Physics

On the basis of a non-local Lagrangian for Maxwell equations in a dispersive medium, the energy-momentum tensor of the field is derived. We obtain the Field equations through variational methods and an extension of Noether theorem for a non-local Lagrangian is obtained as well. The electromagnetic energy-momentum tensor obtained in the general context is then specialized to the case of a field with slowly varying amplitude on a rapidly oscillating carrier.

The aim of this paper is to estimate the damage of railway carriage wheels caused by lock-up braking. First, we investigate the typical characteristics of wheel-rail contact including the parameters of contact patch, longitudinal creep and coefficient of adhesion. Then we show the current requirements concerning the wheel-slide protection systems and determine the points which should be reviewed. To predict the effect of the sliding phenomenon, material properties are needed, as well. Afterwards, a simplified thermal simulation is built to estimate where martensite formation may occur.

The nonlinear damping characteristics of friction wedges in the secondary suspension of a freight bogie are investigated considering unidirectional contact and non-smooth frictional forces. In the proposed MultiBody System model, each wedge has six Degrees of Freedom with corresponding inertial properties. The geometry of wedge, as well as, wedge angle, toe-in condition, and clearances between wedge/bolster and wedge/side frame are also considered in the modeling. The methodology for the identification of contact parameters is presented to achieve a smooth response and efficient numerical solutions. The response of the system with the proposed method is compared with the quasi-static methodology of wedge simulation and limitations of alternative methods are highlighted. The model is then applied to evaluate the effects of different design parameters of the wedge system on vertical and lateral response of the system.

In this paper, an exact analytical solution for free vibration analysis of thick laminated annular circular plates is presented.

We present exact closed-form expressions and complete asymptotic expansions for the electrostatic force between two charged conducting spheres of arbitrary sizes. Using asymptotic expansions of the force we confirm that even like-charged spheres attract each other at sufficiently small separation unless their voltages/charges are the same as they would be at contact. We show that for sufficiently large size asymmetries, the repulsion between two spheres increases when they separate from contact if their voltages or their charges are held constant. Additionally, we show that in the constant voltage case, this like-voltage repulsion can be further increased and maximised though an optimal lowering of the voltage on the larger sphere at an optimal sphere separation.

Traveling wave tube (TWT) is a powerful vacuum electronic device used to amplify radio-frequency (RF) signals with numerous applications, including radar, television and telephone satellite communications. TWT design in a nutshell comprises of a pencil-like electron beam (e-beam) in vacuum interacting with guiding it slow-wave structure (SWS). In our studies here the e-beam is represented by one-dimensional electron flow and SWS is represented by a transmission line (TL). The interaction between the e-beam and the TL is modeled by an analytic theory that generalizes the well-known Pierce model by taking into account the so-called space-charge effects particularly electron-to-electron repulsion (debunching). Many important aspects of the analytic theory of TWTs have been already analyzed in our monograph on the subject. The focus of the studies here is on degeneracies of the TWT dispersion relations particularly on exceptional points of degeneracy and their applications. The term exceptional point of degeneracy (EPD) refers to the property of the relevant matrix to have nontrivial Jordan block structure. Using special parameterization particularly suited to chosen EPD we derive exact formulas for the relevant Jordan basis including the eigenvectors and the so-called root vector associated with the Jordan block. Based on these studies we develop constructive approach to sensing of small signals.

We study beam transmission and refection resulting from the resonant excitation of forward and backward surface waves in a metamaterial. We predict that exciting these waves at an interface this medium and free space led to transmitted and reflected beams with high amplitudes. The FDTD simulated results demonstrate clearly propagation of these waves and their energy-trapping ability as described by theory. The resonant coupling of the refracted waves and reflected waves at the first interface led to the excitation of the backward surface waves. The forward surface waves transfer incoming energy in the direction of incidence and the backward surface transfer incoming energy in a direction opposite to the direction of incidence (i.e. negative direction). More also, the parameters used in these simulations were carefully chosen to minimize the effects of interference between the incident and the reflected beams earlier reported. In overall, these numerically simulated results clearly demonstrate the propagation of these waves and may serve as numerical approximations to what might be observed in experimental setups. Putting this simply, they may find applications in optical and photonic devices involving enhancing beam transmission and reflection using negative-index metamaterial.

The amplification of evanescent waves by flat superlens requires a near-resonance coupling which has been linked to resonant surface plasmons. A subtle interplay has been proposed to exist between the excitation of well-defined resonant surface plasmons and the focusing capability of a superlens. To gain insights into these resonant modes and their contributions to the amplification and recovery of evanescent waves, we performed simple but robust full-wave FDTD simulations on causal negative index Lorentz models. We found that well-defined pair of resonant surface plasmons is excited whenever a coupling of a diverging transmitted beam and a converging refracted beam occurs at the second interface. The resonant coupling of these modes at the interface led to the excitation of a single interface resonance predicted by the theory. The physical consequence of this resonance is that incident wave energy is pumped into the negative-index material medium and beyond it. These phenomena contribute substantially to the amplification and recovery of the near-fields in the image plane. It is noteworthy that, for evanescent wave refocusing to be achieved in a flat superlens, its thickness, and the source-to-superlens distance should be optimized. This optimization is critical in that an optimal thickness alongside optimal source-to-superlens distance will allow evanescent wave refocusing to dominate material lost. The FDTD simulated result showed an image of the point source inside the superlens and beyond it when its thickness was optimized. The resolution of the image beyond the superlens was ??58λ and this superlens behaves like a near-perfect lens. Overall, these numerically simulated results could serve as useful approximations to the degree of resonant amplification and refocusing of near-fields that can be achieved by flat superlens in near-fields experimental imaging setups.

Port-Hamiltonian systems theory provides a structured approach to modeling, optimization and potential-based control of multiphysical systems. Yet, it often seems to be unclear how the port-Hamiltonian structure relates to thermodynamics. One reason is that the Hamiltonian of a dissipative system is traditionally referred to as an energy function, although it is an exergy function. Clarity on this aspect leads benefits: 1. Links to the GENERIC structure of systems with local equilibrium are identified making it relatively easy to borrow ideas from a widely-accepted and actively-developed approach to nonequilibrium thermodynamics. 2. The port-Hamiltonian structure combined with a suitable bond-graph syntax is expected to become a main ingredient in thermodynamic optimization methods akin to exergy analysis and beyond. The intuitive nature of exergy and diagrammatic language facilitates interdisciplinary communication that is necessary for implementing sustainable energy systems and processes. Port-Hamiltonian systems are cyclo-passive, meaning that a power-balance equation immediately follows from their definition. For exergetic port-Hamiltonian systems, cyclo-passivity is synonymous with degradation of energy and follows from the first and the second law of thermodynamics being encoded as structural properties.

Passive parity-time-symmetric medium provides a feasible scheme to investigate non-Hermitian systems experimentally. Here, we design a passive PT-symmetric acoustic grating with a period equal to exact PT-symmetric medium. This treatment enhances the diffraction ability of a passive PT-symmetric grating with more compact modulation. Above all, it eliminates the first-order disturbance of previous design in diffraction grating. Additional cavities and small leaked holes on top plate in a 2D waveguide are used to construct a parity-time-symmetric potential. The combining between additional cavities and leaked holes makes it possible to modulate the real and imaginary parts of refractive index simultaneously. When the real and imaginary parts of refractive index are balanced in modulation, asymmetric diffraction can be observed between a pair of oblique incident waves. This demonstration provides a feasible way to construct passive parity-time-symmetric acoustic medium. It opens new possibilities for further investigation of acoustic wave control in non-Hermitian systems.