Molecular dynamics simulation of gaseous ion-motion in electrostatic fields

A molecular dynamics (MD) method has been developed for the simulation of the motion of ions in neutral gases under the influence of homogeneous electrostatic fields. The method treats the translational motion of the ions and gas molecules classically and thus requires as input the ion-molecule interaction potentials. The continuous dissipation of a part of the ion-energy to a thermal bath is accomplished through the introduction of "iconical interactions" between ions and images of the neutrals created and stored in the memory of a computer during ion-atom encounters. The steady ion motion is then simulated by usual equilibrium MD methods using periodic boundary conditions. The resulting ion mobilities, effective temperatures, and third other velocity-distribution moments, expressed as skewness parameters, compare well with available results derived from the moment solution of Boltzmann equation and Monte Carlo simulations using the same interaction potentials in the cases of K+ in He and Ar, as well as of Ba+ in Ar. The additional reproduction of experimental data provides an independent test for the accuracy of the employed model potentials. Although the method has been applied to atomic systems it is easily extendable to the more complex molecular systems but at the expense of computer time. © 1995 American Institute of Physics

Third-order transport properties of ion-swarms from mobility and diffusion coefficients

A method is presented for the calculation of third order transport properties of ions drifting in gases under the action of an electrostatic field with the use of mobility and ion-diffusion coefficients. The approach is based on a three-temperature treatment of the Boltzmann equation for the ion transport and follows the development of generalized Einstein relations (GER), between diffusion coefficients and mobility. The whole procedure is tested by comparison with numerical and molecular dynamics simulation results for three available alkali ion-noble gas systems. Extension to systems involving internal degrees of freedom and inelastic collisions is shown to follow the development of molecular GER. © 2005 Elsevier B.V. All rights reserved

Velocity correlation functions, Fickian and higher order diffusion coefficients for ions in electrostatic fields via molecular dynamics simulation

The dynamic and transport properties of swarms of ions in a uniform electrostatic field are studied by using a molecular dynamics method. For a representative system, K+ in Ar, using a universal interaction model potential, second and third order ion-velocity correlation functions are determined at various field strengths. From them, Fickian diffusion coefficients parallel and perpendicular to the field, as well as higher order diffusion coefficients, Qzzz, are obtained within estimated overall accuracy 5% and 7%, respectively. Comparisons of the Fickian diffusion coefficients against results of the moment solution of Boltzmann kinetic equation and a Monte Carlo simulation method using the same interaction potential as well as against experimental data, reveal consistency among all calculation procedures and in addition agreement with drift tube measurements. These comparisons provide new tests for the accuracy of the employed interaction potential. The method has been applied for up to third order velocity correlations and diffusion coefficients but it is extendible to higher order dynamic and transport properties. © 1996 American Institute of Physics

Galloping of overhead transmission lines.

This Thesis describes a technique for collecting, moulding and testing naturally occurring ice accretions. Faithful reproductions of the ice shapes were cast in silicone rubber from which wind tunnel models were made. They were tested using a specifically designed wind tunnel rig which measured the aerodynamic lift, drag and pitching moment of the models. From the aerodynamic data the gradients of lift, drag and pitching moment of each ice shape were calculated. The aerodynamic data were consequently used in a two-dimensional two degree of freedom theoretical aerodynamic model which included aerodynamic lift, drag, moment, ice eccentricity, conductor wake effects and the mechanical properties of the conductor. Wind tunnel tests were carried out on a specifically designed wind tunnel dynamic rig. Instabilities of the coupled vertical/torsional galloping were established. Regions of instability were also predicted using a two-dimensional theoretical conductor model. The initial theoretical analysis formed the basis upon which a more sophisticated three-dimensional finite element aeroelastic model was developed. The effects of ice and wind on the natural frequencies and the stability of the conductor were investigated. The use of galloping control devices, the pendulum detuners was also examined. Results showed that the pendulums had a stabilising effect in controlling the vertical/torsional frequency ratio of twin bundles. The vibration characteristics and the stability of quad bundles were investigated using finite elements. In this case, the pendulums shifted the torsional frequencies of the bundle to higher values close to the corresponding vertical frequencies, thus enhancing coupling and having an adverse effect on stability. Finally, limitations in the performance of the pendulum detuners were predicted

Dynamic properties and third order diffusion coefficients of ions in electrostatic fields

Velocity correlation functions and third order diffusion coefficients of ions moving in a buffer gas under the influence of an electrostatic field are determined via molecular dynamics simulation. For the closed shell system of K+ in Ar using a universal interaction model potential, the general form of the third order correlation functions is found to be monotonically decaying in time except in the cases of (ΔvZ(0)ΔvX(t)2), (ΔvZ(0)ΔvY(t)2), and (ΔvZ(0)ΔvZ(t)2), with Δv(t)=v(t) -(v(t)) and the field in the z direction. These functions acquire positive slope at short times showing enhancement of correlations between instantaneous VZ components of the ions and their future kinetic energies or velocity measures. This feature is shown to quantify the dynamics of correlations between velocity components suggested in the past by Ong, Hogan, Lam and Viehland [Phys. Rev. A 45, 3997 (1992)] in order to explain the form of an ion velocity distribution function calculated through a Monte Carlo simulation method. In addition, within a stochastic analysis which establishes a relation between velocity correlation functions and third order diffusion coefficients, only two independent components of the diffusion tensor, Q∥ and O⊥, are predicted. We thereby calculate the O⊥ component, which has not been determined so far, over a wide field range. The magnitudes of the resulting third order diffusion coefficients indicate that their contribution to the ion transport in usual drift-tube measurements should be very small. © 1997 American Institute of Physics

Transport and dynamic properties of O2+(X 2IIg) in Kr under the action of an electrostatic field: Single or multiple potential energy surface treatment

Ion transport and dynamic properties are calculated through molecular dynamics simulation of the motion of O2+ in Kr under the action of an electrostatic field. The two lower potential energy surfaces X̃2A′ and Ã2A′ are considered for the interaction of the ground state of the ion with a closed shell noble gas. First, we study the reproduction of experimental mobility data through the use of single and multiple potential energy surfaces and establish the contribution of both lower energy states to the interactions. Further, we obtain mean energies and components of the diffusion coefficient parallel and perpendicular to the field, the latter through calculation of the velocity correlation functions. We also calculate components of the angular momentum which provide a measure of the collisional rotational alignment of the ions at high field strength. © 2011 American Institute of Physics

Ion dynamics in electrostatic fields

We study the dynamic motion of ions moving in a gas under the action of an electrostatic field via a nonequilibrium molecular dynamics simulation method and kinetic theory calculations. In the case of Rb+ in Ar, we find from simulation of the ion motion using an accurate ion-atom interaction potential that velocity-kinetic energy correlation functions acquire a positive slope at short times revealing the mean microscopic motion of the ions in velocity space. The results are supported by kinetic theory calculations of the ion energy rate of change due to collisions of the ions with the neutral gas