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Traditionally, solitary waves are generated in the laboratory setups using the Goring's methodology that considers a wave of permanent form during the generation process. In this study, we propose a new methodology for solitary wave generation using pistontype wavemakers by considering the evolving nature of the wave during the generation process. This proposed methodology is tested by conducting a series of experiments in a wave tank. In the experiments, generation of solitary wave profiles predicted by Boussinesq's and Rayleigh's theoretical solutions are accomplished using both the New and Goring methodologies. Waves generated using the Goring methodology served as a benchmark to assess the performance of the proposed New methodology. Comparisons of experimental observations and theoretical solutions for various wave characteristics have shown that the New methodology is capable of generating more accurate solitary waves with less attenuation as waves propagate compared to the Goring methodology. Proposed solitary wave generation methodology is expected to be useful in various laboratory investigations to study different aspects of long wave theories and to simulate tsunamis and internal waves, among others. DOI: 10.1061/(ASCE)WW.1943-5460.0000046 CE Database subject headings: Solitary waves; Tsunamis; Wave generation; Wave tanks. Author keywords: Solitary wave; Tsunami; Laboratory wave generation; Wave tanks.

It is well established that wave propagation in the heart is anisotropic and that the ratio of velocities in the three principal directions may be as large as [v.sub.f] : [v.sub.s] : [V.sub.n] [approximately equal to] 4(fibers):2(sheets): 1(normal). We develop an alternative view of the heart based on this fact by considering it as a non-Euclidean manifold with an electrophysiological(el-) metric based on wave velocity. This metric is more natural than the Euclidean metric for some applications, because el-distances directly encode wave propagation. We develop a model of wave propagation based on this metric; this model ignores higher-order effects like the curvature of wavefronts and the effect of the boundary, but still gives good predictions of local activation times and replicates many of the observed features of isochrones. We characterize this model for the important case of the rotational orthotropic anisotropy seen in cardiac tissue and perform numerical simulations for a slab of cardiac tissue with rotational orthotropic anisotropy and for a model of the ventricles based on diffusion tensor MRI scans of the canine heart. Even though the metric has many slow directions, we show that the rotation of the fibers leads to fast global activation. In the diffusion tensor MRI-based model, with principal velocities 0.25:05:1 m/s, we find examples of wavefronts that eventually reach speeds up to 0.9 m/s and average velocities of 0.7 m/s. We believe that development of this non-Euclidean approach to cardiac anatomy and electrophysiology could become an important tool for the characterization of the normal and abnormal electrophysiological activity of the heart. cardiac arrhythmias | cardiac electrophysiology | diffusion tensor MRI | patient-specific cardiac models | Riemannian geometry doi/ 10.1073/pnas.1008837107

The problem in the creation of traveling waves is approached here from an unconventional angle. The formulation makes use of normal vibration modes, which are standing waves, to express both traveling waves and the required force distribution. It is shown that a localized force is required at any discontinuity along the structure to absorb reflected waves. This convention is demonstrated for one--and two-dimensional structures modeled as continua, and as discretized numerical approximation of the mass and stiffness matrices. Harmonic vibrations can be characterized as standing or traveling waves or as a combination of both. By applying forces that have been specially designed for the purpose, the vibratory response can become a pure traveling wave. The force distribution is important for the design of ultrasonic motors and in control applications, attempting to absorb and create outgoing and incoming waves. [DOI: 10.1115/1.3176999]

Metallic nanoscale structures are capable of supporting surface plasmon polaritons (SPPs), propagating collective electron oscillations with tight spatial confinement at the metal surface. SPPs represent one of the most promising structures to beat the diffraction limit imposed by conventional dielectric optics. Ag nanowires have drawn increasing research attention due to 2D sub-100 nm mode confinement and lower losses as compared with fabricated metal structures. However, rational and versatile integration of Ag nanowires with other active and passive optical components, as well as Ag nanowire based optical routing networks, has yet to be achieved. Here, we demonstrate that SPPs can be excited simply by contacting a silver nanowire with a Sn[O.sub.2] nanoribbon that serves both as an unpolarized light source and a dielectric waveguide. The efficient coupling makes it possible to measure the propagation-distance-dependent waveguide spectra and frequency-dependent propagation length on a single Ag nanowire. Furthermore, we have demonstrated prototypical photonic-plasmonic routing devices, which are essential for incorporating low-loss Ag nanowire waveguides as practical components into high-capacity photonic circuits. waveguides | subwavelength | Ag nanowires | Sn[O.sub.2] nanoribbons | propagation length doi/10.1073/pnas.0902064106

In the design of wind turbines--onshore or offshore--the prediction of extreme loads associated with a target return period requires statistical extrapolation from available loads data. The data required for such extrapolation are obtained by stochastic time-domain simulation of the inflow turbulence, the incident waves, and the turbine response. Prediction of accurate loads depends on assumptions made in the simulation models employed. While for the wind, inflow turbulence models are relatively well established; for wave input, the current practice is to model irregular (random) waves using a linear wave theory. Such a wave model does not adequately represent waves in shallow waters where most offshore wind turbines are being sited. As an alternative to this less realistic wave model, the present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on an offshore wind turbine with a focus on the fore-aft tower bending moment at the mudline. We use a 5 MW utility-scale wind turbine model for the simulations. Using, first, simpler linear irregular wave modeling assumptions, we establish long-term loads and identify governing environmental conditions (i.e., the wind speed and wave height) that are associated with the 20-year return period load derived using the inverse first-order reliability method. We present the nonlinear irregular wave model next and incorporate it into an integrated wind-wave response simulation analysis program for offshore wind turbines. We compute turbine loads for the governing environmental conditions identified with the linear model and also for an extreme environmental state. We show that computed loads are generally larger with the nonlinear wave modeling assumptions; this establishes the importance of using such refined nonlinear wave models in stochastic simulation of the response of offshore wind turbines. [DOI: 10.1115/1.3160647]

Hysteretic damping is often modeled by means of linear viscoelastic approaches such as 'nearly constant attenuation' models (e.g. NCQ model). These models do not take into account nonlinear effects either on the stiffness or on the damping, which are well known features of soil dynamic behavior. The aim of this paper is to propose a mechanical model involving nonlinear viscoelastic behavior for isotropic materials under dynamic excitations. This model simultaneously takes into account nonlinear elasticity and nonlinear damping. On one hand, the shear modulus is a function of the excitation level; on the other, the description of viscosity is based on a generalized Maxwell body involving nonlinearity. This formulation (X-NCQ) is implemented into a one-dimensional finite-element approach for a dry soil. The validation of the model shows its ability to retrieve low amplitude seismic ground motion. For larger excitation levels, the analysis of seismic wave propagation in a nonlinear soil layer over an elastic bedrock leads to results which are physically satisfactory (lower amplitudes, larger time delays, higher frequency content). DOI: 10.1061/(ASCE)0733-9399(2009)135:11(1305) CE Database subject headings: Inelasticity; Viscoelasticity; Damping; Wave propagation; Earthquake engineering; Ground motion: Nonlinear response; Finite element method.

Extensive experimental evidence is now available to support the observation that there are two components of jet mixing noise. They are the fine-scale turbulence noise and the noise from the large turbulence structures of the jet flow. The large turbulence structure's noise radiates primarily in directions with a large inlet angle around the downstream axis of the jet. The fine-scale turbulence noise dominates in the sideline and upstream directions. This study investigates the mechanism by which large turbulence structures radiate noise. It is believed that the mechanism is Mach wave radiation. Theoretical model results and physical reasoning are presented to support the Mach wave mechanism. They are further supported by experimental measurements both in the far field and in the near acoustic field. These measurements include peak noise direction, noise-source distribution along the jet column, and near-field pressure-contour pattern. A signature pattern of the near-field pressure contours associated with Mach wave radiation is identified. DOI: 10.2514/1.42644

Close agreement between theory and experiment is critical for adequate understanding and implementation of the cross-well radar (otherwise known as cross-borehole ground penetrating radar) technique, mentioned in a previous paper by the authors. Comparison of experimental results to simulation using a half-space dyadic Green's function in the frequency domain requires development of transfer functions to transform the experimental data into a compatible form. A channel transfer function (CTF) was developed to avoid having to model the transmitting and receiving characteristics of the antennas. The CTF considers electromagnetic wave propagation through the intervening media only (soil in this case) and hence corresponds to the simulation results that assume ideal sources and receivers. The CTF is based on assuming the transmitting antenna, soil, and receiving antenna as a cascade of three two-port microwave junctions between the input and output ports of the vector network analyzer used in the experimental measurements. Experimentally determined CTF results are then compared with computational model simulations for cases of relatively dry and saturated sandy soil backgrounds. The results demonstrate a reasonable agreement, supporting both the model and CTF formulation. DOI: 10.1061/(ASCE)GT.1943-5606.0000029 CE Database subject headings: Radar; Antennas; Saturated soils; Computer aided simulation; Transfer functions.

This paper proposes a stochastic multipath model that is useful for generating multiple-input-multiple-output (MIMO) channel matrices in time-variant environments. The multipath model is developed by first extracting the relevant multipath cluster characteristics from measured indoor channel data and subsequently capturing these characteristics in an autoregressive stochastic model. This model is then used to generate channel matrices whose space-time characteristics closely match those of realistic scenarios, particularly when birth and death of multipath clusters are included in the stochastic representation. Computational examples reveal the applicability and the accuracy of the approach. While the current implementation is based on data taken assuming that propagation is confined to the horizontal plane, this paper also discusses the extension of the model to describe 3-D propagation, enabling its application to a wide range of physical scenarios and antenna characteristics. Index Terms--Communication channels, multiple-input-multiple-output (MIMO) systems, propagation.

Broad area maritime surveillance (BAMS) is a current interest area for the application of unmanned aerial vehicles (UAVs). Robust communications is a primary concern that impedes the general acceptance of UAVs by the Federal Aviation Administration (FAA), as loss of communications link is generally perceived as a loss of vehicular control. Thus, to gain an increased understanding of the communications channel UAVs' experience during low-level maritime operations, a channel-modeling effort using the finite-difference time domain method (FDTD) is conducted. The focus of this effort has been to assess the effects of sea surface shadowing conditions on the marine communications channel. A 2-D electromagnetic (EM) simulator has been developed, utilizing modified Pierson-Moskowitz (PM) spectral models to generate a random sea surface in a deep-water location from which multipath scattering is produced. Data analysis conducted on the transient EM simulation results has produced generalized path loss exponent, standard deviation, mean excess delay, and root mean square delay models as a function of frequency and observable sea surface height for fixed transmitter and receiver locations. Index Terms--Finite-difference methods, marine vehicle communications, microwave propagation, multipath channels, overwater radio propagation, unmanned aerial vehicles (UAVs).

In order to get the information about multipath propagation, this communication presents two estimation methods for slowly varying fading coefficients. With known numbers of the far-field sources and multipaths from each far-field source, the fading coefficients in multipath propagation can be well estimated. The methods not only have low complexity but also have high performance, and the fading coefficients can be resolved directly without a searching procedure. Afterwards, we make an extension to further improve the estimation accuracy of both direction of arrival (DOA) and fading coefficients after obtaining their estimations. The Cramer-Rao bound (CRB) for this problem is also deduced. Simulation results demonstrate the effectiveness and efficiency of the proposed methods. Index Terms--Array signal processing, coherent signals, fading coefficients, multipath propagation, uniform linear array (ULA).

A generalized definition is presented of multipath angular spread, used to study the spatial correlation of the signal envelope in a three-dimensional wireless channel, both in the cases of a three-dimensional and a horizontal travel direction. Previous results from two-dimensional modeling are shown to be a special case of this general analysis when the multipath power and the travel direction are confined to a single plane in space. The results show that the main lobe of the spatial correlation of the signal envelope at a given distance has an approximately Gaussian dependence on the three-dimensional multipath directivity metric proposed in this paper, for both cases for the travel direction. Index Terms--Fading channels, mobile communications, multipath channels, propagation, scattering.

In this paper, we introduce an approach to analyze the interaction between antennas and the propagation channel. We study both the antennas and the propagation channel by means of the spherical vector wave mode expansion of the electromagnetic field. Then we use the expansion coefficients to study some properties of general antennas in those fields by means of the antenna scattering matrix. The focus is on the spatio-polar characterization of antennas, channels and their interactions. We provide closed form expressions for the covariance of the field multimodes as function of the power angle spectrum (PAS) and the channel cross-polarization ratio (XPR). A new interpretation of the mean effective gains (MEG) of antennas is also provided. The maximum MEG is obtained by conjugate mode matching between the antennas and the channel; we also prove the (intuitive) results that the optimum decorrelation of the antenna signals is obtained by the excitation of orthogonal spherical vector modes. Index Terms--Channel modeling, cross-polarization ratio, mean effective gain, scattering matrix, spherical vector wave expansion.