Wave Zone Research Articles

TRANSITION EXCITATION OF A PERFECTLY CONDUCTING CYLINDER BY RELATIVISTIC ELECTRON BUNCHES

Transition radiation of a relativistic electron bunch, which arises when it collides with the surface of an infinite perfectly conducting cylinder, is considered. The electron bunch moves perpendicular to the cylinder surface. Expressions for the field strength of electromagnetic radiation in the wave zone are obtained and investigated.

Applicability Study of a Sunken Vessel as an Artificial Reef in a High Wave Energy Zone

The purpose of this study is to analyze the applicability of a sunken vessel as an artificial reef in a high wave energy area in a shallow water zone. Artificial reefs in general shapes are unlikely to be installed and maintained stably in high-energy wave zones. To solve this problem, a method for using a large 2000-ton class sunken vessel as an artificial reef was proposed in this study. Accordingly, we analyzed the applicability, including stability, of the sunken vessel as an artificial reef on the east coast of South Korea, where high-energy waves and swells are observed frequently. In the analysis process, various methods, such as numerical simulation and a hydraulic model experiment, were utilized. 2D and 3D numerical simulations were conducted to simulate the wave energy in the area where the sunken vessel was installed as an artificial reef to identify wave force applied to the vessel. Moreover, a hydraulic model experiment using a sunken vessel model as an artificial reef was performed to compare the stability and motion properties of the vessel model. The experimental results indicated that the stability and motion properties of the sunken vessel differed depending on the water depth. Additionally, the water depth that ensured the stability of the sunken vessel was identified.

Dynamic positioning of ROV in the wave zone during launch and recovery from a small surface vessel

Dynamical positioning (DP) control of a remotely operated vehicle (ROV) using thrusters is investigated as a means to increase operability of its launch and recovery system (LARS) on relatively small surface vessels. The ROV uses its DP control system to dynamically track the wave-driven motion of the surface vessel that may also be under DP control. We consider the critical phase when the ROV position is very close to the LARS. In this phase, the ROV’s motion is strongly influenced by first-order surface waves and hydrodynamic couplings from the surface vessel motion. Hence, there is risk of collision between the ROV and the surface vessel and its LARS. We employ a time-domain dynamic system simulator that calculates the coupled motions of both the surface vessel and the ROV to analyse and optimize the operability of the system. It is based on coupled added mass, damping and wave force response amplitude operators (RAO) for both the surface vessel and the ROV, which can be calculated using WAMIT coupled hydrodynamic multi-body computations, accounting also for the moonpool water column dynamics. Simulation results show the positive effect of the ROV DP on the operability of the LARS using statistical environmental data from the North Sea.

Microstructure characterization and tensile shear failure mechanism of the bonding interface of explosively welded titanium-steel composite

The dissimilar joining of TA2/Q235 had attracted great attention because of its important applications in the nuclear fusion, aerospace, petrochemical industry. The possibility of joining TA2 and Q235 irrespective of different thermal expansion coefficients, different melting temperatures, or different mechanical properties was restricted with the conventional welding methods. Explosive welding, as a well-known composite processing technology, could be used to weld two or more similar and dissimilar plates. Which had been used to successfully fabricate TA2/Q235 composite in this work. The failure often occurred near the bonding interface of transition joints and mechanical parts were fabricated by explosive welding in the industrial application and production. Then the tensile-shear test was conducted to investigate the tensile shear failure mechanism at the bonding interface with in-situ SEM, and also nanoindentation was used to identify the mechanical properties. The interface bonding shear strength was around 345 MPa. Further, the microstructure characterization of interfacial zone and the melting and mixing of welding materials were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results demonstrated that the failure of the bonding interface occurred along the interface wave direction. It was because that the defects, such as cavities, cracks and brittle intermetallic, in the melted zone of the interface wave. The discontinuities and stress concentration points would result in degraded mechanical properties. The fracture morphology showed microcracks, cleavage plane, dimples, fragmentation of brittle intermetallic compounds, which presented mainly brittle fracture, while ductile fracture existed in some zone. Meanwhile, there were the serious lattice distortion and high internal stress in Ti matrix, while the deformation and elongation of the grain and fine grains formed in Fe matrix.

Linking separation sharpness with the characteristics of axial velocity wave zone in a hydrocyclone

Maximizing separation sharpness is critical for the design and operation of hydrocyclones but remains difficult to achieve. This work presents a numerical study on the relationship between separation performance and the characteristics of axial velocity wave zone to understand the reasons for the limited separation sharpness. The results showed that this zone is featured as an inherent transition region, with extensive secondary vortices formed between inner and outer spiral flows. The presence of this zone adversely affects the fluid-solid momentum transfer, causing prolonged residence time and accumulation of intermediate-sized particles. The separation sharpness shows a strong dependence on the characteristics of axial velocity wave zone, which are further controlled by geometric parameters. Increasing the symmetry of flow field and optimizing the spatial distribution of this zone can help increase the separation sharpness while its size shows little effect.

Reversal of laser wake phase velocity generates high-power broadband Cherenkov signal

As a femtosecond terawatt laser pulse propagates along a positive density gradient within a stratified plasma column, it drives a low-frequency electromagnetic wake wave, the period of electron fluid oscillations in the wake gradually shrinking. Phase velocity of the wake promptly exceeds the vacuum speed of light, setting in near-forward emission of terahertz Cherenkov radiation. As the wake accelerates further, the Cherenkov emission ray rotates by 180∘. Emission from a given plasma locality is sustained for a finite interval of time, in the middle of which the wake experiences a ‘reversal,’ its phase velocity becoming singular and changing sign (Zhang C J et al 2017 Phys. Rev. Lett. 119 064801) At this instant, the electromagnetic energy flows at 90∘, the emission power reaching its peak. After the reversal, the wake keeps radiating into the rear hemisphere until its phase velocity becomes subluminal. Experimentally capturing evolution of the Cherenkov signal may thus shed light onto the plasma wake dynamics. Far away from the plasma, the radiation fills an expanding, almost spherical shell, the shell thickness increasing with an increase in the observation angle. The length of the terahertz signal sampled in the wave zone thus ranges from zero (forward emission) to a few tens of picoseconds (backward emission). The signal is positively chirped, its frequency increasing from the Langmuir frequency at the foot of the column to the Langmuir frequency at the top. Theoretical estimates for the regimes involving 10 TW-class drive pulses promise a few-kW emission power; the energy conversion efficiency, from optical to terahertz, of order 10−7; and an MV m−1-scale electric field strength centimeters away from the plasma.

Displacement speed analysis of surface propagation in moderately turbulent premixed reacting waves

The propagation of premixed reacting waves can be characterized by a displacement speed Sd at which the local surface of the reaction progress scalar moves respective to flow. Often, Sd is considered through decomposition into three parts of contribution due to the tangential diffusion of curvature, normal diffusion, and reaction. A set of recently derived transport equations for Sd and three of its decomposed parts provides new diagnostics for better understanding reaction wave propagation in a turbulent environment. In this work, those diagnostics are applied on four similarly setup direct numerical simulation cases studying the propagation of moderately perturbed planar reaction waves into homogeneous turbulence, and the reaction waves differ by the density ratio between fresh and burned gases. The data analysis reveals four self-acceleration behaviors: (i) surfaces propagating at large positive (negative) Sd tend to advance (retreat) faster, (ii) surfaces having large positive (negative) curvature tend to become more curved positively (negatively), (iii) thicken wave zones tend to become thicker, and (iv) surface elements accelerate toward their destruction. The extent of the above accelerations all reduces in the reaction wave having a high density ratio. This can be attributed to the turbulence inhibition due to the flow dilatation and viscosity increase across a thermal-expansion enabled reaction wave. The distribution of curvature for the reaction-zone surface skews toward a negative value, i.e., the curvature center pointing to the burned product.

Sensitive Dependence of Ultrarelativistic Electron Precipitation on EMIC Wave Frequency

AbstractElectromagnetic ion cyclotron (EMIC) waves have long been considered to cause atmospheric precipitation loss of relativistic electrons, the quantitative evaluation of which is critical for understanding the radiation belt electron flux dropouts. In this study, we report test particle calculation results on precipitation loss rates of 5 MeV electrons with initial pitch angles of 4–10° at the equator by He+ band EMIC waves. We find a sensitive dependence of the precipitation loss rates on the wave frequency in that a preferred wave frequency (maximum loss frequency) exists at which the loss rate peaks predominantly and away from which it declines substantially. According to a cold plasma dispersion relation, a small difference in wave frequency just below the He+ gyrofrequency corresponds to a significant difference in wavenumber. Different frequencies lead to phase trapping of different extents, which determines the significance of loss cone scattering at the exit latitude from the wave zone. At frequencies below the maximum loss frequency, phase trapping causes large advective pitch angle changes, prohibiting loss cone scattering. At the maximum loss frequency, the phase‐trapping effect is optimized such that diffusion is dominant over advection and large enough to allow effective loss cone scattering. At frequencies above the maximum loss frequency, the phase‐trapping effect disappears, and the resulting (both diffusive and advective) scattering is too weak to drive loss cone scattering unless the initial pitch angle is close enough to the loss cone angle.

2D Analysis for Computing Supersonic and Subsonic Flow Field by Using Schlieren Light.(Dept.M)

A new 2D analysis of light for computing the Mach number distribution in a flow field from Schlieren images taken across the shock wave and expansion zone is proposed and verified by a 2D numerical computation Specially, inserting experimental measuring sensors inside shock wave zone and associated flow field is difficult. Across the shock wave, the pressure and density of the fluid change at extremely high rates, which affect the light refraction. The intensity of the Schlieren light is a function of the medium density, which is a function of the light ray path position. The contrast of each pixel is determined by analyzing the image. The equations of Schlieren intensity light are derivated to be a function of Mach number. These functions are applied on an image of shockwave around hemisphere-cylinder at M=1.96. To verify the results two-dimensional Navier-Stokes equations and conservation of energy equation are used to model the flow field and determine Mach number distribution around a hemisphere-cylinder of the supersonic flow, shock waves and associated flow field at the same previous conditions. The governing equations are discretized on a structured grid using an upwind difference scheme. The computational and experimental results show that there is a good matching in the iso-contour of the Mach number with difference less than 3%. The new proposed technique successfully represents the upstream region, the shock wave region, the subsonic region and the expansion fan region. This difference may be due to the noise in the image. This noise can be eliminated if Schlieren light received directly on a CCD camera. The results show that this analysis can be applied for both bright and dark regions of the Schlieren image with correcting the phase shift in the dark area. This optical technique has the advantages of optical measurements. This optical technique has all the advantages of optical measurements as does not disturb the flow and the high sensitivity to capture rapid variation.

Modified rigorous coupled-wave analysis for grating-based plasmonic structures with a delta-thin conductive channel: far- and near-field study.

The modified rigorous coupled-wave analysis technique is developed to describe the optical characteristics of the plasmonic structures with the grating-gated delta-thin conductive channel in the far- and near-field zones of electromagnetic waves. The technique was applied for analysis of the resonant properties of AlGaN/GaN heterostructures combined with a deeply subwavelength metallic grating, which facilitates the excitation of the two-dimensional plasmons in the terahertz (THz) frequency range. The convergence of the calculations at the frequencies near the plasmon resonances is discussed. The impact of the grating's parameters, including filling factor and thickness of the grating, on resonant absorption of the structure was investigated in detail. The spatial distributions of the electromagnetic field in a near-field zone were used for the evaluation of total absorption of the plasmonic structures separating contributions of the grating-gated two-dimensional electron gas and the grating coupler.

ВОПРОСЫ РЕКОНСТРУКЦИИ КРУПНЫХ КАНАЛОВ И ОЦЕНКА ИХ ЭФФЕКТИВНОСТИ

Purpose: to define the efficiency of large irrigation canals on the example of the Donskoy main canal and the Bolshoy Stavropol canal during their reconstruction. Materials and methods. The use of geocomposite materials is of interest to creating combined linings of large canals. A geocomposite anti-seepage structure made without a protective layer at the bottom and part of the slope and a concrete-film element in the zone of wave and ice impact is considered. The application of such a combined lining makes it possible to reduce the cost of the anti-seepage screen, to shorten the construction time, to reduce the canal roughness and to increase its water-carrying capacity. The method of hydraulic research is based on the field data of the Bolshoy Stavropol and the Donskoy main canals for average and maximum values of hydraulic parameters. In particular, it was necessary to determine the roughness coefficients of the canal in the area between the sections. Results. A feature of the reconstruction of the most dangerous sections of the Donskoy main canal is the close location of the groundwater level to the surface, which leads to the transition of free filtration to a banked-up one. According to the results of calculations, the total losses are 13.7×10³ cubic meters/day in the section 57–70 km, and with canal lining – 0.504 cubic meters/day, which is 2.09 times less than with banked-up filtration in the earthen bed. Conclusions. The most effective types of linings with the use of geocomposites which, as shown by field surveys, are reliable combined coatings, are presented. Anti-seepage facilities made of concrete and reinforced concrete have high impermeability, though at the same time they are destroyed over time under the influence of water and freezing-thawing processes.

ADVANCES IN THE UNSTRUCTURED WAVEWATCH III AND APPLICATION TO HURRICANE DORIAN

The spectral wave generation and propagation model WAVEWATCH III (WW3) is undergoing rapid development to extend capability and applicability. An option for unstructured grids and implicit solution provides WW3 with the flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup, and wave-driven sediment transport with multi-scale spatial coverage over approximately three orders of magnitude. The model is compatible with community-based coupling infrastructure to facilitate two-way coupling with circulation models for simulating hurricane storm surge and waves. Unstructured WW3 is applied for 2019 Hurricane Dorian and validated with National Data Buoy Center buoys and nearshore gauges at the US Army Corps of Engineers Field Research Facility.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/kz9G46xUD0k

Complex Permittivity Measurement of Dielectric Substrate in Sub-THz Range

In this article, three frequency independent optical paths are designed and analyzed. A two-parabolic-mirror system and a four-parabolic-mirror system are studied and developed over 140–220 GHz to achieve precision complex permittivity measurements of a dielectric substrate. To achieve a wide plane wave zone for the center of the four-parabolic-mirror system, two 80-mm-length corrugated horns are designed and fabricated for the measurement systems. The Gaussicity of the corrugated horn is larger than 97.4%. For the multiple reflection model and direct wave model, two closed-form expressions of loss tangent are derived from the transmission parameters (insertion losses) of the measurement systems. Meanwhile, the resolution and uncertainty of loss tangent can be calculated according to the working frequency, the thickness of the wafer, the real part of the relative permittivity, and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$| {{S_{21}}} |$</tex-math></inline-formula> measurement uncertainty. The complex permittivity of the Rogers/Duroid series PCB substrates, which are commonly used at microwave frequencies, and silicon wafers are measured in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> -band.

Wave motion compensation in dynamic positioning of small autonomous vessels

Conventional dynamic positioning (DP) systems on larger ships compensate primarily for slowly time-varying environmental forces. In doing so, they use wave filtering to prevent the DP from compensating for the first-order wave motions. This reduces wear and tear of the thruster and machinery systems. In the case of smaller autonomous vessels, the oscillatory motion of the vessel in waves may be more significant, and the thrusters can be more dynamic. This motivates the use of DP to compensate for horizontal wave motions in certain operations. We study the design of DP control and filtering algorithms that employ acceleration feedback, roll damping, wave motion prediction, and optimal tuning. Six control strategies are compared in the case study, which is a small autonomous surface vessel where the critical mode of operation is launch and recovery of an ROV through the wave zone.

Application of Network Analysis to Flow Systems with Alternating Wave Channels: Part B. (Superimposed Drag-Pressure Flows in Extrusion).

Due to progress in the development of screw designs over recent decades, numerous high-performance screws have become commercially available in single-screw extrusion. While some of these advanced designs have been studied intensively, others have received comparatively less attention. We developed and validated a semi-numerical network-theory-based modeling approach to predicting flows of shear-thinning polymer melts in wave-dispersion screws. In the first part (Part A), we systematically reduced the complexity of the flow analysis by omitting the influence of the screw rotation on the conveying behavior of the wave zone. In this part (Part B), we extended the original theory by considering the drag flow imposed by the screw. Two- and three-dimensional melt-conveying models were combined to predict locally the conveying characteristics of the wave channels in a discretized flow network. Extensive experiments were performed on a laboratory single-screw extruder, using various barrel designs and wave-dispersion screws. The predictions of our semi-numerical modeling approach for the axial pressure profile along the wave-dispersion zone accurately reproduce the experimental data. Removing the need for time-consuming numerical simulations, this modeling approach enables fast analyses of the conveying behavior of wave-dispersion zones, thereby offering a useful tool for design and optimization studies and process troubleshooting.

Gravitational radiation by magnetic field: application to millisecond magnetars

ABSTRACT We investigate the direct contribution of the magnetic field to the gravitational wave (GW) generation. To do so, we study the post-Newtonian (PN) energy–momentum tensor of the magnetized fluid and the PN expansion of the gravitational potential in the wave zone. We show that the magnetic field appears even in the first PN order of the multipole moment tensor. Then, we find an explicit relativistic correction containing the magnetic field contribution to the well-known quadrupole formula. As an application of this derivation, we find that the B-field part of the GWs released in the early stages of a millisecond magnetar’s life can be as much as one-hundredth of the signals due to the deformed rotating neutron stars. We show that although the event rate of this system is small, the signal would lie in the sensitivity range of the next generation of detectors.

A new approach to simulate the fluid dynamics in a wiped film evaporator using Modelica

Wiped film evaporators (WFE) or agitated thin film evaporators are efficient equipment for separating highly thermosensitive and viscous mixtures. There are different approaches to tailor residence time distribution (RTD) for specific applications and to maximise heat and mass transfer by equipment design or process parameter adjustment, like liquid load or wiper speed. However, the fundamental principles of fluid dynamics are only known to a limited extent. Although flow patterns and mass and heat transfer are highly interdependent, the effects of different wipers on liquid flow and, finally, on residence time behaviour are poorly understood. In this work, a new model describing fluid dynamics and its impact on RTD is presented. The model is implemented in the equation-based modelling language Modelica. Several system variables – e.g. film thickness, hold-up or velocity profiles within the liquid film – can be estimated. The effect of varying process parameters, like liquid load and fluid temperature, can also be evaluated. The model has been calibrated with results from RTD experiments in a WFE using a roller wiper. It is capable of describing the residence time behaviour of the roller wiper system with good accuracy. Mean residence times can be reproduced within approx. ±20%. Further results, such as film thickness or hold-up, provide additional insight into the fluid dynamics of characteristic regions – like bow wave, wiping and thin film zone – and the effects of several process parameters.

Integrated design optimization of spar floating wind turbines

A linearized aero-hydro-servo-elastic floating wind turbine model is presented and used to perform integrated design optimization of the platform, tower, mooring system, and blade-pitch controller for a 10 MW spar floating wind turbine. Optimal design solutions are found using gradient-based optimization with analytic derivatives, considering both fatigue and extreme response constraints, where the objective function is a weighted combination of system cost and power quality. Optimization results show that local minima exist both in the soft-stiff and stiff-stiff range for the first tower bending mode and that a stiff-stiff tower design is needed to reach a solution that satisfies the fatigue constraints. The optimized platform has a relatively small diameter in the wave zone to limit the wave loads on the structure and an hourglass shape far below the waterline. The shape increases the restoring moment and natural frequency in pitch, which leads to improved behaviour in the low-frequency range. The importance of integrated optimization is shown in the solutions for the tower and blade-pitch control system, which are clearly affected by the simultaneous design of the platform. State-of-the-art nonlinear time-domain analyses show that the linearized model is conservative in general, but reasonably accurate in capturing trends, suggesting that the presented methodology is suitable for preliminary integrated design calculations.

Synchrotron radiation in odd dimensions

In odd space-time dimensions, the retarded solution of the massless wave equation has support not only on the light cone, but also inside it. At the same time, a free massless field should propagate at the speed of light. The apparent contradiction of these two features is resolved by the fact that the emitted part of the field in the wave zone depends on the history of motion up to the retarded moment of proper time. It is shown that in the case of circular motion with ultrarelativistic velocity, the main contribution to the radiation amplitude is made by a small interval of proper time preceding the retarded time, and thus the tail term is effectively localized. We obtain a tentative formula for scalar synchrotron radiation in $D$ dimensions: $P =g^2(\omega_0\gamma^2/\sqrt{3})^{D-2}$, which is explicitly verified in $D=3,\,4,\,5$.

Numerical analysis of an Uppsala University WEC deployment by a barge for different sea states

Wave energy converters (WECs) have been deployed onshore, nearshore, and offshore to convert ocean wave movement into electricity. The exploitation of renewable energy sources has restrictions; in the case of wave energy, high installation, maintenance, and decommissioning costs have limited their commercial use. Moreover, these offshore operations can be compromised by safety issues. This paper draws attention to offshore operation safety of a WEC developed by Uppsala University. Specifically, this paper investigates what sea states are suitable for the safe deployment of a WEC from a barge. This study follows recommendations in DNV-RP-H103 for analysis of offshore operations, namely lifting through the wave zone. ANSYS Aqwa is used to find hydrodynamic forces acting on a typical barge, using frequency domain analysis. Based on these hydrodynamic simulation results and methodology given in DNV-RP-H103, tables are created to show the sea states that would allow for the safe installation of a WEC using a typical barge. Considered sea states have significant wave heights varying between 0 m and 3 m and the wave zero crossing periods varying between 3 s and 13 s. The WEC submersions are considered between 0 m and 7 m, i.e. when the WEC is in the air until it is fully submerged.