Tunneling Of Waves Research Articles

Optical Bistability in Ag/Dielectric Multilayers

We numerically study the optical bistability (OBIS) in periodic multilayers of Ag/SiO2. The calculated dependence of the output on the input intensity shows two possible OBIS states at the test wavelength. One is due to the field localization effects in silver layers with nonlinear refractive index modifying resonant tunneling of electromagnetic waves; the other, with about a three times lower threshold input intensity, is attributed to the intensity dependence change of the Ag/SiO2 composite's effective dielectric constant from metallic-like (negative) to dielectric-like (positive). With appropriate design engineering the Ag/SiO2 multilayers could find broad applications in all-optical information processes.

Steady streaming and sediment transport at the bottom of sea waves

AbstractThe flow and sediment transport in the boundary layer at the sea bottom due to the passage of surface waves are determined by considering small values of the wave steepness and of the ratio between the thickness of the boundary layer and the local water depth. Both the velocity field and the sediment transport rate are determined up to the second order of approximation thus evaluating both the steady streaming and the net (wave-averaged) flux of sediment induced by nonlinear effects. The flow regime is assumed to be turbulent and a two-equation turbulence model is used to close the problem. The bed load is evaluated by means of an empirical relationship as function of the bed shear stress. The suspended load is determined by computing the sediment flux, once the sediment concentration is determined by solving an appropriate advection–diffusion equation. The decay of the wave amplitude, which is due to the energy dissipation taking place in the boundary layer, is taken into account. The steady streaming and the sediment transport rate at the bottom of sea waves turn out to be different from those which are observed in a wave tunnel (U-tube), because of the dependence on the streamwise coordinate of the former flow. In particular, in the range of the parameters presently investigated, the sediment transport rate at the bottom of sea waves is found to be always onshore directed while, in a water tunnel (U-tube), the sediment transport rate can be onshore or offshore directed.

Characteristic analysis of air pressure wave generated by high-speed trains traveling through a tunnel

It is a complicated dynamic phenomenon when a transient pressure pulse is triggered by two high-speed trains passing each other in a tunnel. The air pressure pulse is a transient excitation to side wall of the car body. It can stimulate almost all vibration modes of the car body and the correlated assemblies, cause serious aerodynamic noise, and have important impacts on car body distortion, train noise, and operation safety. This article analyzes the time-frequency characteristics and main parameters of field-measured the air pressure wave, and its relationship with the train velocity as well as the vibration of the car body. Cepstrum analysis concludes that in the process of the meeting, the air pressure wave in tunnel crossing is a multiplying pressure wave instead of a superposed wave. The pressure pulse during the meeting is non-symmetrical one featured with a sharp front, large amplitude, fluctuating central section, and less sharp tail. The pulse width is inversely proportional to the train speed. As the speed increases, the impulse amplitude is amplified, and the speed of pulse front is advanced.

On the treatment of transient area variation in 1D discontinuous Galerkin simulations of train‐induced pressure waves in tunnels

SUMMARYTo simulate the pressure wave generated by a train travelling through a tunnel, we implement a discontinuous Galerkin (DG) method for the solution of the one‐dimensional equations of variable area flow. This formulation uses a spatial discretisation via Legendre polynomials of arbitrary degree, and the resulting semi‐discrete system is integrated using an explicit Runge–Kutta scheme. A simulation of subsonic steady flow in a nozzle shows that the scheme produces stable solutions, without the need for artificial dissipation, and that its performance is optimal for polynomial degrees between 5 and 7. However, when dealing with an unsteady area, we report the presence of numerical oscillations that are not due to the steep pressure fronts in the flow but rather to the projection of a moving area, with piecewise continuous derivatives onto a fixed grid. We propose a reformulation of the DG method to eliminate these oscillations that, put in simple terms, amount to splitting the integrals where the derivatives of the cross‐sectional area are discontinuous into subintegrals where they are continuous. The resulting method does not exhibit oscillations, and it is applied here to two practical cases involving train‐induced pressure waves in a tunnel. The first application is a validation of the DG method through comparison of its computational results with pressure data measured during transit at the Patchway tunnel near Bristol (UK). The second application is a study of the influence of the nose shape and length on the pressure wave gradients responsible for sonic boom at tunnel exit portals to show that the proposed modification is able to deal with realistic train shapes. Copyright © 2012 John Wiley & Sons, Ltd.

Fe/MgO/Fe (100) textured tunnel junctions exhibiting spin polarization features of single crystal junctions

Crystallographic and spin polarized transport properties of (100) textured and (100) epitaxial Fe/MgO/Fe magnetic tunnel junctions are compared. Strong similarities in the transport properties show that structural coherence and magnetic quality at the 25 nm grain scale in textured junctions are sufficient to issue signatures of the spin polarized transport specific to a single crystal junction. This demonstrates that the lateral coherence of the Bloch tunneling wave function is identically limited in both systems. Our analysis leads to model the textured tunnel junction as a juxtaposition of nanometer sized single crystal junctions, placed in parallel.

A transparent 90° polarization rotator by combining chirality and electromagnetic wave tunneling

A three-layer chiral metamaterial is constructed by using two layers of four mutually rotated resonators and a subwavelength mesh sandwiched symmetrically between these layers. The resulting structure is an ultrathin, transparent, and polarization angle independent 90° polarization rotator. Due to the electromagnetic tunneling effect exerted by the negative permittivity mesh, a cross-polarization conversion efficiency of 99% and 93% is achieved numerically and experimentally. The structure is modeled using the effective medium theory and then the transfer matrix method is applied to demonstrate the existence of the tunneling resonance theoretically.

Tunneling and filtering characteristics of cascaded ɛ-negative metamaterial layers sandwiched by double-positive layers

In this paper, we examine the electromagnetic (EM) wave tunneling and filtering characteristics of multi-layer structures composed of an arbitrary number of ɛ-negative (Epsilon-negative or ENG) metamaterial layers sandwiched by very thin double-positive (DPS) layers with high dielectric constant values. We explain the phenomenon of EM wave tunneling through this propagation barrier by drawing an analogy between this problem and a generalized coupled resonator system. Using this analogy, we demonstrate that a multi-layer structure composed of N DPS layers (N ≥ 2 is an integer number) that sandwich N-1 ENG layers can not only be made transparent in a frequency range where the ENG layers are normally opaque but also be designed to provide a desired spectral filtering characteristics. Furthermore, we present an analytical method for synthesizing such multi-layer spectral filters from the characteristics of their desired responses. The proposed synthesis procedure can be used to develop spatial filters operating throughout the microwave and mm-wave frequency bands as well as multi-layer metallo-dielectric filters at optical wavelengths. We demonstrate the validity of the proposed analytical synthesis method using full-wave numerical electromagnetic simulations. Finally, using the duality principle, we expand these analytical results to the problem of EM wave tunneling through multiple μ-negative layers surrounded by thin DPS layers with high relative permeability values.

The Feature of Weak Shock Wave Propagated in a Overlong Tunnel

Many experiment researches have been developed before. But most of them were carried out with the condition that the tunnel’s ratio of length and diameter (x/D) is under 1000. Recently, the process that compression wave convents into shock wave in the overlong tunnel has also been paid attention. In this paper, features of shock wave as it propagates through a overlong tunnel is investigated, rupturing thin films at the entrance to obtain three kinds of shock wave with different intensities. Then study their features respectively during they propagates through a overlong tunnel with x/D over 6000 at most. Comprehend shock wave more deeply by comparing the results of experiments.

Wind-generated water waves in a wind tunnel: Free surface statistics, wind friction and mean air flow properties

This paper describes the systematic measurements of wind and water waves in a wind tunnel with a water tank inside. The velocity fields are measured using a 2-D Laser Doppler velocimetry (LDV) in air and the instantaneous water levels are measured using resistive twin-wire probes. The paper gives details on the free surface statistics and the mean airflow structures, including the statistics of the wind-generated water waves, phase velocity and group velocity of the waves, waves grouping, and interface friction. Comparisons are made between these results with those for wind flows over a fixed solid surface. The wind-generated waves show the typical growing trend with fetch and wind speed and are also asymmetric, with crests more pronounced than troughs. A model is developed to account for the relative variation of phase velocity and group velocity, which includes a dependence of the drift velocity from the wave steepness. The statistics of the wave groups suggest that a separate treatment of the envelopes of the crests and of the troughs should be necessary. The air flow boundary layer over the water waves shows a logarithmic profile with a wake near the free stream and the apparent roughness is related to the wave amplitude. The transition to turbulence occurs at an earlier stage with respect to the transition of a boundary layer over a smooth, flat rigid wall. The thickness of the boundary layer over the water waves grows much faster than for the boundary layer over a plain, smooth solid wall.

Examination of the Suspended Sediment Concentration Formulae Using Full-Scale Rippled Bed and Sheet-Flow Data

A set of theoretical models is deduced for computing suspended sediment concentration in coastal waters under the influence of suspension on the bottom boundary layer due to turbulent motion over full-scale sand ripples and sheet-flow, following on from the work of Jayaratne and Shibayama [2007]. Dimensional analysis and best-fit technique are the main methods for the formulation of reference concentrations and vertical distribution of diffusion coefficients of the models. Model parameters (e.g. calibration coefficients) are calibrated with the help of full-scale measured data. Time-averaged concentration profiles are derived from the steady diffusion equation. Two distinct suspension layers (i.e., lower and upper) were identified within the suspension over rippled bed, therefore predictive models were given separately for each layer. In the case of sheet-flow regime, predictive models were given for suspension and upper-sheet-flow layers. Published experimental data from 4 different data sources in the SANTOSS database (75 and 80 full-scale experiments for rippled bed and sheet-flow regimes respectively) from 1994 to 2007 [Van der Werf et al., 2009] are better explained by the proposed formulae, however different sets of calibration coefficients were assigned in each data set primarily due to the flow type (regular or irregular flows) and nature of the laboratory experiments (wave flume and wave tunnel).

Comparison of Micro-Pressure Wave Radiated Out of Tunnel Exit between Slab Track and Ballast Track

Based on the one-dimensional unsteady compressible non-isentropic flow theory, micro-pressure wave radiated out of tunnel exit generated by a high-speed train entering a tunnel was investigated. In calculation process, the track roadbed and tunnel length were considered. The results, which were qualitatively and quantitatively analyzed, show that the ballast track has reduction effect of micro-pressure wave in long tunnel.

New exact travelling wave solutions of bidirectional wave equations

The surface water waves in a water tunnel can be described by systems of the form [Bona and Chen, PhysicaD116, 191 (1998)] $$ \label{BWE} \left\{ \begin{array}{l} v_t+u_x+(uv)_x+au_{xxx}-bv_{xxt}=0, \\ u_t+v_x+uu_x+cv_{xxx}-du_{xxt}=0, \end{array} \right. $$ (1) where a, b, c and d are real constants. In general, the exact travelling wave solutions will be helpful in the theoretical and numerical study of the nonlinear evolution systems. In this paper, we obtain exact travelling wave solutions of system (1) using the modified \(\tanh\)–\(\coth\) function method with computerized symbolic computation.

Corner effect and separation in transonic channel flows

An investigation into parameters affecting separation in normal shock wave/boundary layer interactions (SBLIs) has been conducted. It has been shown that the effective aspect ratio of an experimental facility (defined as δ*/tunnel width) is a critical factor in determining when shock-induced separation will occur. Experiments examining M∞ = 1.4 and 1.5 normal shock waves in a wind tunnel with a small rectangular cross-section have been performed and show that a link exists between the extent of shock-induced separation on the tunnel centre-line and the size of corner-flow separations. In tests where the corner-flows were modified ahead of the shock (through suction and vortex generators), the extent of separation around the tunnel centre-line was seen to vary significantly. These observations are attributed to the way corner flows modify the three-dimensional shock-structure and the impact this has on the magnitude of the adverse pressure gradient experienced by the tunnel wall boundary layers.

Modeling of Electron Cyclotron Heating in Large Tokamaks and ITER with the 3-D Full-Wave Code STELEC

We present modeling results of basic electron cyclotron heating (ECH) scenarios in several tokamaks and ITER performed with the most recent version of the three-dimensional (3-D) full-wave STELEC code (stellarator ECH, including tokamaks as a special case). This code includes all basic wave physics such as interference, diffraction, wave tunneling, mode conversion to electron Bernstein waves at the upper hybrid resonance (UHR), and appropriate boundary conditions. The code solves the wave equations in real 3-D magnetic geometry and thanks to the use of massive parallel teraflop computers, it is the first to provide full-wave solutions of the problem in toroidal plasmas. Several important new results are thus obtained that cannot be predicted with codes based on ray-tracing techniques, such as the influence of diffraction effects and the importance of the UHR for both X- and O-mode antenna excitation at fundamental harmonic. This last result also shows that the so-called “O and X” modes are coupled solutions. The coupling of these modes, partly supported by experiments in the DIII-D tokamak showing similar heating efficiencies for both radiated modes, leads to different power deposition profiles and spatial distribution, compared to results from ray-tracing codes. Coupling between the O-mode and the X-mode (launched at the low-field side) reveals the importance of electron Bernstein waves in ECH calculations for high-density ITER plasmas. These results not only could influence the predictions for neoclassical tearing mode suppression for ITER using electron cyclotron current drive but could also lead to important simplifications in ECH hardware (converters, polarization, etc.) and to a reduced cost of the ECH system on ITER. The code also allowed investigation of the urgent issue of the O-X-B ECH scenario for overdense tokamak/stellarator plasmas.

Tunneling of magnetoacoustic waves through a gap in ferromagnetic crystals with a relative longitudinal displacement

The tunneling of a plane monochromatic acoustic wave through a gap between two ferromagnets under conditions of their relative longitudinal displacement has been considered. It has been shown that, when the gap thickness h is comparable to the wavelength, the complete transmission of the acoustic wave at the Damon-Eshbach frequency becomes possible. When the gap thickness is smaller than the wavelength, the complete transmission of the acoustic wave occurs but already at two resonant frequencies. The inclusion of the longitudinal displacement of one of the crystals, in all cases, leads to the violation of resonance conditions and, consequently, to a noticeable decrease in the coefficient of transmission of the acoustic wave through the gap between two ferromagnets; this effect is more pronounced, the higher is the velocity of motion of the crystal. It has been demonstrated that there is a possibility of the wave front reversal occurring with the amplification of the reflected magnetoacoustic wave.

Superresolution via enhanced evanescent tunneling

We here propose the concept of enhanced evanescent tunneling (EET). Our analysis indicates that by means of a suitable control field, the transmission of evanescent waves across a forbidden gap can be enhanced by several orders of magnitude-well beyond the ordinary frustrated total internal reflection case. We show how such a phenomenon can be used to probe both the amplitude and phase of the evanescent portion of the angular spectrum, thereby allowing target superresolution. In principle EET can be manifested in other areas of physics where wave tunneling is involved.

APPLICABILITY OF THE PREDICTIVE FORMULAE FOR SUSPENDED SEDIMENT CONCENTRATION ON FULL-SCALE RIPPLED BED AND SHEET FLOW

A significant proportion of the sediment transported in coastal waters consists of sand, which moves in suspension. Transport of sediment suspension in and outside the surf zone occurs predominantly due to vortical motion over wave-generated sand ripples, from sheet flow layer and turbulence under breaking waves. A new set of explicit theoretical formulae was deduced to predict sediment concentration over full-scale rippled beds and sheet flows from the predictive models given in Jayaratne and Shibayama (2007). Previous predictive models were derived using 325 test cases from 1977-1996. The new formulae were also established using dimensional analysis and a best-fit technique. Parameterisation of formulae was carried out with the help of large-scale wave, sediment and concentration data appeared in SANTOSS database (see Van der Werf et al., 2009) which covered 298 test cases from 1987-2007. Reference level was taken where the concentration could be measured without affecting the bed.
 
 Formulae for predicting bed reference concentration (ca), diffusion coefficient (εs) and concentration profile [c(z)] on full-scale rippled bed and sheet flow from Jayaratne and Shibayama (2007) were initially assessed with the new datasets. It was identified that the measured and predicted concentration do not agree well in magnitudes and distribution profile since the measured field-scale wave tunnel data have stronger bed shear effect than in the wave flume data used for the previous formulation. Therefore, some variables in previous equations were replaced appropriately with suitable parameters such as Shields parameter with the mobility number (ψ) etc. Bed reference levels (z0) were adjusted based on the value of ca. Empirical constants and exponents in those equations were carefully modified in order to match the measured values using the regression analysis. The applicability of modified empirical formulae on full-scale rippled bed and sheet flow depends highly on z and ψ. Two different suspension regions were identified within the datasets over the rippled beds due to the enhanced mixing processes in the upper layer and the coherent vortex structure in the osillatory boundary layer, hence two types of equations were proposed. Conversely, four distinct suspension regimes were identified over full-scale sheet flow and predictive models were given for upper sheet flow (exponential) and suspension layers (power relationship). The cause for this different behaviour was identified as the presence of different mixing processes in the upper sheet flow and suspension layer. 
 
 REFERENCES
 Jayaratne, M.P.R. and Shibayama, T. (2007). Suspended sediment concentration on beaches under three different mechanisms, Coastal Engineering Journal, 49(4), 357-392.
 
 Van der Werf, J.J., Schretlen, J.J.L.M., Ribberink, J.S. and O'Donoghue, T. (2009). Database of full-scale laboratory experiments on wave-driven sand transport processes, Coastal Engineering, 56, 726-732.

Interplay between quantum interference and electron interactions in a Rashbasystem

We investigate theoretically a nanoscopic device in which quantum interference of electronwaves takes place in the presence of their mutual Coulomb interaction. The device consistsof interacting quantum dots coupled to spin-polarized leads via quantum point contactswith Rashba interaction. The Rashba spin-flip-assisted inter-subband mixing in quantumpoint contacts induces quantum interference between the tunneling waves, which interactby Coulomb repulsion inside the dot. The spin-dependent Fano resonances, which appearin the conductance through the device, are significantly modified by Coulombinteractions. Their width and shape depend on the quantum dot spin-up and spin-downoccupancies, controlled by electron interactions. On the other hand, correlatorscalculated for the quantum dot spin sub-levels are not influenced by quantuminterference between them and depend rather on the degree of localization of theselevels.

Salecker–Wigner–Peres clock and average tunneling times

The quantum clock of Salecker–Wigner–Peres is used, by performing a post-selection of the final state, to obtain average transmission and reflection times associated to the scattering of localized wave packets by static potentials in one dimension. The behavior of these average times is studied for a Gaussian wave packet, centered around a tunneling wave number, incident on a rectangular barrier and, in particular, on a double delta barrier potential. The regime of opaque barriers is investigated and the results show that the average transmission time does not saturate, showing no evidence of the Hartman effect (or its generalized version).

Tunable hybridization at midzone and anomalous Bloch–Zener oscillations in optical waveguide ladders

We have studied the optical oscillation and tunneling of light waves in optical waveguide ladders (OWLs) formed by two coupled planar optical waveguide arrays. For the band structure, a midzone gap is formed owing to band hybridization, and its wavenumber position can be tuned throughout the whole Brillouin zone, which is different from the Bragg gap. By imposing a gradient in the propagation constant in each array, Bloch-Zener oscillation (BZO) is realized with Zener tunneling between the bands occurring at the midzone, which is contrary to the common BZO with tunneling at the center or edge of the Brillouin zone. The occurrence of BZO is demonstrated by using the field-evolution analysis. The tunable hybridization at the midzone enhances the tunability of BZO in the OWLs. This Letter may offer new insights into the coherent phenomena in optical lattices.