the Lagrange equation with dissipation function. For the silicon tip and glass assemblage in the SPAM the frequency response is simulated and it is in agreement with the experimental result. The living myoblast cells on the glass slide are imaged at resonance frequencies of the SPAM system, which are 20 kHz, 30 kHz and 120 kHz. It is shown that good contrast of SPAM images could be obtained when the system is operated at the resonance frequencies of the system in high and low-frequency regions.