Waves

The flux variability of Jovian decametric radio emission (DAM) is analyzed to search for its possible modulations by magnetohydrodynamic waves of ultra-low frequencies (ULF). The ULF modulating waves are found in DAM dynamic spectra as a moving pattern (moire) on the background of Jovian millisecond radio bursts (S-bursts). Their frequency drift (∼55 MHz/s) corresponds to the wave motion to Jupiter with Alfvén velocity (∼4 × 10 4 km s −1). There are whistler-like drifting details in the dynamic power spectra of ULF variations of S-burst emission flux at fixed radio frequency. Their frequency drift rates are consistent with whistlers that are dispersed mainly in the Io torus. Formally our analysis of DAM reveals the specific 2s-modulation of S-burst emission corresponding to the known magnetic pulsations near gyrofrequencies of heaviest ions (SO + 2 and SO +) in the Io torus.

Problems of constructing a sediment budget for an inlet is often a challenge due to constraints on data collection, quality of data, and assumptions one is willing to consider. Existing literature does not provide a systematic foundation for equations considered and often engineers and geologists typically do not properly consider the mathematical constraints placed on the problem of sediment budget construction. The present technical note presents a simple methodology for assessing sediment budget, i.e. what is possible, and what is not possible via a matrix equation system and linear algebra. It is hoped that engineers and geologists will not impose unrealistic expectations on the sediment budget system via using such an approach. Some simple examples are given of both pitfalls and correct approaches to sediment budget development.

1] The Maldives experience seasonal shifts in monsoon winds from the west-northwest to northeast. The morphologic response of reef island beaches and shorelines to these predictable shifts in climate was examined on eight islands in South Maalhosmadulu Atoll based on global positioning system surveys of island shoreline planform in January and June 2002 and February 2003. Surveys show that islands exhibit large gross changes (31-120% of beach area) in shoreline position between seasons. Such changes reflect large reversals in sediment flux of 9-23 Â 10 3 m 3 on a biannual basis driven by seasonal reversals in wind and wave conditions. Annual net change is small (2-15%), suggesting islands spatially exhibit a dynamic equilibrium. An island oscillation index (I o ) is defined that describes the spatial extent of shoreline change around islands. Island shape (ellipticity, e) is found to be positively correlated with I o in the expression I o = 1.021e À 0.275, suggesting island shape is a better indicator of the susceptibility of island shorelines to morphological change than wave energy exposure.

1] A parametric study is presented on the temporal evolution of the phase space density (PSD) of the outer radiation belt energetic electrons driven by the superluminous right-hand extraordinary (R-X) mode waves at the location L = 4.5. Bounce-averaged diffusion rates in pitch angle and momentum are calculated by varying the peak wave frequency, the wave normal angle distribution, and the wave latitudinal distribution. Those diffusion rates are used as inputs to solve a 2-D momentum-pitch angle diffusion equation. In particular, three cases are considered: momentum diffusion rates alone, momentum +pitch angle diffusion rates, and momentum +pitch angle +cross diffusion rates. Numerical results show that at 24 h, electron PSDs can enhance substantially for 1 MeV energy at higher pitch angles. Momentum diffusion dominates the dynamic evolution of energetic electrons, whereas the contribution of pitch angle or cross-diffusion rates is insignificant using the specified wave model. In addition, PSD evolutions are sensitively dependent on the assumed different wave normal angle distributions and tend to be located in lower pitch angles when wave normal angles move to smaller regions. Diffusion coefficients and PSD evolution are largely determined by the wave latitudinal distributions. High-latitude R-X mode waves primarily contribute to pitch angle scattering of energetic electrons, whereas equatorial (or lower latitude) R-X mode waves yield efficient acceleration of electrons. This result supports the previous findings that superluminous R-X mode waves potentially contribute to dramatic variation in the outer radiation belt electron dynamics under appropriate conditions.

The paper aims at providing a simple model for evaluating piling-up, overtopping, filtration and rip currents for low crested structures. The model is essentially based on mass and momentum balance and on resistance laws that relate piling-up and mass fluxes. A procedure is suggested and validated against 2D and 3D experimental data. This procedure is innovative with respect to existing formulations since it applies both to emerged and submerged structures and it simultaneously accounts for structure permeability, return flow through gaps and dissipative effects.

Drill-string waves can be successfully used as reference pilot signals for drill-bit seismic-while-drilling (SWD) purposes. The seismic signals obtained by correlating pilot and geophone measurements are disturbed by the drill-string reverberations because the pilot waves are reflected at each interface between string sections with different acoustic impedances. Inverse filtering of these reflections, using a reference-pilot deconvolution calculated in the presence of additional noise, may cause signal distortion. To overcome this problem, we consider using dual-sensor measurements in the drill string to remove the reflections of the drill-bit waves in the acquisition phase and to improve pilot deconvolution. We measure acceleration and strain of drill-string dual fields, which have opposite reflection coefficients and, in a string of constant elastic properties, the same transmission coefficients. These quantities are scaled to fit the amplitude of the direct arrivals, summed to remove the reflections in the drill string and in the rig, and may be deconvolved by Einstein deconvolution to characterize the reflection coefficient between the drill bit and the formation. Synthetic numerical examples and real measurements acquired downhole in a location close to the bit show that upgoing and downgoing drill-string pilots can be separated using dual fields and jointly used to improve the SWD seismograms.

In this paper, we use geometric algebra to describe the polarization ellipse and Stokes parameters. We show that a solution to Maxwell's equation is a product of a complex basis vector in Jackson and a linear combination of plane wave functions. We convert both the amplitudes and the wave function arguments from complex scalars to complex vectors. This conversion allows us to separate the electric field vector and the imaginary magnetic field vector, because exponentials of imaginary scalars convert vectors to imaginary vectors and vice versa, while exponentials of imaginary vectors only rotate the vector or imaginary vector they are multiplied to. We convert this expression for polarized light into two other representations: the Cartesian representation and the rotated ellipse representation. We compute the conversion relations among the representation parameters and their corresponding Stokes parameters. And finally, we propose a set of geometric relations between the electric and magnetic fields that satisfy an equation similar to the Poincaré sphere equation.

Coastal boulder accumulations are often mentioned in the literature, even though their interpretation remains difficult, especially along rock coasts affected both by storms and tsunamis. Studies on the geomorphic impact of such high-energy events are actually of great interest, since their intensity and frequency are key issues for the future evolution of coasts in the framework of the global change. The southwest coast of Iceland faces the powerful storms of the North Atlantic Ocean, with wave heights of more than 15 m. The probability for past and present tsunamis to hit this coast is very low. In this paper, we describe boulder accumulations along the volcanic rock coast of Reykjanes (southwest Iceland). They consist of cliff-top boulders, clusters and ridges, beaches, and boulder fields. Large boulders, up to 70 t in weight, have been transported and deposited up to 65 m inland (6 masl). The maximum limit of boulder deposition and driftwood was found respectively 210 m and 550 m inland. Storms appear to be a predominant factor in the geomorphic evolution of Reykjanes coasts. Our observations also give new insight for the interpretation of coastal boulder accumulations. Processes of erosion and deposition by tsunamis are a rising topic in the literature, and the effects of recurrent and powerful storms are neglected.

PURPOSE: The purpose of this Coastal and Hydraulics Engineering Technical Note (CHETN) is to demonstrate the use of BOUSS-2D in three example applications. The BOUSS-2D is a Boussinesq wave model that is now part of the U.S. Army Corps of Engineers (USACE) Surface-Water Modeling System (SMS). Demirbilek et al. (2004) gives a detailed description of the SMS interface of BOUSS-2D. This second technical note in BOUSS-2D series is a step-by-step tutorial that shows the usage of the model through illustrative examples. The next CHETN in the series will describe the time-series and frequency-domain analysis utilities and techniques developed for post-processing BOUSS-2D model results. Details of the BOUSS-2D model theory, numerical implementation, and example applications are presented in a technical report (Nwogu and Demirbilek 2001). BACKGROUND: As a phase-resolving nonlinear wave model, BOUSS-2D can be used in the modeling of various wave phenomena including shoaling, refraction, diffr...

Cet article présente une méthode originale pour estimer l'amplitude de la houle réfléchie et transmise en milieu confiné tel que dans un canal à houle où les signaux à analyser sont de courte durée. Il s'agit d'une extrapolation à 3 sondes de la méthode à 2 sondes de Goda et Suzuki. Ce papier propose également des solutions pour calculer les coefficients de transmission et de réflexion fiables malgré les problèmes liés à l'apparition des harmoniques de houle et aux interférences entre les différentes ondes qui se propagent alors dans le canal.

Hopkinson bar experimental techniques have been extensively employed to investigate the mechanical response and fracture behavior of engineering materials under high rate loading. Among these applications, the study of the dynamic fracture behavior of materials at stress-wave loading conditions (corresponding stress-intensity factor rate ϳ10 6 MPa ͱ m / s) has been an active research area in recent years. Various Hopkinson bar loading configurations and corresponding experimental methods have been proposed to date for measuring dynamic fracture toughness and investigating fracture mechanisms of engineering materials. In this paper, advances in Hopkinson bar loaded dynamic fracture techniques over the past 30 years, focused on dynamic fracture toughness measurement, are presented. Various aspects of Hopkinson bar fracture testing are reviewed, including (a) the analysis of advantages and disadvantages of loading systems and sample configurations; (b) a discussion of operating principles for determining dynamic load and sample displacement in different loading configurations; (c) a comparison of various methods used for determining dynamic fracture parameters (load, displacement, fracture time, and fracture toughness), such as theoretical formula, optical gauges, and strain gauges; and (d) an update of modeling and simulation of loading configurations.

Most wave monitoring buoys and systems conventionally rely on the direct acquisition of wave height data. This paper introduces an alternate wave monitoring method that uses acceleration and angular deviation data. A threshold level crossing counting scheme is used as a means for data compression and as a basis for an alarm system. The sensor modules pre-process data at predetermined sampling periods before sending out telemetry to more efficiently manage power. The MEMS based sensors used provide a smaller footprint lower cost alternative to direct wave height measurement systems.

The objective of this paper is to provide a succinct review of past and current research in developing wave rotor technology. This technology has shown unique capabilities to enhance the performance and operating characteristics of a variety of engines and machinery utilizing thermodynamic cycles. Although there have been a variety of applications in the past, this technology is not yet widely used and is barely known to engineers. Here, an attempt is made to summarize both the previously reported work in the literature and ongoing efforts around the world. The paper covers a wide range of wave rotor applications including the early attempts to use wave rotors, its successful commercialization as superchargers for car engines, research on gas turbine topping, and other developments. The review also pays close attention to more recent efforts: utilization of such devices in pressure-gain combustors, ultra-micro gas turbines, and water refrigeration systems, highlighting possible further efforts on this topic. Observations and lessons learnt from experimental studies, numerical simulations, analytical approaches, and other design and analysis tools are presented. Fig. 26 Historical perspective of wave rotor technology. Red: gas turbine application. Green: IC engine supercharging. Blue: refrigeration cycle. Pink: pressure divider and equalizer. Purple: wave superheater. Orange: internal combustion wave rotors. Black: general applications.

Full-waveform inversion ͑FWI͒ is a challenging data-fitting procedure based on full-wavefield modeling to extract quantitative information from seismograms. High-resolution imaging at half the propagated wavelength is expected. Recent advances in high-performance computing and multifold/multicomponent wide-aperture and wide-azimuth acquisitions make 3D acoustic FWI feasible today. Key ingredients of FWI are an efficient forward-modeling engine and a local differential approach, in which the gradient and the Hessian operators are efficiently estimated. Local optimization does not, however, prevent convergence of the misfit function toward local minima because of the limited accuracy of the starting model, the lack of low frequencies, the presence of noise, and the approximate modeling of the wave-physics complexity. Different hierarchical multiscale strategies are designed to mitigate the nonlinearity and ill-posedness of FWI by incorporating progressively shorter wavelengths in the parameter space. Synthetic and real-data case studies address reconstructing various parameters, from V P and V S velocities to density, anisotropy, and attenuation. This review attempts to illuminate the state of the art of FWI. Crucial jumps, however, remain necessary to make it as popular as migration techniques. The challenges can be categorized as ͑1͒ building accurate starting models with automatic procedures and/or recording low frequencies, ͑2͒ defining new minimization criteria to mitigate the sensitivity of FWI to amplitude errors and increasing the robustness of FWI when multiple parameter classes are estimated, and ͑3͒ improving computational efficiency by data-compression techniques to make 3D elastic FWI feasible.

This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mechanism of a modern transonic fan rotor with a forward travelling wave. To induce an initial travelling wave for the blade structure, an initial BC that can facilitate each blade to vibrate with a time lag by a given nodal diameter (ND) is implemented. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved with a system of structure modal equations in a fully coupled manner. The 5th order WENO scheme with a low diffusion E-CUSP Riemann solver is used for the inviscid fluxes and a 2nd order central differencing is used for the viscous terms. A half annulus sector is used for the flutter simulations with a time shifted phase lag boundary condition at the circumferential boundaries. The present FSI simulations show that the shock instability causes the flutter. When the detached normal shock moves further upstream in a direction normal to the blade chord, the interaction of the detached normal shock with tip leakage vortex creates more serious blockage to the blade passage that can introduce an aerodynamic instability to the blade structure due to the incoming flow disturbance, resulting in flutter. The flutter of the transonic fan observed in this study occurs at the 1st mode before the stall. The predicted flutter boundary agrees well with the experiment.

Underwater pipeline inspections are conducted using ultrasonic cylindrical guided waves in the laboratory environment. Three different types of mechanical defects-gouge, removed metal, and dent-are fabricated in small-diameter, 22.22-mm, aluminum pipes and tested. To efficiently propagate the antisymmetric (flexural) cylindrical guided waves through the aluminum pipe in water, a new transducer holder device is designed. The device uses commercially available ultrasonic transducers that generate compressional ultrasonic waves in the water. The device can change the striking angle of the incident beam from 0 to 51 deg. With the help of this device, the incident angle adjustment and frequency sweeping can be carried out. This is necessary for obtaining the time history of the received signals for various incident angles and signal frequencies; then these time histories are converted to V(f) curves, or received signal amplitude versus frequency curves. From the amplitude of these V(f) curves, the type and extent of the mechanical defects can be estimated. This investigation shows that the new coupler device can be effectively used for health monitoring of underwater pipelines using guided waves.

The Ground Penetrating Radar (GPR) method has been extensively used to map shallow subsurface features. Such kind of information is very important for different types of studies, ranging from archaeological to groundwater search. Almost all GPR surveys for groundwater exploration are usually conducted in sedimentary terrains. In this paper, we demonstrate the applicability of the GPR method in the exploration of underground water in a crystalline terrain. An example of fixed offset data collected at one known spring in the district of Petrópolis (Brazil) is given. Common Mid Point (CMP) data were also collected to estimate the velocity of the radar waves. The saturated region is clearly outlined in the GPR section as a zone of attenuation and inversion of the wavelet phase polarity.

We demonstrate that surface acoustic waves ͑SAWs͒, nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and micro/bioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at 1 -10 cm/ s velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micro/nanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the drop free surface giving rise to inertial liquid jets that persist over 1 -2 cm in length or atomization of the entire drop to produce 1-10 m monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymer/protein solutions can also be used for the rapid synthesis of 150-200 nm polymer/protein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physical/chemical templating.

Aims:The effects of ion and neutral drag forces on stability of low-frequency wave modes is investigated from the point of view of parameters relevant to the plasma environment in the vicinity of the main Saturn rings. The electrons and ions are considered to be magnetized and corotate with the planet, while the dust grains are not magnetized and move around the planet on Keplerian orbits. Methods: The wave modes of low-frequencies in such dusty plasma, propagating along the ring in the azimuthal direction are analyzed based on the susceptibilities derived from the standard fluid approach and model presentation of the drag forces valid in the parameter regime of the dusty plasma of planetary rings. Results: It is found that the ion drag force can crucially change the stability of dust-acoustic perturbations. Inside the co-rotation distance the ion drag force can be responsible for the excitation of dust-acoustic waves, while for the region outside the synchronous orbit this mode can hardy be excited (at radial distances corresponding to the main rings). The instability due to the ion drag force could be also of some importance for perturbations in remote rings, where the relative ion-dust velocities exceed their thermal speed.

The concept of equilibrium plan form and equilibrium profile has been widely used as an engineering tool in order to design beach nourishment projects. The scope of this paper is to further explore this “equilibrium beach” concept in crenulated bays, as a long-term tool for beach nourishment projects. The proposed methodology is based on González and Medina (2001) and combines the static equilibrium plan and profile for long-term analysis. This methodology includes a modified equilibrium plan form, which is able to define the orientation of the local wave front in the diffracting point, and also to locate the downcoast starting point of the static equilibrium beach from which the parabolic plan form of Hsu and Evans (1989) is valid. This methodology permits the application of any equilibrium profile formulation. An example of the application of this methodology and long-term formulations to the design of the Spanish nourishment project of Poniente Beach (Gijón) is presented. Ten years after its construction, the beach has still got a static equilibrium and remains pretty close to the predicted equilibrium beach in plan and profile.

This report documents a study performed for the U.S. Army Engineer District, Seattle to identify and evaluate feasible methods for reducing annual maintenance dredging in the outer Federal navigation channel at Grays Harbor, WA, by modification of the north jetty. Main interest was in potential reduction of southward sand bypassing the north jetty and preservation of the jetty should the neighboring beach erode. Considerable information and predictive capability were generated concerning the behavior of the Federal navigation channel and adjacent coastal and inlet shorelines. The study was conducted as a coordinated multi-disciplinary approach involving field measurement, physical modeling of the hydrodynamics and potential sediment pathways, geomorphic analysis and sediment budget formulation, and numerical modeling of waves, circulation and sediment transport, including modeling of shoreline change and bypassing. Numerous alternatives were considered and subjected to a screening p...

A numerical method for non-hydrostatic, free-surface, irrotational flow governed by the nonlinear shallow water equations including the effects of vertical acceleration is presented at the aim of studying surf zone phenomena. A vertical boundary-fitted grid is used with the water depth divided into a number of layers. A compact finite difference scheme is employed for accurate computation of frequency dispersion requiring a limited vertical resolution and hence, capable of predicting the onset of wave breaking. A novel wet-dry algorithm is applied for a proper handling of moving shoreline. Mass and momentum are strictly conserved at discrete level while the method only dissipates energy in the case of wave breaking. The numerical results are verified with a number of tests and show that the proposed model using two layers without ad-hoc assumptions enables to resolve propagating nonlinear shoaling, breaking waves and wave run-up within the surf and swash zones in an efficient manner.

Ultrasonic measurements of acoustic wavefields scattered by single spheres placed in a homogenous background medium ͑water͒ are presented. The dimensions of the spheres are comparable to the wavelength and the wavelength and represent both positive ͑rubber͒ and negative ͑teflon͒ velocity anomalies with respect to the background medium. The sensitivity of the recorded wavefield to scattering in terms of traveltime delay and amplitude variation is investigated. The results validate a linear ͑first-order͒ diffraction theory for wavefields propagating in heterogeneous media with anomaly contrasts on the order of ±15%. The diffraction theory is compared further with the exact results known from literature for scattering from an elastic sphere, formulated in terms of Legendre polynomials. To investigate the 2D case, the firstorder scattering theory is tested against 2D elastic finite-difference calculations. As the presented theory involves a volume integral, it is applicable to any geometric shape, and the scattering object does not need to be spherical or any other specific symmetrical shape. Furthermore, it can be implemented easily in seismic data inversion schemes, which is illustrated with examples from seismic crosswell tomography and a reflection experiment.

The study of electromagnetic wave propagation in a coaxial cable can be a powerful approach to the study of waves at an undergraduate level. This study can explore different experimental situations, going from those where the finite velocity of propagation must be considered (distributed or transmission line behaviour), to those where this velocity may be considered infinite (lumped behaviour). We believe that the student observation of the existence of these two regimes can be important for the understanding of wave phenomena in general. In this work we show that this can be achieved using low-cost equipment and a set of quite simple experiments, such as the measurement of wave propagation velocity or the study of standing waves and resonance. The results obtained in a coherent set of selected experiments are discussed.

A model describing the dynamic evolution of waves on laminar falling wavy films at low to moderate Reynolds numbers (Re < 30) over corrugated surfaces is presented. This model is based on the integral balance method, which is used to simplify the analysis by assuming a parabolic velocity profile within the bulk of the film. The predictions of this model are compared to those obtained from a computational fluid dynamics (CFD) code for which no assumptions regarding the film velocity profile are made. The film evolution profiles, obtained via numerical solution of the model equations, are used to calculate the ratio of film interfacial area to that of the substrate. The model predicts suppression of wave growth on a corrugated surface. The model is also used to predict the effect of packing geometry on the flow characteristics, which may improve packing design. Solutions obtained for Re ∼ 200 using the CFD code demonstrate the degree of fluid accumulation within the 'valleys' of the structured substrate wherein re-circulation occurs.

This paper presents an update of the Corps of Engineers tsunami disaster mitigation research in the United States. Because the U.S. has not had any major tsunami disasters in many years, the Corps does not have a formal mission or policy for incorporating tsunami runup considerations in the design wave height for coastal structures. In the 1990's the Corps' Coastal and Hydraulics Laboratory did participate in substantial physical modeling of tsunami runup on beaches, vertical walls, and islands, as part of the National Science Foundation's Joint Tsunami Runup Study. These data have been extensively published and used by the international tsunami community to provide a better understanding of the physical phenomena and verify numerical models. The University of Southern California has recently been working with the Corps, and other federal and state agencies in California to develop tsunami inundation maps based on farfield and local tectonic and landslide sources. An example using the ports of Los Angeles and Long Beach in southern California is presented to illustrate the significance of these local co-seismic sources.

In this paper, the obtained results from the experimental studies describe the basic characteristics of wave propagation and ability of possible measures to reduce the impact of soil vibrations on structural response for both active and passive isolation cases. A series of field tests on the foundation vibrations generated by electrodynamic shaker are performed to examine the screening efficiency of open and in-filled trench barriers which are constructed for full-scale measurement. From field measurements of amplitude with and without the barrier, the amplitude reduction ratio is estimated at different points of interest. Wave propagating characteristics and frequency-dependent screening effects of the wave barriers are investigated according to various isolation material stiffnesses. The remarkable outcomes from these experimental studies can be briefly generalized as follows: backfilled trench with softer material than soil is more effective for the passive isolation than the active one. The reduction effects of wave barriers depend on the frequency of vibration source for both passive and active isolation cases. In-situ measurements confirm that vibration screening systems using open or in-filled trench barriers can be applied as a reduction measure for soil vibrations due to a moving load which is considered as stationary wave source in this problem.

Wave overtopping nearshore coastal structures, such as shore-parallel breakwaters, can significantly alter the current circulation and sediment transport patterns around the structures, which in turn affects the formation of tombolos and salients in the nearshore area. This paper describes the implementation of a wave overtopping module into an existing depth-averaged coastal morphological mode: COAST2D and model applications to investigate the effect of wave overtopping on the hydrodynamics and morphodynamics around a group of shore-parallel breakwaters. The hydrodynamic aspects of the model were validated against a series of laboratory conditions. The model was then applied to a study site at Sea Palling, Norfolk, UK, where 9 shore-parallel segmented breakwaters including 4 surface-piercing and 5 low-crested breakwaters are present, for the storm conditions in Nov 2006. The model results were compared with laboratory data and field measurements, showing a good agreement on both hydrodynamics and morphological changes. Further analysis of wave overtopping effect on the nearshore hydrodynamics and morphodynamics reveals that wave overtopping has significant impacts on the nearshore circulation, sediment transport and the resulting morphological changes within such a complex breakwater scheme under the storm and macro-tide conditions. The results indicate the importance of including the wave overtopping in modelling nearshore morphodynamics with the presence of coastal structures.

We tested the usefulness of acquiring multicomponent GPR data to detect cracks in a historical building, and to monitor their dynamics, caused by a slowly and irregularly moving landslide. We used 2 GHz bipolar antennas in a configuration that allows for acquiring the in-line and cross-line electric field components with x-and y-directed antennas. The 2 × 2 data matrix was collected on a floor in the building along transects at four different times over a period of one year. The data were processed with a standard 2D scalar algorithm and with the latest 3D single component vector algorithm that corrects for antenna effects. We have implemented a 3D single component vector migration algorithm in a 2.5D sense to produce 2D slices of a 3D vector migration image by applying the algorithm on line data. This procedure allows for migrating single component line data taking into account all vector effects as well as three-dimensional wave propagation. We show that the 2.5D vector migration images have a much better in-line resolution than the migration images obtained by applying a standard 2D scalar migration algorithm. The GPR profiles agree with the a priori information about the structure of the floor. In particular, we detected two different types of anomalies, only a few of which can be due to utilities and to metallic mesh. Some shallower anomalies agree well with the cracks visible on the tiling, suggesting that some cracks can be directly detected using GPR. Visually there were no changes in the cracks on the floor and no clear changes in the GPR data could be attributed to possible subsurface changes in the cracks. The variations in the GPR images seemed primarily caused by changes in the coupling of the antennas with the investigated structure (floor) depending on the season when the measurements were made. For this reason the monitoring aspect of the survey is not successful.

This study proposes a new technique to separate incident and reflected higher harmonic waves using four or more spatially separated probes. Both the free and locked modes in the higher harmonics of the regular waves can be isolated. The complex waves are decomposed into individual frequency components using the Fourier transform. The least squares method is applied to minimize the error caused by possible signal noise and to obtain the equations for solving the unknown parameters related to the wave amplitudes. Probe spacing condition for preventing singularity in the calculation is provided. The accuracy of this method is verified by applying it to resolve artificial waves with arbitrarily selected amplitudes. The sensitivity of this method to the noise inevitably associated with the experiments is also tested. The free surface elevation data collected from probes located upstream and downstream of a submerged breakwater in a numerical wave tank are analyzed to demonstrate the applicability of this method. The results are compared with those obtained using the methods of Goda and Suzuki Estimation of incident and reflected waves in random wave experiments. Proc. 15th Costal Eng. Conf., ASCE, pp. 828 -845] and Mansard and Funke [Mansard, E.P.D., Funke, E.R., 1980. The measurement of incident and reflected spectra using a least squares method. Proc. 17th Costal Eng. Conf., ASCE,. The comparison indicated that this method gives exactly the same first harmonic incident and reflected wave amplitudes as the other two methods. The full modes in the higher harmonics can be determined using this method, while the higher harmonics are presumed to be free waves in the other two methods.

Splitter or baffle silencers are commonly used, for example, in heating ventilation and air conditioning (HVAC) systems and as inlet/outlet silencers on gas turbines. Another application is to reduce noise from the cooling fan inlet for large IC-engines. A splitter silencer can be seen as a periodic arrangement of parallel rectangular absorbers, which can be placed in a rectangular duct. The noise reduction afforded by parallel splitters depends not only on the physical properties of the lining but also upon the angle of incidence of the impinging sound waves, and the splitter and duct dimensions. In this paper, the potential of using splitters made of microperforated plates (MPPs) is investigated, with a particular focus on cooling fan inlet/outlet applications.

Various weather related mechanisms for noise generation during marine seismic acquisition is addressed from a fluid dynamical perspective. This is done by analyzing a number of seismic lines recorded on modern streamers, during non-optimal weather conditions. In addition we provide a description of some of the complex fluid mechanical processes associated with flow that surrounds seismic streamers. The main findings are that noise in the 0-2 Hz range mostly is the result of direct hydrostatic pressure fluctuations on the streamer caused by wave motion. For normal 'swellnoise' above 2 Hz, and for 'cross-flow noise', a significant portion of the observed noise probably comes from dynamic fluctuations caused by the interaction between the streamer and fluid structures in its turbulent boundary layer. This explanation differs from most previous works which have focused on streamer oscillations, bulge waves inside old fluid filled seismic streamers or strumming/tugging as the main source for the generation of weather related noise. Even though modern streamers are known to be less sensitive to such sources of noise, their ability to tackle the influence on 'turbulent flow noise' has not improved. This implies that noise induced by turbulent flow has increased its relative importance on modern equipment, and that in order to improve the signal-to-noise (S/N)-ratio on seismic data, design issues related to flow noise need to be addressed. NOMENCLATURE a = streamer radius (m) α = streamer incidence angle (deg) C D = tangential drag coefficient δ = boundary layer thickness (m) δ ij = Kronecker delta = viscous dissipation rate (m 2 /s 3 ) F D = drag force (N) η κ = Kolmogorov micro-scale (m) l = integral length scale (m) L = streamer length (m) ν = kinematic viscosity (m 2 /s) Re = Reynolds number. Re L = U L/ν, Re δ = U δ/ν ρ = fluid density (kg/m 3 ) τ = shear stress (N/m 2 ) τ w ≡ µ(∂U/∂y) y=δ , shear stress on surface (N/m 2 )

1] Lightning return-stroke models are needed for specifying the source in studying the production of transient optical emission (elves) in the lower ionosphere, the energetic radiation from lightning, and characterization of the Earth's electromagnetic environment, as well as studying lightning interaction with various objects and systems. Reviewed here are models based on Maxwell's equations and referred to as electromagnetic models. These models are relatively new and most rigorous of all models suitable for computing lightning electromagnetic fields. Maxwell's equations are numerically solved to yield the distribution of current along the lightning channel. Different numerical techniques, including the method of moments (MoM) and the finite difference time domain (FDTD) method, are employed. In order to achieve a desirable current-wave propagation speed (lower than the speed of light in air), the channel-representing wire is embedded in a dielectric (other than air) or loaded by additional distributed series inductance. Capacitive loading has been also suggested. The artificial dielectric medium is used only for finding the distribution of current along the lightning channel, after which the channel is allowed to radiate in air. Resistive loading is used to control current attenuation with height. In contrast with distributed circuit and so-called engineering models, electromagnetic return-stroke models allow a self-consistent full-wave solution for both lightning-current distribution and resultant electromagnetic fields. In this review, we discuss advantages and disadvantages of four return-stroke channel representations: a perfectly conducting/resistive wire in air, a wire embedded in a dielectric (other than air), a wire in air loaded by additional distributed series inductance, and a wire in air having additional distributed shunt capacitance. Further, we describe and compare different methods of excitation used in electromagnetic return-stroke models: closing a charged vertical wire at its bottom with a specified grounded circuit, a delta-gap electric field source, and a lumped current source. Finally, we review and compare representative numerical techniques used in electromagnetic modeling of the lightning return stroke: MoMs in the time and frequency domains and the FDTD method. We additionally consider the so-called hybrid model of the lightning return stroke that employs a combination of electromagnetic and circuit theories and compare this model to electromagnetic models. Citation: Baba, Y., and V. A. Rakov (2007), Electromagnetic models of the lightning return stroke,

A rotary piezoelectric motor design with simple structural components and the potential for miniaturization using a pretwisted beam stator is demonstrated in this paper. The beam acts as a vibration converter to transform axial vibration input from a piezoelectric element into combined axial-torsional vibration. The axial vibration of the stator modulates the torsional friction forces transmitted to the rotor. Prototype stators measuring 6:5 6:5 67:5 mm were constructed using aluminum (2024-T6) twisted beams with rectangular cross-section and multilayer piezoelectric actuators. The stall torque and noload speed attained for a rectangular beam with an aspect ratio of 1.44 and pretwist helix angle of 17.7 were 0.17 mNm and 840 rpm with inputs of 184.4 kHz and 149 mW, respectively. Operation in both clockwise and counterclockwise directions was obtained by choosing either 70.37 or 184.4 kHz for the operating frequency. The effects of rotor preload and power input on motor performance were investigated experimentally. The results suggest that motor efficiency is higher at low power input, and that efficiency increases with preload to a maximum beyond which it begins to drop.

This work presents a mathematical model for the two-phase flows in the mortar systems and demonstrates the application of approximate Riemann solver on such model. The mathematical model for the two-phase gas-dynamical processes in the mortar tube consists of a system of first-order, nonlinear coupled partial differential equations with inhomogeneous terms. The model poses an initial value problem with discontinuous initial and boundary conditions that arise due to the design complexity and nonuniformity of granular propellant distribution in the mortar tube. The governing equations in this model possess characteristics of the Riemann problem. Therefore, a high-resolution Godunov-type shock-capturing approach was used to address the formation of flow structure such as shock waves, contact discontinuities, and rarefaction waves. A linearized approximate Riemann solver based on the Roe-Pike method was modified for the twophase flows to compute fully nonlinear wave interactions and to directly provide upwinding properties in the scheme. An entropy fix based on Harten-Heyman method was used with van Leer flux limiter for total variation diminishing. The three-dimensional effects were simulated by incorporating an unsplit multidimensional wave propagation method, which accounted for discontinuities traveling in both normal and oblique coordinate directions. A mesh generation algorithm was developed to account for the projectile motion and coupled with the approximate Riemann solver. The numerical method was verified by using exact solutions of three test problems. The specific system considered in this work is a 120 mm mortar system, which contains an ignition cartridge that discharges hot gas-phase products and unburned granular propellants into the mortar tube through multiple vent-holes on its surface. The model for the mortar system was coupled with the solution of the transient gas-dynamic behavior in the ignition cartridge. The numerical results were validated with experimental data. Based on the close comparison between the calculated results and test data, it was found that the approximate Riemann solver is a suitable method for studying the two-phase combustion processes in mortar systems. to 68.9.31.77. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm 3.5 Three-Dimensional Wave Propagation. Unsplit methods ͓20͔ were used for multidimensional wave propagation. Both the increment waves ͑i.e., flux vectors͒ and correction waves ͑i.e., second-order terms͒ were split into parts propagating in both the direction normal to the interface between two spatial locations and transverse direction by solving Riemann problems in coordinate directions tangential to the interfaces. This treatment calculates the cross derivative terms necessary for achieving both a stable and second-order accurate scheme. One-dimensional Riemann problems are solved at the interfaces. The scheme used in this