Irregular Waves Research Articles

Experimental validation of Orcaflex-based numerical models for the PEWEC device

To design a Wave Energy Converter mooring system that ensures maintainability, resistance, and low costs, without affecting productivity, it is necessary to foster reliable numerical models. In the present paper, the validation of Orcaflex© simulations against the experiment results of the Pendulum Wave Energy Converter (PEWEC) 1:25 scale model, conducted at the University of Naples Federico II, is presented. The experiments consist of free-decay and static pull-out tests to assess the inertial properties of the model and mooring system; tests in operative and extreme regular and irregular waves to fully characterize the mooring system and the device dynamics. The same wave records measured in the towing tank have been used in numerical simulations. PEWEC motion results from numerical simulations and experiments are given in terms of Response Amplitude Operators (RAOs) for regular waves, as statistical values in tabular form, and in Taylor's diagrams for irregular waves. The most probable fairleads tensions for the load dimensioning have been obtained from the Generalized Extreme Value (GEV) distribution of experimental and numerical data. The obtained differences show that the numerical model accurately predicts the experimental data and correctly estimates the dimensioning load of the mooring lines.

Hydrodynamic Tests of Innovative Tourist Submarine

This paper deals with the resistance, towing, seakeeping, and open water propeller tests of an innovative tourist submarine model. Tests were performed in a 276 m long towing tank. As the submarine model is a complex structure composed of various parts attached to the pressure hull, the largest possible model, in the scale of 1:5.0, was produced, considering the towing tank depth and the capabilities of the measurement equipment. Resistance tests were performed in deep water and on the surface in calm water. The tested speed range in both cases was up to 5.5 knots. To ensure the avoidance of free surface effects, resistance tests in deep water were performed for different draughts and then extrapolated to infinite water depth. Smaller effective powers were found for the surface condition. The results are compared to an independently performed computational fluid dynamics (CFD) analysis using OpenFOAM. A fair agreement between the experimentally and numerically predicted effective power is found, while the reasons for the differences found are explained. The free submarine model was towed with a rope performed for the speed range 1.7 kn–3.5 kn, and the towing force in the rope was measured. Seakeeping tests in irregular beam waves at zero speed were performed to check the flooding risk on open hatches. Open water tests of the main thrusters for propelling the submarine were conducted, indicating that both power demand and propeller thrust are slightly larger compared to the initial estimates.

Non-spectral linear depth inversion using drone-acquired wave field imagery

We present an efficient approach for video-based bathymetry survey. The proposed algorithm consists of preprocessing for drone-acquired video, extracting incident gravity wave, estimating celerity vector field, operating linear water depth inversion, and mapping bathymetry. The image preprocessing includes resizing, masking, barrel distortion correction, ortho-rectification, and georeferencing. The challenges posed by noisy, non-stationary, and irregular wave field data was resolved using ensemble empirical mode decomposition (EEMD), which is capable of extracting incident gravity wave structure without arbitrary initial guessing of frequencies. The extracted wave structures were used to estimate their celerity vector field using large-scale particle image velocimetry (LSPIV) technique. Given that wave celerity depends on water depth in nearshore based on linear dispersion relation, spatial distribution of water depths was determined from wave celerity estimates. The proposed algorithm was tested using the wave field videos and ground truth data, collected during the field experiment at Jangsa beach, located in the east coast of South Korea, and good agreement was achieved.

Interfacial microstructure of multi-layered Al–Cu joint by electromagnetic pulse welding

The necessity of the multi-layered aluminium (Al) tabs to copper (Cu) busbars for electric vehicle (EV) batteries is increasing. Electromagnetic pulse welding (EMPW) is applicable to the welding of conductive dissimilar/similar materials. In this work, three Al sheets and a single Cu sheet are joined by the EMPW technology. The layer-wise tensile lap shear properties of the multi-layered Al–Cu EMPW joint are evaluated. The layer-wise interfacial microstructures of the joint parallel to and perpendicular to the welding direction are studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). The results showed that every layer achieved metallurgical bonding. The Al sheets were formed to be whole. There were three types of interfaces at the Al–Cu interface parallel to the welding direction: flat interface, wave interface, and irregular wave interface. The transition zone is found at the Al–Cu interface perpendicular to the welding direction. Due to the impact, the grains near the Al–Cu interface and Al–Al interface are both refined. Thus, the well-bonded multi-layered Al–Cu joint is successfully obtained via EMPW. The results can offer help in the manufacturing of EVs.

An experimental investigation of the effect of ride control systems on the motions response of high-speed catamarans in irregular waves

To ensure high-quality passenger comfort and enhance the operability and performance of high-speed catamarans, reducing the severity of motions and structural loads using Ride Control Systems (RCS) is beneficial. To optimize ride control algorithms, it is essential to have a thorough understanding of the RCS in actual ship sailing conditions. In this paper, a set of towing tank tests was undertaken to study the effectiveness of different control algorithms, including linear and nonlinear versions of the heave control, pitch control, and local control, on motion responses of a 2.5 m scaled model of a 112 m INCAT Tasmania high-speed catamaran in irregular waves. The RCS included a centre bow-fitted T- Foil and two transom-mounted stern tabs. Obtained results were also compared with the scale model responses with passive RCS and with no RCS fitted. Heave and pitch motion responses as well as vertical accelerations of the catamaran were calculated in head seas at a model speed of 2.89 m/s (37 knot full scale), a modal period of 1.5 s (10 s full scale) and two significant wave heights of 60 mm and 90 mm, simulating full scale wave heights of 2.7 m and 4 m, respectively. It was demonstrated that deploying a passive RCS led to modest reductions in the peak motion responses. The most significant reductions in ship motions took place in the nonlinear modes of the heave and pitch control algorithms at a significant wave height of 60 mm (full-scale 2.7 m). In this condition, the nonlinear pitch control mode was demonstrated to be the most effective algorithm since this control mode mitigated the peak pitch RAO by 41% and vertical accelerations by 46%. This was a substantial reduction in ship motions with the RCS at an equivalent full-scale speed of 37 knot at 2.7 m wave height in the random sea condition.

Experimental study on the effect of flare barriers on wave run-up and motion response of a semi-submersible platform

Semi-submersible platforms can suffer slamming damage from extreme waves, making it crucial to improve their airgap performance. To address this issue, this paper proposes an innovative flare barrier that can be attached to the surface of a column at a certain height above the waterline, drawing on bow flares used in ship design. This study conducts a series of experiments to investigate the effectiveness of the flare barrier in mitigating wave run-up and motion response of a semi-submersible platform. The experiment uses irregular waves with a 100-year return period, incident at angles of 180° and 135° to simulate the wave field. To reduce the transient effect, two different wave seeds are considered for each wave-heading condition. The results show that the flare barrier significantly mitigates wave run-up, especially at the height of the lowest edge of the barrier. The addition of flare barrier has no negative effect on surge, heave and pitch response. At incident angle of 180°, it even has a slight mitigation effect on heave and pitch. The mitigation effect is also demonstrated in the power spectral analysis. Considering the wave run-up and motion response, a flare barrier is recommended to mitigate wave run-up on the column and improve the airgap performance under extreme waves.

Evaluation of Geometrical Influence on the Hydrodynamic Characteristics and Power Absorption of Vertical Axisymmetric Wave Energy Converters in Irregular Waves

Abstract To obtain the mechanical energy of waves from arbitrary directions, the vibration absorbers of wave energy converters (WEC) are usually vertically axisymmetric. In such case, the wave-body interaction hydrodynamics is an essential research topic to obtain high-efficiency wave energy. In this paper, a semi-analytical method of decomposing the complex axisymmetric boundary into several ring-shaped stepped surfaces based upon the boundary approximation method (BAM) is introduced and examined. The hydrodynamic loads and parameters, such as the wave excitation forces, added mass and radiation damping of the vertical axisymmetric oscillating buoys, can then be achieved by using the new boundary discretisation method. The calculations of the wave forces and hydrodynamic coefficients show good convergence with the number of discretisation increases. Comparison between the constringent results and the results of the conventional method also verifies the feasibility of the method. Then, simulations and comparisons of the hydrodynamic forces, motions and wave power conversions of the buoys with series draught and displacement ratios in regular and irregular waves are conducted. The calculation results show that the geometrical shape has a great effect on the hydrodynamic and wave power conversion performance of the absorber. In regular waves, though the concave buoy has the lowest wave conversion efficiency, it has the largest frequency bandwidth for a given draught ratio, while in irregular waves, for a given draught ratio, the truncated cylindrical buoy has the best wave power conversion, and for a given displacement of the buoy, the concave buoy shows the best wave power conversion ability.

Effects of turbulent wind and irregular waves on the dynamic characteristics of a floating offshore wind turbine platform

Effects of turbulent wind and irregular waves on the dynamic characteristics of a floating offshore wind turbine platform

Comparative study of spread and vertical mooring configuration in floating collar net cage under irregular waves and current

Offshore aquaculture is one of the national strategic programs of the Ministry of Fishery and Ocean Affairs of Indonesia to increase national fish cultivation capacity. One solution is using a simple flexible floating collar net which is suitable for small to medium cage volume. In designing such floating net, its mooring line configuration is vital to maintain fixed position of structural part of the cage volume. The purpose of this study is to compare design aspect of two types of mooring systems, i.e., spread and vertical configuration, by investigating selected parameters: mooring line tension, collar offset, and volume reduction. The mooring configuration and floating collar net cage are analysed through a numerical model based on Morison-force equations, under environmental condition adopted from Kangean field East Java. Load cases from collinearly in-line and between-line directions under extreme waves and current condition are checked against relevant safety factor. The dynamics analysis using the time domain simulation is based on 900 seconds cut-off according to its maximum wave elevation profile to generate the hydrodynamics forces to the mooring systems.

Laboratory experiments on the evolution of a submerged berm driven by low-energy irregular waves

Laboratory experiments on the evolution of a submerged berm driven by low-energy irregular waves

Comparison of schlieren-based techniques for measurements of a turbulent and wavy free surface

The free surface at an air–water interface can provide information regarding bathymetric complexities, as well as the subsurface flow. We present a comparison of the performance of two recent advances in light-based optical techniques for free surface measurements, total internal reflection-deflectometry and moon-glade background-oriented schlieren, with the more established method of free surface synthetic schlieren. We make use of an optical flow algorithm over the more traditional digital image correlation, in order to obtain higher spatial resolution data across the imaged free surface domain. The optical flow algorithm presents additional benefits, such as computational efficiency and robustness in capturing large displacements and straining of tracked features. The three optical techniques are assembled in synchronization to image two free surface conditions: (1) a free surface being impinged upon by an underlying turbulent, free-shear flow and (2) a random and irregular wave field induced by a free jet. Using the high-resolution measurements, we provide insight on the emergence of multiple free surface dynamics for a turbulent free surface. We present a comprehensive discussion on the benefits and drawbacks of each technique, including suggestions on the suitability of each technique for several experimental constraints.Graphical abstract

Prediction of irregular wave (current)-induced pore water pressure around monopile using machine learning methods

Prediction of irregular wave (current)-induced pore water pressure around monopile using machine learning methods

Modulation effects of submarine internal wake waves on free surface divergence field

Disturbances of a submarine in a stratified flow stimulate an infinite mode of internal waves with multiple dispersion relationships. The surface convergence and divergence field are then modulated by such trailing waves, affecting the spatial distribution of microscale waves and leaving traces that are easily detected by Synthetic Aperture Radar. The present modified model combines the source-caused internal waves theory with the wind-waves spectrum model and coupling wave components of the full wave systems decomposed by the Fourier series method based on the second-order multi-directional irregular waves interactions theory. The effects of maximum Brunt–Vaisala frequency amplitude and depth, internal wave modes, submarine dive depth and velocity, wind speed, and wind direction are discussed in this study. It is shown that after the appearance of mode 0, the wake waves have an appreciable effect on the modulation of the free surface wind-wave field. However, this effect is not fixed to manifest as an enhancement of the free surface convergence or divergence field intensity. The free surface field has the maximum divergence intensity amplitude under the downwind sailing condition when the submarine is sailing at low speed. At high speed, the maximum divergence intensity amplitude is at an angle of 30° between the sailing direction and the wind direction, while the minimum divergence intensity is downwind.

An experimental study on the performance of combined membrane breakwaters against incident waves

A series of experimental model tests are conducted to investigate the performance of a combined membrane breakwater consisting of horizontal and vertical flexible membranes against regular and irregular incident waves. A dimensional analysis is carried out to determine the dimensionless parameters used to study the performance of the breakwater. The dependence of the reflection, transmission and dissipation coefficients on non-dimensional breakwater design parameters and wave characteristics such as membrane porosity, submergence depth, membrane length, mooring line stiffness, number of vertical membranes, spacing between horizontal and vertical membranes, spacing between vertical membranes, wave period, and wave height are analysed. Further, the efficiency of the present combined membrane breakwater is analysed by comparing different membranes combinations under different wave conditions. The analysis of the reflection and transmission coefficients as well as the dissipation coefficient of the proposed breakwater will be helpful for better designing of combined membrane protecting infrastructures in coastal and ocean engineering.

Numerical simulation of the effect of embedded gap on impact initiation of explosive PBX-9502

ABSTRACT Aiming at the influence of the embedded gap on the impact initiation of the flat flyer of PBX-9502, the finite element simulation software LS-DYNA and the I-G model are used to establish the numerical calculation model of the cylindrical PBX-9502 with the embedded gap for the impact initiation of flat flyer, and analyze the influence of different impedance. The effect of the thickness and position of the large impedance gap on the reduction of the flyer initiation threshold is studied, the initiation velocity thresholds under different conditions are obtained, and the causes of abnormal detonation at the threshold velocity are analyzed. The results show that, under certain conditions, the small impedance gap will increase the impact initiation velocity threshold while the large impedance gap will decrease; the reduction effect of the position and thickness of the large impedance gap on the detonation velocity increased first and then decreased; both embedded gaps and deformed flying plates will produce Irregular reflection waves that affect the explosive initiated at a critical speed.

DYNAMIC SIMULATION OF AN OFFSHORE AQUACULTURE STRUCTURE SUBJECTED TO COMBINED WAVE AND CURRENT CONDITIONS

Abstract The reliable design of offshore aquaculture structure (OAS) for fish farming in the open ocean is vital to the marine aquaculture industry in the future. However, the lack of easy-to-access numerical tools for the dynamic analysis of OAS challenges the development of marine aquaculture. This article presents a newly developed numerical library under an open-source, finite-element analysis code, Code_Aster, enabling the dynamic analysis of OAS. A numerical model of OAS is first developed using the present numerical library, and then validated against published experiments. The validation shows a good agreement in terms of structural motions and tensions in mooring lines. Subsequently, the dynamic responses of this model are analyzed under irregular waves and current conditions from field measurements on an offshore fish farm site. The results indicate that a negative mean pitch angle will occur when the current velocity is large.

The effect of the operational environment on the survivability of passenger ships

The in-force probabilistic framework for passenger ship survivability assessment covers collision hazards. The framework primarily pertains to a static approach. Nonetheless, more complex dynamic analyses usually employ the same damage definitions, adding besides the breach characteristics, the environmental condition selection or, more precisely, the irregular wave environment necessary to simulate the damage scenarios. The traditional dynamic approaches to survivability consider only the significant wave height sampled from statistical formulations, with the wave period deriving from a constant steepness assumption. However, wave height and period influence ship dynamics in waves differently, especially concerning survivability after damage. Therefore, aiming at a direct assessment of ship survivability and the probability of loss of lives determination in realistic operational scenarios, it is essential to properly study the influence of combined variations of wave height and periods and their occurrence. The present study proposes a methodology for dynamic simulations in site-specific conditions derived from the Global Wave Statistics. The study documents the process in two critical collision damages for a reference passenger ship, using wave height and period combinations typical of the main sea areas of interest for passenger ships and performing a sensitivity analysis on the simulations needed to evaluate survivability. This enhanced analysis allows identifying the limiting environmental conditions for the critical damage cases, including the effect of heading variations, determining the ship’s survivability to specific damage in an operational area.

Experimental performance of multi-layered membrane breakwaters

A series of experimental tests are conducted to investigate the performance of a newly proposed multi-layered flexible membrane breakwater under regular and irregular waves in a two-dimensional wave flume. The effect of various parameters, such as edge conditions, spring stiffness of mooring lines, wave height, wave period, breakwater height, submergence depth of the breakwater, membrane porosity and membrane length, on the efficiency of breakwaters is detailly studied and discussed. The performance of the proposed flexible breakwater is estimated by measuring the wave transmission, reflection and dissipation coefficients through a four-point method based on the least squares method with variable weights and the fast Fourier transform technique. Spectral analysis for irregular waves is also carried out to help understand the experimental results. The results show that the overall trend of the transmission and dissipation coefficients is not significantly different between regular and irregular waves. However, in regular waves, the reflection coefficient fluctuates periodically with respect to the wave period, which does not occur in irregular waves. The experimental data presented here can be used to verify and validate analytical models dealing with similar floating breakwaters.

Wave forced vorticity and dissipation scaling on a rip channeled beach

Rip-currents, commonly observed on natural beaches, are vorticity induced and part of large scale near-shore circulations. The questions arise: how do bathymetric gradients magnitudes relate to rip velocities? how does rip current vorticity scale with wave characteristics and dissipation? What is the dynamics of the large scale 2D vorticity? To address these questions, we utilize a Non Linear Shallow Water model with a shock-capturing scheme. It is validated with preexisting experiments of wave induced rip-currents on uneven bathymetries generated by irregular waves. To do so the enstrophy (spatially averaged square of the vorticity) is shown to be a relevant metric to calibrate the bottom friction coefficient of the model. The numerical study based on a large number of simulations with monochromatic wave forcing shows that the more non-uniform the bathymetry is, the stronger the gradients in wave dissipation are and the stronger the enstrophy is. The rip current velocity is shown to linearly increase with the square root of the local enstrophy. The wave-averaged shallow water vorticity equation terms are evaluated. It is suggested that large scale 2D vorticity dynamics mainly result from an equilibrium between vorticity production, vorticity advection by the circulation and dissipation by bottom friction.

Shape parameters optimisation of a quayside heaving rectangular wave energy converter

In order to increase the competitiveness of wave energy industry, device’s efficiency and cost have to be both optimised. This study is a contribution to this approach by presenting the optimisation of a quayside rectangular float. This configuration has been chosen since quayside systems present the advantage of greatly reducing maintenance and grid connection costs. Moreover, rectangular shapes enable us to validate the optimisation algorithm with a linear potential model. Thus, the evolutionary algorithm CMA-ES (Covariance Matrix Adaptation-Evolutionary Strategy) is used to perform Capture Width (CW) optimisations in regular and irregular waves. Results with and without a vertical wall are compared. In an open sea configuration, at a given frequency, the shape that maximises efficiency is a horizontal flat rectangle with a minimum draft and a breadth equals to a half wavelength. The presence of the wall doubles the CW compared to the case in open sea. Moreover, for a quayside case, one optimal shape for a given draft has a breadth twice smaller than the open sea case. In addition, the resonances of the water column in the gap between the float and the dike, that occur in regular waves, lead to various possible optimised shapes. However, only the shape corresponding to a minimum draft and a quarter of the wavelength wide is also recovered in irregular waves. This shape also appears to be less dependent to the clearance distance between the float and the vertical wall. Finally, bi-objective optimisation results are realised in irregular waves for a rectangular quayside float in order to maximise the CW and minimise the WEC (Wave Energy Converter) cost. A significant cost reduction is possible by reducing the float immersed surface and volume. It is shown that a reduction of the float’s breadth of 50% from the optimal value only reduces the CW of about 7%.