Wave Surface Research Articles

Manipulation of sub-terahertz waves using digital coding metasurfaces based on liquid crystals.

This paper presents a novel sub-terahertz liquid crystal (LC) phase shifter based on digital coding metasurfaces. The proposed structure consists of metal gratings and resonant structures. They are both immersed in LC. The metal gratings function as reflective surfaces for electromagnetic waves and electrodes for controlling the LC layer. The proposed structure changes the state of the phase shifter by switching the voltage on every grating. It allows the deflection of LC molecules within a subregion of the metasurface structure. Four switchable coding states of the phase shifter are obtained experimentally. The phase of the reflected wave varies by 0°, 102°, 166°, and 233° at 120 GHz. Due to the presence of the transverse control electric field, modulation speed is approximately doubled compared to the free relaxation state. This work provides a novel idea for wavefront modulation of phase.

Seakeeping criteria of a moored and articulated multibody floating platform in head seas

Within the Space@Sea project, a multi-use floating concept was proposed by following the analogy of standardization to enlarge a floating platform using a multitude of smaller structures. An operability analysis must be performed to identify its seakeeping criteria and how the criteria limit its functionality. A two-step strategy is proposed to calculate the responses of the target floating concept in head waves, where nonlinearities of the mooring lines and mechanical joints are taken into account. The weakly nonlinear time-domain procedure relies on a diffraction-radiation model in the frequency domain. The motions of floating bodies are solved in the time domain, in which nonlinear Froude-Krylov and hydrostatic forces are estimated under instantaneous incident wave surface. Multibody interactions are resolved under consideration of catenary mooring lines and hinged joints. Wave-induced motions and loads are expressed in terms of transfer function for determining the limiting criteria, including the translational and rotational motions velocities and accelerations; relative motions, velocities and accelerations between module floaters; mooring tensions; as well as hinge forces. Assessments are carried out for various sea conditions against the prescribed values, for the chosen criteria, to address safety issues due to sever platform responses. Different operational requirements for trained workers and untrained passengers of motions and accelerations are taken into consideration. Results show that rotational motions have a significant influence on the platform’s seakeeping performance. Nevertheless, the root mean square (RMS) value of vertical acceleration dominates the comfort of persons onboard, defining the final limiting criterion of the entire platform.

Wave propagation in a circular channel: sloshing and resonance

Surface wave resonance of a liquid (water) layer confined in a circular channel is studied both experimentally and numerically. For the experiment, eight unevenly distributed ultrasonic distance sensors measure the local height of the wave surface. The resonance curves show maxima only for odd multiples of the fundamental resonance frequency f_0. We explained this behavior using a simple intuitive “ping-pong” like model. Collision of wave fronts can be observed for higher frequencies. Also, the wave reflection on the walls can be treated as wave collision with itself. The non-linearity seems to be weak in our study so the delay in the wave propagation before and after the collision is small. Time-space plots show localized propagating waves with high amplitudes for frequencies near resonance. Between the peaks low amplitude and harmonic patterns are observed. However, for higher frequencies, the frequency band for localized waves becomes wider. In the Fourier space-time plane, this can be observed as a point for the harmonic patterns or a superposition of two lines: one line parallel to wave-vector k axis corresponding to the excitation frequency f_0 and a second line with inclination given by wave propagation velocity sqrt{gh}. For planned future work, this result will help us to reconstruct the whole water surface elevation using time-series from only a few measurement points

Development of an overtopping wave absorption device for a wave-making circulating water channel

With the improvement of the stable flow generation function of circulating water channels (CWCs), CWCs are gradually developing toward multi-functionality. The wave test capability of a CWC is limited by its short test section; so, an efficient wave absorption device is required. This paper proposes the design concept of an overtopping wave absorption device (OWAD). An optimization design of the device by combining CFD and simulation-based design (SBD) is presented. The SBD system included optimal Latin hypercube design (Opt LHD), SVR, and particle swarm optimization (PSO). The reflection coefficient was used to estimate the wave absorption device performance. Numerical analysis was performed using the Unsteady Reynolds Averaged Navier–Stokes (URANS) solver and the boundary wave generation method, and was verified and validated via a procedure recommended by International Towing Tank Conference (ITTC). Under two wave conditions, four different OWAD design variables (slope angle, porosity, extension depth, and extension height) were optimized using the SBD system to minimize the reflection coefficient. Based on optimization results, the OWAD was assembled and its wave dissipation performance was confirmed experimentally. Installing an OWAD caused the reflection coefficient to be reduced from 0.161 to 0.089 for short-wave design conditions and from 0.528 to 0.128 for long-wave design conditions. A wave absorption test also showed that the wave absorption performance of the OWAD is significantly better than that of honeycomb and multiple porous plate wave absorption devices. The OWAD produces not only lower reflection coefficients but also smoother wave surfaces due to its two-dimensional structures.

Ocean shallow-water studies on a generalized Boussinesq-Broer-Kaup-Whitham system: Painlevé analysis and similarity reductions

Today, shallow seas are the focus of scientists and explorers. We look into a generalized Boussinesq-Broer-Kaup-Whitham system, which describes certain gravity water waves in a shallow ocean scenario in terms of the wave height and surface velocity of the water wave, by proving that the system is Painlevé integrable and constructing two families of similarity reductions. Each family results in a well-studied ordinary differential equation that depends on the coefficients in the system.

Vehicle stochastic response prediction of sea-crossing railway bridges under correlated wind and wave via machine learning

Offshore railway bridges are exposed to the complex coastal environment, and vehicles could experience significant responses when vehicles cross the sea-crossing bridge. To predict the vehicle response subjected to the stochastic excitations (wind and wave actions), Archimedes copulas are applied to identify the correlation between wind speed and wave height. Based on the Gibbs sampling, an algorithm of Markov Chain Monte Carlo (MCMC), training samples are sampled from the optimal copula model (e.g., Clayton copula) as the input parameters. Taking a representative vehicle-bridge model as the study object, wind tunnel and wave flume scaling tests are performed to obtain the aerodynamic characteristics with the influence of wave surface. The external loads are calculated using the Morison equation and the MacCamy-Fuchs equation. Vehicle indexes as the output parameters are quantified using Support Vector Machine (SVM), Gaussian Process (GP), and Neural Network (NN) Machine Learning (ML) method. Training and testing results indicate that the NN algorithm outperforms other training strategies even though SVM and GP reasonably predict the vehicle dynamic response. Since the wind and wave action is lateral, vertical acceleration performs insensitivity to input parameters and is mainly influenced by track irregularity.

Comparative studies of terahertz-based optical properties in transmission/reflection modes of GaSe: S and measurement of its scattering losses due to surface roughness for efficient terahertz generation

The paper reports the efficient terahertz generation using nonlinear optical rectification process and comparative studies of optical, dielectric, and surface scattering properties of 1.0 mm thin GaSe: S crystal in transmission/reflection modes. We employed 800 nm wavelength of 140 fs and 50 fs pulse durations obtained from Ti: Sapphire oscillator and amplifier systems at 80 MHz and 1 kHz repetition rates, respectively, and evaluated the THz generation potential and optical properties such as refractive index, absorption coefficients, relative permittivity in transmission mode. In the next step, the crystal was subjected to a commercial THz system of 0.3 picosecond pulses at 100 MHz repetition rate in reflection geometry and once again recorded the similar parameters and compared with the transmission mode data. In addition, these data were used to measure the scattering losses from the surface of the crystal in terms of surface roughness, optical impedance, and Fresnel’s reflection coefficients. The maximum generated THz power was of the order of 4.5 μW with conversion efficiency (η) ∼ 2.2 × 10−3%. The measured THz efficiency was found equivalent to 20 mm thick GaSe crystal used in the difference frequency mixing technique. Finally, we have measured the Rayleigh roughness factor (g) of the crystal surface at different THz frequencies. When g < 1 (for small roughness) the Fresnel reflection loss factor becomes 0.75 at 1.4 THz frequency range which helps to enhance the generated signal.

Experimental and Numerical Investigation on the Interactions between the Weakly Three-Dimensional Waves

The results of laboratory experiments and numerical simulations were performed to investigate the interactions between the weakly three-dimensional waves in an ‘X’ configuration, which has a 16-degree approaching angle. In addition, another oblique two-dimensional experiment was also conducted for comparison with the ‘X’ configuration but in one single channel by removing a dummy wall in the interaction region. Our experimental results show that as the wave trains propagate into the interaction region, it is obvious that there is an increase in the wave height which reaches a maximum height of about 1.37H0 for different initial wave steepness at the center of the interaction region, and then decreases thereafter, where H0 is the input wave height. Then wave elevations at different positions downstream of the interaction region were also studied, indicating that the frequency and initial wave steepness were highly correlated with the wave-wave interaction between the weakly three-dimensional waves. For the wave with low frequency (f = 0.8 Hz), a crescent wave surface formed at the beginning of the interaction and then separated into two two-dimensional waves after the interaction, which illustrates that the waves can still keep their initial characteristic and propagate as their initial directions downstream of the interaction region. While the frequency increased (f = 1.2 Hz), three-dimensional effects appeared to dominate the interaction of weakly three-dimensional waves, especially for the large initial steepness, and the wave surfaces were also three-dimensional after interactions. Finally, numerical simulations with larger approaching angles were conducted to further understand the influence of propagation direction on the interactions between the weakly three-dimensional waves. The results suggest that intense interactions and strong three-dimensional characteristics of the wave trains downstream interactions can result from larger approaching angles.

Способ проактивной компенсации мешающих отражений при радиолокационном наблюдении малоразмерных объектов на фоне подстилающей поверхности

The low probability of radar detection of a small-sized aircraft (MLA) is determined both by its small effective scattering surface of radio waves and by the presence of interfering (re-reflected) radio signals generated by the reflection of the radar sounding signal from the un-derlying earth's surface, against which the search and observation are carried out MLA. The power of such reflected signals can significantly exceed the power of the signals reflected from the MLA itself, and the multi-lobe nature of the radar antenna pattern further contrib-utes to masking the useful signal (reflections from the MLA) during its search. To increase the probability of radar detection of MLA, many different methods are used, associated with the formation of special probing signals, with various types of spatial and frequency filtering, with the development of special designs of antennas and antenna arrays. However, most of the methods used require rather complex technical solutions, the practical implementation of which is often impossible or excessively difficult. A method is proposed for increasing the probability of detecting MLA against the background of signals reflected from the relief by using a proactive method for calculating the interference field in the algorithms for second-ary processing of radar signals, based on the synthesis of a signal generated using a relief model of the underlying surface and compensation in the output signal. radar receiver com-ponent due to reflection from the terrain. The implementation of the proposed method is based on the use of the mathematical apparatus of the theory of radio wave propagation in the approximation of geometric optics. The requirements for the main characteristics of the digital model of the underlying surface and the resources of the computing device for the im-plementation of this method are given. An expert assessment of increasing the probability of detecting MLA is given. The results of the work can be used in on-board computing devices and radar systems for the tasks of detecting small aircraft, determining coordinates and pre-dicting their trajectory.

Retrieval of Sea Surface Currents and Directional Wave Spectra by 24 GHz FMCW MIMO Radar

This article investigates the capabilities of 24 GHz frequency-modulated continuous-wave (FMCW) multiple-input–multiple-output (MIMO) radar technology to retrieve sea surface currents and directional wave spectra. A procedure based on the dispersion relation, which was previously applied to process X-band marine radar data, is here exploited. The estimation performance of the radar sensor is first assessed by numerical tests in the case of synthetic sea wave fields with known characteristics in terms of wave direction and surface currents. Finally, the estimation procedure is assessed on real data collected at San Vincenzo quay in the port area of Naples, Italy. The achieved results are encouraging and highlight that 24 GHz FMCW MIMO radar is a viable technology for sea wave monitoring.

Control-oriented wave surface elevation forecasting strategies: Experimental validation and comparison*

Optimal control strategies are a key development step towards commercialization of wave energy converters (WECs). Most of these rely on optimization routines to find a suitable control action to maximize WEC power production. Nevertheless, most of these solutions make use of device dynamical models, with the free-surface elevation as the external (uncontrollable) input, effectively representing the incoming wave field. Consequently, predictive strategies, such as model predictive control, strongly depend on the availability of future wave information, and hence suitable forecasters are commonly used to ‘restore’ the causality of the optimization problem. Motivated by the intrinsic requirement of suitable forecasting strategies within optimal WEC control, this study provides a validation and comparison of different algorithms, including adaptive and non-adaptive techniques, based on experimental data. The paper focuses on the adaptability of each algorithm, which must be capable to fit properly each wave surface elevation signal, thus not affecting the optimality condition by providing poor prediction results.

A Sea Surface–Based Drag Model for Large-Eddy Simulation of Wind–Wave Interaction

Abstract Monin–Obukhov similarity theory (MOST) is a well-tested approach for specifying the fluxes when the roughness surfaces are homogeneous. For flow over waves (inhomogeneous surfaces), phase-averaged roughness length scales are often prescribed through models based on the wave characteristics and the wind speed. However, such approaches lack generalizability over different wave ages and steepnesses due to the reliance on model coefficients tuned to specific datasets. In this paper, a sea surface–based hydrodynamic drag model applicable to moving surfaces is developed to model the pressure-based surface drag felt by the wind due to the waves. The model is based on the surface gradient approach of Anderson and Meneveau applicable to stationary obstacles and extended here to the wind–wave problem. The wave drag model proposed specifies the hydrodynamic force based on the incoming momentum flux, wave phase speed, and the surface frontal area. The drag coefficient associated with the wind–wave momentum exchange is determined based on the wave steepness. The wave drag model is used to simulate turbulent airflow above a monochromatic wave train with different wave ages and wave steepnesses. The mean velocity profiles and model form stresses are validated with available laboratory-scale experimental data and show good agreement across a wide range of wave steepnesses and wave ages. The drag force is correlated with the wave surface gradient and out-of-phase with the wave height distribution by a factor of π/2 for the sinusoidal wave train considered. These results demonstrate that the current approach is sufficiently general over a wide parameter space compared to wave phase-averaged models with a minimal increase in computational cost. Significance Statement Understanding the physics of wind waves plays an important role in the context of numerous geophysical and engineering applications. A drag-based model is developed that characterizes the effect of the sea surface waves on the wind above. The model is validated with existing experimental datasets and is shown to be effective in predicting the average wind velocity and stress over waves with varied steepnesses and phase speeds. The ease of implementation and low computational cost of the model make it useful for studying turbulent atmospheric-scale flows over the sea surface important in offshore wind energy research as well as for modeling air–sea fluxes of momentum, heat, and mass.

Investigation on mechanisms of fast opposing water waves influencing overlying wind using simulation and theoretical models

We use large-eddy simulation (LES) and theoretical analyses to study the turbulent flow over fast opposing water waves. A novel nonlinear viscous model for airflow perturbations induced by Stokes waves is developed, which can account for wave-perturbation viscous stress and the nonlinear forcing by multimode interactions of wave-correlated quantities in the wind field. Overall, the nonlinear viscous model can describe the wave-coherent airflow in the LES results for high-order Stokes waves, which demonstrates the negligible effects of wave-perturbation turbulent stress in the wind. According to the model, the dominant components of the fundamental mode of the airflow perturbation primarily result from the linear response of the wind to the wave and, thus, are not substantially affected by the nonlinear forcing. However, the weak components of the fundamental mode, which produce the form drag on the wave, are created by the combined effects of the nonlinear forcing and the wave-perturbation viscous stress. We found that the main mechanism for generating the nonlinear forcing is the interaction between the second harmonic and the fundamental mode of the wave-correlated quantities in the air. In this mechanism, wave nonlinearity exerts its effects mainly through the second harmonic of the wave surface, instead of the second harmonic of the wave kinematics. Therefore, it is further demonstrated that a second-order Stokes wave is sufficient to capture the wave nonlinearity effects on the form drag.

HIBISCUS: Simulator for the development of control strategies for coherent beam combining lasers with PISTIL technique

CBC (Coherent Beam Combining) is a key technology for the realisation of intense lasers. In this context, PISTIL (PISton and TILt interferometry), a precise metrology tool for measuring segmented wave surfaces, has been developed and used in particular to characterise and diagnose CBC ultrafast and digital laser in the framework of the XCAN (X Coherent Amplification Network) project at the École Polytechnique. We propose here to use it in a way to help the optimisation of control techniques by including PISTIL in an XCAN type CBC laser simulator. This will allow an easy tuning of the control laws, outside the clean rooms in which these large lasers are deployed and without the need to start them up.

Effects of heave motion of an elastically supported floating oscillating water column device on wave energy harvesting efficiency

The main aim of this paper is to analyze the effects of the heave motion of an elastically supported floating oscillating water column device (OWC) on wave energy harvesting efficiency through two-dimensional computational fluid dynamics simulations. After the numerical model is validated using experimental data, it is used to analyze the effect of the natural frequency ratio on the hydrodynamic efficiency of the OWC. The natural frequency ratio is the ratio of the natural frequency to the wave frequency. The numerical results prove that the natural frequency ratio must be greater than 3 for achieving the best hydrodynamic efficiency. The best hydrodynamic efficiency decreases with the decrease in the natural frequency ratio when the latter is 3. If the natural frequency ratio is smaller than 3, both the amplitudes of wave surface elevation and the displacement of the OWC chamber increase but the harvested energy reduces because the relative motion of the wave surface and the OWC device is small. The energy dissipation due to vortices does not have a strong contribution to the reduction of energy at small natural frequencies.

IRREGULAR WAVEMAKER (PISTON TYPE) IN A NUMERICAL AND PHYSICAL WAVE TANK

This paper describes the design and execution of a wavemaker system (piston type) in the Dokuz Eylül University Hydraulic Laboratory flume to perform hydraulic model tests using regular and irregular waves. In this study, we will focus on generating the irregular waves from the target JONSWAP spectrum using an adjusted random phase method; also, the control software of the wavemaker is described. An identical numerical channel wave to the physical flume was modeled to compare and observe the irregular wave profile using Flow 3D, one of the advanced computational fluid dynamics (CFD) software. The paddle movement was validated by comparing the experimental converted irregular wave surface elevations to the wave spectrum to the numerical models of several waves.

Artificial Intelligence (AI)-Based Evaluation of Bolt Loosening Using Vibro-Acoustic Modulation (VAM) Features from a Combination of Simulation and Experiments

The detection of bolt loosening using vibro-acoustic modulation (VAM) has been increasingly investigated in the past decade. However, conventional nonlinear coefficients, derived from theoretical analysis, are usually based on the assumption of ideal wave–surface interactions at the joint interfaces. Such coefficients show a poor correlation with the tightening torque when the joint is under the combined influences of structural and material nonlinearities. A reliable inspection method of residual bolt torque is proposed in this study using support vector regression (SVR) with acoustic features from VAM. By considering the material intrinsic nonlinearity (MIN) and dissipative nonlinearity (DN), the responses of aluminum–aluminum and composite–composite bolted joints during the VAM test were accurately simulated. The SVRs were subsequently established based on the database built by combining simulated and experimental nonlinear spectral features when the joints were inspected at different scenarios. The results show that the evaluation of residual torque using the SVR models driven by the acoustic nonlinear responses had higher accuracy compared to the conventional nonlinear coefficients. Requiring limited experimental data, the proposed method can achieve a reliable inspection of bolt torque by including the simulated data in the machine training.

Artificially engineered metaconcrete with wide bandgap for seismic surface wave manipulation

The energy of ground-born vibration is transported mostly by surface waves. Therefore, engineered structures which absorb externally applied energy and improve structural performance against explosive dynamic loading are highly desired. In this paper, we demonstrate the application of designed metamaterial barriers with engineered metaconcrete for reflecting and redirecting surface Rayleigh waves to avoid disastrous consequences. The metabarrier is constructed by embedding metaconcrete cubes in a semi-infinite space. The local resonance type metaconcrete cell is composed of a heavy lead ball coated by a soft rubber layer and mortar. One significant limitation of metaconcrete is the narrow bandwidth. Here, we apply an array of metaconcretes where the radii of the lead ball and soft coating increase or decrease simultaneously. Via transmission ratio analysis, we show that the metaconcrete-based metabarrier is able to achieve broadband wave attenuation. More interestingly, the elastic wave trapping and surface wave mode conversion can be realized and controlled by the metabarrier. The mechanism of trapping and conversion is fully explained with an unconventional dispersion relation. A time history analysis based on several actual earthquake records obtained from public database is also presented. The result in this paper highlights new applications of metaconcretes and it can be applied to novel seismic shields for protecting civil infrastructures against seismic destruction.

Hydro-optic interaction study on submerged unmanned underwater vehicles regarding to Snell's window

The underwater creatures have always desired to collect information about what is in beyond, for instance, the fishes that hunt birds with precise calculations. Possessing a subsurface device capable of imaging the outside area of water without any distortion can be notified of modern demand. To be acquainted with this phenomenon, a series of submerged hydrofoils with various geometrical properties are mentioned in different submerged distance and incidence angles to recognize the recondite limits upon its aberrated eyesight. The behavioral changes of the wave's surface have been numerically discussed in detail by ray length. The primary outcomes reveal that Snell's window varies under different conditions. In brief, the emitted ray undergoes more severe distortions when the hydrofoil is positioned near the water's surface. However, interestingly enough, other parameters such as hydrofoil's geometry, the angle of attack, and the flow's velocity affect the wave's shape, and that wave's geometry sets different distortions patterns in different areas that the camera spots.

Integrated design and analysis of inlet and missile with two side layout

Abstract Based on the flow field characteristics of the missile body, an integrated design method for missile and inlet with two side layout is proposed under asymmetric inflow. The result of numerical simulation shows that shock waves hit on cowl lip on the symmetry plane at the design condition, which verifies the method of integrated design. And the flow characteristics of inlet under the influence of the missile body are analyzed. Under the influence of asymmetric incoming flow and missile body, the first shock wave surface of the two-dimensional inlet presents the characteristics of a three-dimensional concave surface, and there is a pair of asymmetric vortex structures in the inner flow path. Finally, a bleed cavity of self-adaptively adjusting bleeding by vortex is proposed to improve the performance of inlet and broaden the inlet start and attack angle boundary. The minimum Mach number of the inlet start is reduced from 2.8 to 2.3 at ɑ = 6° and the maximum attack angle of inlet start is widened from ɑ = −1° to ɑ = 3° at Ma = 2.1.