Large Steepness Research Articles

Prediction of near-field uni-directional and multi-directional random waves from far-field measurements with artificial neural networks

Predictions of several minutes of phase-resolved waves ahead of time from the measurements of wave-time-series at upstream locations are important for many marine operations. Making use of the knowledge of wave propagation, we design the input-output data and fine-tune the architecture of an artificial neural network (ANN) to push its capacity for the wave predictions. The study cases include uni-directional and multi-directional waves with mild to large wave steepness. The ANN models are trained on simulated random waves of JONSWAP spectrum with significant wave height and peak spectral wave period taken from a North Sea wave statistical data. Results demonstrate that the ANN models with single or multi-inputs from a 1 km upstream far-field locations can predict near-field waves with errors ranging from 2.6% to 12% for both uni- and multi-directional waves. The model accuracy is dependent on the wave steepness. Sensitivity studies show that the network model's hyper-parameters might need to be changed for different wave conditions. There are also possible optimal locations for far-field wave probes that will give the optimal prediction at the near-field.

Investigation of higher-harmonic wave loads and low-frequency resonance response of floating offshore wind turbine under extreme wave groups

When floating offshore wind turbines (FOWT) encounter extreme waves in operation, it is a highly nonlinear system involving floater motion and extreme wave–structure interactions. The behavior is crucial for floating offshore wind turbine safety. With the aid of a high-fidelity CFD solver, a harmonic separation method through a ‘Stokes-like’ formulation was adopted to obtain the parameters for each harmonic response. The present study focuses on analyzing higher harmonic load, dynamic motion response, and tension load of the mooring line the DeepCwind semi-submersible FOWT. The results show that the higher-harmonic wave load cannot be ignored in the extreme marine environment, the second harmonic can be over 16% of the linear wave load, and the third harmonic can be over 10% of the linear wave load with large wave steepness. The duration of a focused wave crest interaction with the platform is a short process of only 1.4 s in model scale, corresponding to about 10 s for the prototype. The wave runup at the upstream column is larger than the other columns, and the flow velocity is double that of the wave without the floater in presence. The dynamic response of the floater contains two phases, ’forced oscillations’ is identified to be drag-driven from odd harmonics, and ’free decay’ mainly comes from second-order difference-frequency influence from even harmonics. The properly normalized wave forces of surge and heave motion are not influenced by steepness kpAlinear. For surge motion, the total motion mainly comes from the nonlinear low frequency component. For pitch motion, the linear response is dominant in small kpAlinear. When the steepness kpAlinear increases gradually, the second-order low-frequency response of pitch motion will increase rapidly and dominate the total response.

Research on planing motion and stability of amphibious aircraft in waves based on cartesian grid finite difference method

The wave resistance of amphibious aircraft planing in waves is an important consideration in the design of seaplanes. Based on the Cartesian Grid Finite Difference Method (CGFDM), this paper investigates the effect of different wave elements and planing speeds on the motion response and vertical overload of amphibious aircraft, and analyzes the conditions for the occurrence of jump motion and stability of the aircraft. The finite difference method (FDM) based on a fixed regular cartesian grid system is used to solve the flow field, and the Tangent of Hyperbola for Interface Capturing with Slope Weighting (THINC/SW) method is used to capture the violent free surface flow, while the immersed boundary method (IBM) is used to effectively solve the problem of large motion of the object. The numerical simulation results are compared with the experimental values and show good agreement. The calculation results demonstrate that the heave and pitch motion responses of amphibious aircraft show a certain regularity with changing wave conditions. As the speed increases, the vertical overload rises sharply as the aircraft enters the complete planing state, and the slamming effect on the bottom of the aircraft's forward fuselage is most significant. When the wavelength reaches 1.38 to 2.76 times the length of the fuselage, high speeds and large wave steepness are more likely to induce jumping motions and reduce the stability of the aircraft's movement. This study can provide a reference basis for amphibious aircraft to effectively avoid unsafe sea state areas.

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.

Numerical and experimental studies on a self-protected energy concentrator for water waves

Wave concentrators have significant value for ocean energy absorbers to reduce power generation costs. Besides, the protection problem of the concentrator itself should also be carefully considered. In this study, a self-protected energy concentrator (SPEC) for water waves consisting of several truncated cylinders arranged in a concentric circle is proposed by adopting the cloaking technology based on the idea of manipulating scattered waves. To design the SPEC, an optimization program built by a combination of the higher-order boundary element method, the wave interaction theory, and the real-coded genetic algorithm is utilized. To address the manipulation mechanism of the scattered waves as well as to investigate the performance of the SPEC against various wave steepness and a spectrum of waves, the scattered waves and the total waves both inside and outside the concentrator are numerical and experimentally analyzed. High energy density inside the concentrator and the self-protection of the concentrator are validated. It is found that the large wave steepness can reduce the efficiency of concentration and self-protection due to the wave breaking. The SPEC has a high performance at a moderate bandwidth. Besides, the wave frequencies perform a more significant effect on self-protection than the concentration characteristic.

Sigmoid Function Model and Dynamic Characteristics Analysis for Inductive Wireless Power Transmission Control System

In this article, an inductive wireless power transmission (IWPT) system is a high-order nonlinear switching system with multiple operating modes. It is challenging to analyze the dynamic characteristics of such a system using the extended describing function, generalized state-space averaging, or other existing modeling methods. To address this challenge, this study proposes a modeling and dynamic characteristics analysis method for a closed-loop IWPT control system based on a sigmoid function model. A sigmoid function with a large steepness factor is adopted to approximate the switching process of the switch and a unified smooth continuous dynamic model is established for a phase-shift controlled IWPT system. Because this model is infinitely differentiable, a stability theory of continuous systems, such as the Floquet theory, can be directly applied to analyze the bifurcation type of the system and stability of periodic solutions. Analysis results reveal the transition of the phase-shift-controlled IWPT system from periodic behavior to chaotic behavior through saddle-node bifurcation and Neimark–Sacker bifurcation. A stable domain in the two-parameter space of the controller proportional coefficient and load is also obtained. Both simulation and experimental results validate the accuracy of the proposed model and theoretical analysis method, which can significantly reduce the difficulty of analyzing IWPT system dynamics.

Study on the characteristics of Trimaran's surf-riding in stern waves by a hybrid method

In this paper, a hybrid method coupling potential flow method and viscous flow method is applied to simulate the autopilot trimaran in stern waves. The aim is to study the process of being captured by waves and the surf-riding characteristics of the trimaran. The FNPT-based QALE-FEM is used to simulate the wave tank of the external field, and the viscous flow method is used to simulate the autopilot of the trimaran in the wave tank. The sailing and 6DOF motion of the trimaran in waves of different parameters is simulated. The autopilot of a trimaran in stern waves of different lengths and steepness is simulated to study the characteristics of the trimaran's surf-riding in waves. When studying the effect of wave steepness, the wavelength is constant, and various forward speeds of trimaran could change the encounter frequency. However, the trimaran's wave steepness and forward speed are kept constant when the wavelength changes. By the computed results, it is found that the high speed and large wave steepness could significantly increase the vulnerability of trimaran's surf-riding in stern waves, and the wave passing the transom stern and side hulls could accelerate the trimaran to reach the wave celerity.

Three-dimensional stitching of stereo-deflectometry based on marker points.

As a non-contact and three-dimensional (3D) mirror surface measurement method, stereo-deflectometry has been developing rapidly in recent years. For solving the problem of the measurement of large surface inclination with traditional stereo-deflectometry, 3D stitching of stereo-deflectometry based on marker points is proposed. In this method, the 3D points in the subregions under different perspectives were measured. Meanwhile, the 3D coordinates of the marker points on the sample table, which were calculated by binocular stereo vision, were used for coarse stitching, and the ICP algorithm was used for fine stitching. In order to verify the 3D stitching algorithm, we built a measurement system for the freeform surface of an ultra-short lens with a diameter of about 100 mm and a steepness of 52.6°. The spherical fitting error of the reflective bowl after stitching is within 60 mm. The experimental results verify the feasibility of the method, leading to potential mass application of stereo-deflectometry in 3D measurement of complex optical surfaces with a large aperture and high steepness.

Parametric study of wave impact pressure impulse and characteristic pressure on a square column with overhanging deck

Semi-submersible platforms may suffer safety accidents due to severe wave impacts on the decks and columns in extreme ocean environment. The pressure impulse considering the cumulative effect of time is significant for the dynamic response analysis of offshore and coastal structures, and has attracted attentions in recent years. In this study, wave impact tests on a square column with overhanging deck were carried out under a series of fifth-order Stokes waves with large steepness to investigate the influence parameters on the impact pressure impulse. A novel definition of characteristic pressure is proposed from the perspective of pressure impulse, meanwhile, its dimensionless relation with the peak impact pressure is presented. The spatial distributions of the pressure impulse and characteristic pressure on the square column with overhanging deck, as well as the influences of several environmental and model parameters are discussed in detail. The results show that the spatial distributions and vertical variations of wave pressure impulse are affected significantly by the wave parameters, initial air gap, column incline angle and wave incidence angle. In general, the wave impact on the overhanging deck and the upper part of column is severer, and the inclination of column can reduce the pressure impulse to some extent. The research findings may provide references for further research and the predesign of the column–deck structure of semi-submersibles.

Термодинамічні властивості і фазові рівноваги в сплавах системи Bi—Tm

The thermochemical properties of the melts of the Bi—Tm system at a temperature of 1100 K in the range of compositions 0 ≤ xTm ≤ 0,2 were determined for the first time by the calorimetry method. It is established that the minimum value of the enthalpy of mixing of these liquid alloys is equal to –75,7 ± 0,5 kJ / mol at xTm = 0,65. = = –150,7 ± 16,7 kJ / mol, = –230,9 ± 21,8 kJ / mol. The activities of the components and molar particles of associates were calculated according to the model of an ideal associated solution (IAR), using data on the thermochemical properties of melts of the Bi—Tm system. It was found that the activities of the components in these metallic solutions show very large negative deviations from ideal solutions with a high content of TmBi and Tm2Bi associates. The obtained dependences of the first i i melts of the Bi—Tm system on temperature showed a large steepness of the Bi Bi curve in contrast to the gradual decrease of exothermic values Tm of Tm. This indicates large changes in the structure of the Bi atom with increasing temperature. Excess integral and partial Gibbs energies of Bi-Tm system melt mixing calculated from component activities The absolute values of G in the whole concentration range are smaller than H (G min = –41,8 kJ / mol at xTm = 0,58), and the function G of is more asymmetric, which is caused by the entropy contribution (entropy of mixing of the studied melts is negative, and Smin min = −30,5 J / mol ∙ K at xTm = 0,65). Keywords: thermochemical properties, compounds, melts, Bi, Tm.

LITHOLOGY AND HYDRODYNAMICS OF THE BLACK SEA ODESSA COAST UNDERWATER PART

Problem Statement and Purpose. The results of lithological studies, including granulometric analysis, are needed to identify the conditions of sedimentation and distribution of rock fragments by size. They are an indicator of the sedimentation processes dynamic structure peculiarities. The study of the sediment regime is important for the development of measures related to the protection of shores from erosion. The aim of the work is to identify the lithological features of the bottom sediment types distribution on the protected section of the Odessa coast near Cape Maly Fontan. Data & Methods. The materials for this work were obtained in the field research in the summer 2019 in the coastal zone between the hydrobiological station of Odessa I. I. Mechnikov National University, and Cape Malyj Fountain. Three measuring profiles were made perpendicular to the shoreline at 1200 m towards the sea to a depth of 18 m. 35 samples were taken from the upper layer of bottom sediments. Field samples processing was performed in the Problem Research Laboratory of Engineering Geology of the Sea Coast, Reservoirs and Mountain Slopes of Odessa I. I. Mechnikov National University. Granulometric classification was performed. Excel, Statistica, Golden Software Surfer programs were used to process the results. Results. Underwater relief analysis showed that the coastal part is an abrasive terrace, on which ancient landslides relics are widespread. There are 7 main lithological types of bottom sediments detected. The largest areas covered by fine-grained loamy sands and shell detritus with gravel, pebbles and sand. Analysis of distribution curves and cumulative curves of granulometric distribution showed that coarse sediments are characterized by multi-vertex distribution curves and stepwise stretched cumulative curves, which indicates the heterogeneity of sediments in granulometric distribution. For fine-grained material, which is well sorted, the distribution curves are almost single-vertex, and the cumulative curves have a large steepness and a simple, non-stepped shape. There are three hydrodynamic detected in bottom sediments lithology according on statistical parameters of grain size. Outer marine part with depth more than 15 meters characterized by low hydrodynamic processes and presented by good-sorted sands and silt. The central part of polygon in 4–15 meters depth covered by shell detritus with gravel and pebble of carbonate rocks and connected with rocky extrusions after coastal landslides. The feature of this part is active hydrodynamic processes. The rocky extrusions presented like natural breakwater and due to waves braking and collection of different types of sediments. The coastal part in 2–4 m depth in front of constructed breakwater leads to decreasing of wave energy and sedimentation of good-sorted sands.

Experimental study on the evolution of mechanically generated waves mixed with space non-uniform wind-driven waves

This study experimentally investigates the evolution of mechanically generated waves mixed with wind-driven waves. The pre-existing wind-driven waves are time stable and space non-uniform during the tests. Empirical formulations for fetch laws of dimensionless wave energy and peak frequency of wind-driven waves at the laboratory scale are given. Experimental results show that the characteristics of mixed waves are affected by the variations in wind speed, fetch, wave period, and wave height. At small fetches, both waves are generally inhabited after mixing. With increasing fetch, the energy of the generated waves is enhanced at a large wave steepness level. When the frequency gap between the two wave types is small, mixed waves tend to have a chaotic wave evolution. However, the changes in the two types of waves are small when the frequency gap between the two waves is large. A frequency lock-in phenomenon of the wind-driven waves at high-order waves of the generated waves is found in the regular wave tests at large wave steepness levels. Such a phenomenon does not exist in present irregular wave tests. With increasing fetch, the spectrum of mixed waves downshifts at the beginning and upshifts at a large fetch.

On Extreme Waves in Directional Seas with Presence of Oblique Current

This paper will address two questions: i) How the fully nonlinear wave-current interactions modify the extreme wave statistics, spectrum characteristics and average shape of extreme waves in directional seas subject to current with different incident angles; ii) Whether the NewWave model is adequate to describe the average shape of nonlinear extreme waves in directional seas with presence of opposing and oblique current. This study employs fully nonlinear numerical simulations, and the results demonstrate that current can enhance the wave crest exceedance probability at distribution tail and kurtosis, broaden the spectra, and cause severe vertical and horizontal asymmetry of extreme wave profiles depending on the incident angle and initial wave steepness. The assessment on the NewWave models reveals that they fail to predict the reduction of the crest width with increasing current incident angle and significantly underestimate the asymmetry parameters for large steepness waves.

Numerical analysis of pitch and rolling motions of trimaran in oblique waves

The coupled motion of roll and pitch is noticeable when trimaran sails in oblique head waves, which will lead to negative influence on the safety and comfort of navigation. In this paper, the numerical simulation of the trimaran's motion in oblique waves was carried out by open source CFD tool OpenFOAM, and the computed motion response was analyzed to study the characteristics of the torsional rolling. The OpenFOAM-based hybrid method used in this paper was briefly introduced first, and the numerical method was validated and verified by both the grid convergence test and the comparison with experimental result. Both the amplitude and the time history of the computed motion are used for analysis, and the influence of wave steepness, wave heading and forward speed of trimaran on the characteristics of torsional rolling was investigated. The result shows that the large wave steepness and the low forward speed will lead to dangerous torsional rolling motion of trimaran in oblique waves, and the dangerous condition is mainly caused by nonlinear rolling motion; the motion state is more sensitive and changing obviously with other factors at large wave steepness wave conditions.

On the statistical properties of surface elevation, velocities and accelerations in multi-directional irregular water waves

Abstract

Numerical simulation of wave run-up on four cylinders in a square configuration

A series of numerical simulations of wave run-up on four-cylinder array is performed at different spacing ratios 1.5 ≤ L/D ≤ 4.25, where L is the center-to-center distance between the adjacent cylinders and D is the cylinder diameter. The effects of spacing ratio and wave steepness on the wave run up ratio and wave force on the four cylinders are investigated. The results show that with variation of the spacing ratios and incident wave angles, different wave pattern regimes are observed among four cylinders. It is interesting to note that the maximum run-up ratio occurs at L/D = 2.0 with the large wave steepness H/λ=1/16, and approaches to 2.4 times of that for the single cylinder in the case of incident wave angle β = 45°. The spacing ratio has a significant effect on the horizontal wave forces for the four cylinders, however, the location of the maximum wave force is found to be not significantly related to that of the maximum run-up ratio.

Numerical Analysis of Wave Characteristic In the Freak Wave-- “New Year Wave” Formation

Freak waves are both extremely large waves and highly transient time. Such a wave may lead to damage of ships to deaths. In this paper, to describe the connection between freak wave and wave essential factor, we use WAVEWATCH III model simulating “New Year Wave” in the North Sea to explore freak wave, with the importing of ECMWF re-analysis wind field. By this way, we successfully simulate the formation of freak wave in the random wave. Analysis shows large wave steepness and small directional spread angle are necessary conditions for freak waves to easily occur. By analyzing the wave spectrum, it is found that the wave energy is distributed in a small range, and the propagation direction is relatively concentrated.

Generating shallow- and intermediate-water waves using a line-shaped mass source wavemaker

The mass source wavemaker in Navier–Stokes equations, which is a frequently used wave generation method, is relatively difficult to control. To resolve this problem, a line-shaped mass source wavemaker is proposed for generating shallow- and intermediate-water waves. Via employing reasonable simplifications, only the enhanced factor, which is introduced to compensate for the energy loss, is unknown during the wave generation process. It is derived that the enhanced factor depends on a weighted combination of wave steepness and relative water depth. Furthermore, the velocity field inside the source region is adjusted at each time step to eliminate the undesirable downward velocity which could contaminate the result via inducing strong and persistent vortical activities near source regions. With the undesirable velocity eliminated, the vortical activity in the vicinity of the source region is weakened significantly. Compared with other mass source wavemakers, the proposed method yields better results when it is used for simulating waves of large steepness. Compared with piston-type wavemakers, the proposed method yields satisfactory results when it is used for generating high-order waves although it is derived under the linear theory. Further simulations indicate that the proposed method is competent to reproduce the process of wave-structure interactions precisely.

Decrease in the Mode Purity of Microwave Beams in the L-2M Stellarator Peripheral Plasma

The linear coupling (transformation) of two wave modes in a peripheral layer of inhomogeneous plasma confined by a magnetic field is theoretically analyzed. This transformation is a cause of a decrease in the purity of the X-mode introduced into plasma, which is optimum for electron—cyclotron plasma heating at the second harmonic of gyrofrequency. It is shown that the mode purity defect in radiation power under L-2M stellarator conditions can be no more than 6%. An increase in the mode purity defect because of the arbitrarily large steepness of the electron density profile in the peripheral layer appears insignificant.

Geotechnical solutions for high-rise construction in the areas with significant elevation

The high attractiveness of the Black Sea coast in the Russian Federation, the ongoing work to create a world-class resort infrastructure lead to the need for the high-rise buildings’ construction and other heavy structures on steep grades that characterize most of the coastal strip. In such cases, they try to find the solutions that do not lead to a significant amount of earthwork when deepening the required number of floors under the stain of a high-rise building. However, with a large steepness of the relief, the excavation volume is always significant. Excavation during the engineering preparation of the territory and the development of deep pits leads to a loss of the slopes’ stability and, consequently, to the complex foundations’ solutions, engineering protection and overhead structures.