Wave Surface Research Articles

Optimization of leaky-wave surface coil current using an analytical approach

In this work, we investigate the optimal coefficients of the exponential current excited on a leaky wave surface coil. The respective functional is first derived analytically and later computed numerically using Python. The results are compared to the same problem modeled in Comsol Multiphysics.

On the energy of nonlinear water waves.

This article presents results concerning the excess kinetic and potential energies for exact nonlinear water waves. In particular, it is proven, for periodic travelling irrotational water waves, that the excess kinetic energy density is always negative, whereas the excess potential energy density is always positive, in the steady reference frame. A characterization of the total excess energy density as a weighted mean of the kinetic energy along the wave surface profile is also presented.

Hybrid Bio-Inspired Structure Based on Nacre and Woodpecker Beak for Enhanced Mechanical Performance.

Materials with high strength and toughness have always been pursued by academic and industrial communities. This work presented a novel hybrid brick-and-mortar-like structure by introducing the wavy structure of the woodpecker beak for enhanced mechanical performance. The effects of tablet waviness and tablet wave number on the mechanical performance of the bio-inspired composites were analyzed. Compared with nacre-like composites with a flat tablet, the strength, stiffness and toughness of the novel hybrid nacre-like composite with tablet wave surface increased by up to 191.3%, 46.6% and 811.0%, respectively. The novel failure mode combining soft phase failure and tablet fracture revealed the key to the high toughness of composites. Finite element simulations were conducted to further explore the deformation and stress distribution of the hybrid brick-and-mortar-like structure. It showed that the hybrid brick-and-mortar-like structure can achieve a much better load transfer, which leads to greater tensile deformation in tablet before fracture, thus improving strength and energy absorption. These investigations have implications in the design of composites with high mechanical performance for aerospace, automobile and other manufacturing industries.

Investigation of the Anisotropic Patterns in the Altimeter Backscatter Measurements Over Ocean Wave Surfaces

This article attempts to analyze the influence of the anisotropic effects of the ocean wave surface on SAR altimetry backscatter coefficient (Sigma-0) measurements, which has not been intensively addressed in publications. Data of Sentinel-3A, Cryosat-2, and Jason-3 altimeters allocated by the WW3 numeric wave model were analyzed, and the patterns of Sigma-0 with respect to the wave direction were acquired under ∼2 m significant wave height. The ocean waves were classified into six categories, among which the moderate swell and short win-wave cases were analyzed intensively. Swell-dominated ocean surface shows less randomness than the wind-wave-dominated ocean surface. Clear and significant sinusoid trends are found in the Sigma-0 and SSB patterns of both operational modes (SAR mode and PLRM mode) of the Sentinel-3A altimeter for the moderate swell case, indicating the sensitivity of Sigma-0 and SSB measurements to the anisotropic features of the altimeter measurements. The anisotropic pattern in the Sentinel-3A PLRM Sigma-0 is somewhat counterintuitive, but the analysis of Jason-3 altimeter data would show similar results. Additionally, by comparing the anisotropic patterns of two orthogonally polarized SAR altimeters (Sentinel-3A and Cryosat-2), we could draw the conclusion that the Sigma-0 measurements are not sensitive to the polarization mode. As for the SSHA patterns, no clear sinusoid could be identified for the moderate swell. A possible explanation is that the SSB pattern may be overwhelmed in the complicated factors that can influence the SSHA pattern.

A simplified method and numerical simulation for wedge-shaped plunger wavemaker

The wave can be obtained by the horizontal movement of the piston-plate or flap-plate, and also could be obtained by the vertical movement of the wedge. In this paper, Firstly, the implicit equations of the relationship between the wedge vertical motion trajectory and the targeted wave including linear wave, stokes-fifth-order wave, solitary wave and irregular wave, are derived. Secondly, the RNG k-ε turbulence model and VOF method are combined to solve the RANS equations for wave-making of targeted wave by wedge-shaped plunger wavemaker using CFD software Fluent. Thirdly, the numerical results are compared with the analytical solutions systematically, also the characteristics of the wave surface, velocity field, water particle trajectories, as well as their evolution and propagation are elaborately explored. Meanwhile, the effect of the initial entry depth and the wedge angle on the wave-making quality was discussed, and the differences in the force and power between wedge-shaped plunger wavemaker and piston-type wavemaker are compared in detail. The velocity field and water particle trajectories present a comprehensive understanding of wave generation and propagation. The results indicate that the proposed simplified method and numerical simulation have high accuracy for linear wave, weakly-nonlinear stokes-fifth-order wave, solitary wave and irregular wave.

Geometry Optimization of Heaving Axisymmetric Point Absorbers under Parametrical Constraints in Irregular Waves

The objective of this work is to identify the maximum absorbed power and optimal buoy geometry of a heaving axisymmetric point absorber for a given cost in different sea states. The cost of the wave energy converter is estimated as proportional to the displaced volume of the buoy, and the buoy geometry is described by the radius-to-draft ratio. A conservative wave-height-dependent motion constraint is introduced to prevent the buoy from jumping out of the free surface of waves. The constrained optimization problem is solved by a two-nested-loops method, within which a core fundamental optimization process employs the MATLAB function fmincon. Results show that the pretension of the mooring system should be as low as possible. Except for very small energy periods, the stiffness of both the power take-off and mooring system should also be as low as possible. A buoy with a small radius-to-draft ratio can absorb more power, but at the price of working in more energetic seas and oscillating at larger amplitudes. In addition, the method to choose the optimal buoy geometry at different sea states is provided.

Robust, high-resolution, indexed 3D slowness surfaces for Rayleigh-type waves on Lithium Niobate via parallelised Newtonian flow phase tracking

Micro-Electromechanical System Surface Acoustic Wave (MEMS SAW) devices are key technological enablers in RF telecommunications filters, inertial systems and biosensors. Recent developments in Focused SAW (FSAW) device design demand high quality, high resolution dispersion data for multiple in-plane directions, typically presented as a slowness curve, to facilitate design. Very few slowness curves are available in the literature for free surfaces not aligned to the crystallographic axes except in a few special cases. The first major novelty in the present work is a bespoke computational approach to solving this problem with a distinct theoretical basis. After a brief summary of the motivation, the mathematical formulation and its theoretical footing are outlined. The most important algorithms are given as pseudocode, and source code, datasets and extended visualisation available online are referenced. The structure of the method is illustrated via application to the bulk wave problem. The model is then applied to Lithium Niobate to generate new results characterising SAW propagation velocity for all in-plane directions over one full circle of plane inclinations. These results are presented as a two-dimensional velocity surface. This new data constitutes the second area of novelty herein. Finally, the applicability of the work to FSAW design is indicated by examining the RMS variation of propagation velocity for different cut plane inclinations.

Generation of interplanetary type II radio emission

Context.Coronal mass ejections (CMEs) are eruptive phenomena that can accelerate energetic particles and drive shock waves. The CME-driven shocks propagate from the low corona to interplanetary space. The radio emission that results from fast electrons energised by shock waves are called type II bursts. This radio emission can provide information on the physical properties of the shock and its evolution as it travels through the corona and interplanetary space.Aims.We present a comprehensive analysis of the shock wave associated with two type II radio bursts observed on 27 September 2012. The aim of the study is to isolate and understand the shock wave properties necessary for accelerating electrons, leading to the production of the radio emission.Methods.First, we modelled the 3D expansion of the shock wave by exploiting multi-viewpoint reconstruction techniques based on extreme ultraviolet imaging. The physical properties of the shock front were then deduced by comparing the triangulated 3D expansion with properties of the background corona provided by a 3D magnetohydrodynamic model. The radio triangulation technique provided the location of radio source on the surface of the modelled wave in order to compare radio sources with the shock properties.Results.This study is focused on the temporal evolution of the shock wave parameters and their role in the generation of radio emission. Results show a close relationship between the shock wave strength and its geometry. We deduce from this analysis that there may be several mechanisms at play that generally contribute to the generation of radio emission.Conclusions.The comparison between the reconstructed sources of radio emission and the ambient shock wave characteristics reveals the complex relationship between shock parameters and show how they can influence the morphology of the observed type II radio emission.

Годовой ход погрешности радиоальтиметрических измерений уровня Черного моря, обусловленной нелинейностью морских волн.

The variability of the error of the altimetric determination of the Black Sea level L due to the nonlinearity of sea waves is analyzed. The nonlinearity leads to deviations in the distribution of elevations of the reflecting radio wave surface from the Gaussian distribution. The error occurs due to the fact that the median of the non-Gaussian distribution of surface elevations does not coincide with the average surface level. The analysis is carried out within the framework of the Brown model, which describes the shape of an altimetric pulse reflected from the sea surface. Data from a numerical operational model of the surface wave field are used for the analysis. When calculating the shape of the reflected pulse of the altimeter, an additional predictor is introduced – the steepness of the waves. It is shown that there is a clearly defined annual variation of the error L. Its highest values are observed in winter, when the average monthly value reaches the level of 0.25 m, in summer this error decreases to 0.08-01 m. The maximum value calculated from the three-hour characteristics of surface waves is 0.4 m, the average value is 0.17 m.

3D Ocean Water Wave Surface Analysis on Airborne LiDAR Bathymetric Point Clouds

Water wave monitoring is a vital issue for coastal research and plays a key role in geomorphological changes, erosion and sediment transportation, coastal hazards, risk assessment, and decision making. However, despite missing data and the difficulty of capturing the data of nearshore fieldwork, the analysis of water wave surface parameters is still able to be discussed. In this paper, we propose a novel approach for accurate detection and analysis of water wave surface from Airborne LiDAR Bathymetry (ALB) large-scale point clouds data. In our proposed method we combined the modified Density-Based Spatial Clustering of Applications with Noise (DBSCAN) clustering method with a connectivity constraint and a multi-level analysis of ocean water surface. We adapted for most types of wave shape anatomies in shallow waters, nearshore, and onshore of the coastal zone. We used a wavelet analysis filter to detect the water wave surface. Then, through the Fourier Transformation Approach, we estimated the parameters of wave height, wavelength, and wave orientation. The comparison between the LiDAR measure estimation technique and available buoy data was then presented. We quantified the performance of the algorithm by measuring the precision and recall for the waves identification without evaluating the degree of over-segmentation. The proposed method achieves 87% accuracy of wave identification in the shallow water of coastal zones.

The performance and optimization of ANN-WP model under unknown sea states

Ocean wave is one of the main factors to cause the 6 degrees of freedom motion of ships. Accurate wave surface prediction is paramount to ship safety. The artificial neural network-based wave prediction (ANN-WP) model is a phase-resolved wave prediction model based on artificial intelligence, which can provide high-precision prediction results of wave surface. However, it needs training with a large dataset to make the model obtain a satisfying prediction performance. When unknown sea states (not included in the training data) appear in practice, the reliability of the prediction results is low. In this study, the performance of the ANN-WP model under unknown sea states is investigated based on wave tank experiment. The data distribution of training data set is considered as one of the factors that affects the performance of ANN-WP model under unknown sea states. Based on this consideration, data mixing is employed to improve the performance of the ANN-WP model under unknown sea states. Through this method, the prediction error of the model for unknown sea states decreased by approximately 5.0%–10.0%.

Omnidirectional light absorption enhancement of perovskite solar cells by an antireflection film with holographic lithography microstructures.

We report an omnidirectional light absorption enhancement of a perovskite solar cell (PSC) using antireflection (AR) film with soft imprinted microstructures from master molds via holographic lithography technology, which has high throughput and repeatability. The PSC's omnidirectional power conversion efficiency (PCE) enhancement is achieved by reducing Fresnel surface reflections and enhancing the optical path length. The maximum PCE of PSCs with AR film is up to 20.27%, corresponding to an absolute increase of 0.93% compared to 19.34% of control devices. Significantly, the enhancements of PCE increase with incident angle enlargement, which attributes to more effective Fresnel surface reflection suppression. Moreover, AR films exhibit water and dust repellent properties due to hydrophobicity, which is beneficial for PSC's long-term stability and light harvesting.

Numerical simulation with a macroscopic CFD method and experimental analysis of wave interaction with fixed porous cylinder structures

Porous structures have been widely applied in the coastal and ocean engineering due to their wave energy dissipation mechanism. The macroscopic computational fluid dynamics (CFD) approach where the quadratic pressure drop condition of porous surface is introduced to model the wave interaction with porous cylinders. A series of CFD simulations of waves interacting with a single porous cylinder and the combined structure of a porous cylinder with a concentric inner solid column are performed, with corresponding tank tests conducted. The CFD method is compared with experiments, linear potential model, and the quadratic BEM (boundary element method) model. The effects of porosity and porous cylinder radius on wave force and wave heights inside porous cylinder are analyzed to evaluate the performance of porous shell reducing wave loads and wave surface elevation, and the wave force variation with incident wave amplitudes are also investigated. The results demonstrate that the established CFD model is reliable for engineering analysis and thereby being of great significance for reference purpose in the CFD simulations of waves interacting with porous structures.

A Design Method to Assess the Primary Strength of the Delta-Type VLFS

Very large floating structure (VLFS) is a sustainable concept centered around creating solid platforms at sea. The Delta is a new type of VLFS, designed to withstand open-sea conditions and to form, in addition to a broad deck areas, a sheltered basin of year-round operability. The design of this unique hull relies on direct calculations in order to identify critical load cases and assess their load effects. This study formulates a theoretical procedure for the initial assessment of the primary strength. The procedure analytically integrates the floatation loads while the hull rests at hydrostatic equilibrium on a wave surface and obtains the vertical and horizontal bending moment. This preliminary assessment tool enables a fast review of many load cases and provides the basic insights necessary for a reasonable initial design. Using the procedure, we conducted a primary load assessment for the design of Delta. By calculating the load response to 588 load cases, we identified the critical load scenario and the maximal axial stress. As the stress was too high, we improved the geometry in order to reduce loads and assessed proper scantlings for the critical section. We present the formulation of the procedure, the validation of the results, and the implementation for the structural design of the Delta VLFS.

Reflection and transmission of waves at the common interface of piezoelectric half-spaces with microstructure

The present article deals with the reflection and transmission of waves at an interface of piezoelectric (ALN & PZT-5T) half-spaces with microstructures. Unlike the classical piezoelectric material, in the considered media, five coupled waves (2 bulk and 3 surface) [namely the Quasi-longitudinal wave (QP), Quasi-transverse wave (QSV), Electric-acoustic wave (EA), P-type surface wave (SP) and S-type surface wave (SS)] are generated in response to an oblique incident Quasi plane wave. The main objective of this study is to investigate the influence of the characteristic length of microstructure, the inertial characteristic length and flexoelectric coefficients on the reflection and transmission coefficients. For this purpose, the linear algebraic equation, continuity conditions at the common interface are contracted. The linear system of equations comprising the reflection and transmission coefficients is derived, which is solved by using the Cramer's rule. The consequence of this electromechanical phenomenon may be advantageous in certain engineering applications that involve smart nano-composites.

Optimization of a three-dimensional hybrid system combining a floating breakwater and a wave energy converter array

Combining wave energy converters (WECs) with a floating breakwater provides a potential approach to help develop commercial-scale wave power operations. This paper aims to design and optimize a three-dimensional floating breakwater integrated with a WEC array by developing a numerical model of a multi-floating-body coupled system based on potential flow theory with viscous correction in the frequency domain. By analyzing the wave surface elevations around the breakwater, the size of the breakwater was optimized and the optimal installation locations of the WECs were determined. Under the condition of coupled constraints and six degrees-of-freedom motion, the interactions between the WEC array and the breakwater were analyzed. Subsequently, the number of WECs and the distance between the WECs and the breakwater were optimized to maximize the wave energy conversion performance of the hybrid system. Results show that the wave focusing areas appear more frequently near the breakwater. These focusing areas significantly improve WEC power, and thus better wave energy conversion performance can be achieved when the WECs are placed close to the breakwater. The vertical forces on the breakwater significantly increase due to the presence of the WECs, however the horizontal forces are decreased. The findings of this paper provide guidance to design and optimize a hybrid WEC-breakwater system in practical engineering applications.

Aerodynamic performance of semi-submersible floating wind turbine under pitch motion

Pitch motion is a critical factor that affects the aerodynamic load of blades and the motion response of the floating platform. To study the effects of pitch motion, the NREL 5-MW is analyzed by using Computational Fluid Dynamics (CFD) method. The motion of the wind turbine and the floating platform is dealt with dynamic fluid–solid interaction technology. The mesh in the blade rotation domain and the motion domain are sliding mesh and overlapped mesh respectively. The Fluid Volume Fraction (VOF) method is utilized to generate and track the free surface of the ocean wave. By setting the frequencies and amplitudes of sinusoidal pitch motion, the distributed aerodynamic pressure coefficient of the cross-sections is studied along the blade spanwise direction, and the instantaneous thrust, the turbine power coefficient and the wake vortex structure are analyzed. For blade only cases, results show that the pressure coefficient near the leading edge of each spanwise blade section reaches a maximum at the half-cycle, and the turbine power output is in phase with the pitch motion. For fully coupled cases, both the frequency of the turbine thrust and pitch motion is determined by the incident wave but in a reversed phase.

Reduction of the wave propagation error of a sigma grid based numerical tank using a vertical spacing based on the constant truncation error

Numerical wave models that solve the Laplace equation for the velocity potential with a σ-grid in the vertical direction have been proven to be flexible and efficient. However, it is observed that the dispersion relation is sensitive to the variation of the vertical σ-grid arrangements. So far, there is no recommended practice to optimise the vertical σ-grid for a correct representation of the dispersion relation. In order to reduce the uncertainty of the dispersion relation and optimise the choice of the vertical grid distribution, a new method based on the constant truncation error of the finite difference scheme is presented in this article. The optimised vertical grids estimated by the novel method are applied in the fully nonlinear potential flow model REEF3D::FNPF to provide simulated wave surface elevations, which are compared with the measured time series in the long wave flume at SINTEF Ocean, Trondheim. Several different wave scenarios are considered, including regular waves, bichromatic waves, trichromatic waves as well as irregular waves. The wavemaker motions in the physical flume are used to generate waves in the numerical wave tank of REEF3D::FNPF. The proposed truncation error method is seen to effectively mitigate the dispersion (phase) error in all scenarios and the simulations show high accuracy in comparison with the experimental data.

Nonlinear wave surface elevation around a multi-column offshore structure

Surface elevation around multiple column offshore structure is an important phenomenon crucial to air gap design of offshore platforms. This paper investigates the competing hydrodynamic phenomena, i.e., wave run-up of surface elevation rising along the column and near-trapping – the increase of surface elevation due to near-resonance among the columns. Both wave run-up and near-trapping have the characteristics of generating surface elevation peak and often impact the offshore structures with nonlinear wave loads and potentially cause slamming to platforms. With the free-surface Keulegan-Carpenter number Kc<O(1) and wave steepness H/L < 0.14 considered, the free surface amplitude primarily depends on the diffraction pattern caused by the multiple columns and potential theory is applicable. The wave run-up and near-trapping due to wave interaction with a platform consisting of four-square columns with different corner ratios are obtained by numerical simulations. It is found that the increasing corner ratio results in a lower wave run-up under 0° incident wave, but a higher wave run-up under 45° incident wave. For near-trapping among four columns, the peak surface elevation decreases with increasing corner ratio. Two mechanisms namely superposition and near-resonance resulting the peak surface elevation are examined in detail for wave interaction with multiple columns.

Density evolution after shock release from laser-driven polystyrene (CH) targets in inertial confinement fusion

The evolution of the plasma density in the rarefaction wave formed after a laser-driven shock is released from a CH foil was measured using optical interferometry. It was found that the plasma density profile is very sensitive to the conditions at the back surface of the foil before the shock release. Dedicated experiments demonstrated that radiation preheat by coronal x rays caused early expansion of the back surface and faster expansion of the rarefaction wave. Radiation-hydrodynamics simulations with accurate modeling of radiation preheat from the plasma corona are in good agreement with the experimental results. The early expansion of material interfaces due to coronal x-ray preheat must be evaluated in designing and interpreting laser-driven inertial confinement fusion experiments.