Extreme wave groups in a wave flume: Controlled generation and breaking onset

Extreme waves in random seas are usually breaking or close to breaking. Understanding the kinematics and evolution of such waves is important for determining loads on offshore structures. Controlled repeatable generation of realistic breaking waves in wave flume experiments is a difficult but important task. It is rather easy to generate an arbitrary breaking wave, but to the authors’ knowledge there is no methodology for accurate generation of a wave group with a pre-defined spectrum related to a modelled sea state with spilling breaking at a prescribed position. Such waves can be used to model extreme breaking waves in a random sea and their interaction with structures. This paper offers such a methodology. The key feature of the method is the application of an iterative focussing procedure to a linearised amplitude spectrum rather than to a full nonlinear spectrum. The linearised spectrum is obtained using a harmonics separation technique and the general derivation of the method is given for an arbitrary number of components. The procedure is applied to generate focussed wave groups with amplitudes increased in small steps until local crest breaking occurs. As a result, the highest non-breaking waves and weakly breaking waves are generated for otherwise identical conditions. The methodology is applied for four different wave spectra of the same peak frequency: JONSWAP, Pierson-Moskowitz, wide and narrow band Gaussian. It is found that steepness of the limiting breaking wave depends strongly on the choice of wave group spectrum. The results demonstrate that neglecting spectral properties of design waves may lead to misrepresentation of their breaking behaviour

Focusing unidirectional wave groups on finite water depth with and without currents

Focused waves are often used in physical and numerical studies as a representative condition for extreme waves or as a mean to generate very steep and breaking waves at a desired location in space and time. A focused wave is in theory created when all the components in a transient wave group come in phase. In the past, linear wave theory and empirical iterative methodologies have been suggested in order to achieve the required phase and amplitude focusing. Nevertheless, their effectiveness decreases as the non-linearity of the wave group increases and thus the generation of very high focused waves was a challenging task. Here, an empirical iterative methodology is suggested which can focus waves of any height at a predetermined temporal and spatial location. The methodology has been successfully applied to wave groups travelling on still water but also on sheared currents and it has been implemented in both physical and numerical wave flumes. The results presented here refer to linear, weakly non-linear and strongly non-linear focused waves generated with a realistic target spectrum

Experimental Generation of Focusing Wave Groups on Following and Adverse-Sheared Currents in a Wave-Current Flume

Focused waves are often used in physical and numerical studies as a representative condition for extreme waves or as a means to generate very steep and breaking waves at a prescribed location in space and time. They have also been combined with depth-varying currents in investigations of incipient wave breaking, wave breaking–induced energy dissipation, and wave–current induced loads on marine structures. A focused wave is created when all the components in a transient wave group come into phase. In the past, linear wave theory and iterative methodologies coupled with the linear Doppler-shifted dispersion relationship have been suggested to account for the presence of a current and achieve the required phase and amplitude focusing. In the majority of cases, linear or constant steepness spectra are used, which, compared to measured or theoretical spectra like the Joint North Sea Wave Project (JONSWAP), Gaussian, and Pierson-Moskowitz (PM) can be termed unrealistic. The effectiveness of these methodologies also decreases as the nonlinearity increases; therefore, in most studies, either weakly nonlinear conditions are used or the focus location is determined empirically. Here, an iterative methodology is suggested that can focus waves of any height at a predetermined temporal and spatial location even for wave groups propagating on a strong following or adverse current. An experimental apparatus developed to generate relatively stable sheared velocity profiles is also described. The depth-varying profile of the resulting currents diverges from that of classical wind-driven currents and comes closer to profiles measured in field sites important for the deployment of, for instance, tidal and wind energy converters. The methodology is successfully applied to wave groups traveling on still water, following, and adverse currents, and the results presented refer to linear, weakly nonlinear, and strongly nonlinear–focused waves generated for a range of realistic target spectra. The capability to generate wave groups with the same amplitude spectrum at a fixed location for a variety of flow conditions—still water, following, and adverse sheared currents—is also illustrated

Simulating breaking focused waves in CFD: Methodology for controlled generation of first and second order

A new methodology is proposed for the generation of breaking focused waves in computational fluid dynamics (CFD) simulations. The application of the methodology is illustrated for a numerical flume with a piston-type wavemaker built in the CFD model olaFlow. Accurate control over the spectral characteristics of the wave group near the inlet and the location of focus/breaking are achieved through empirical corrections in the input signal. Known issues related to the spatial and temporal downshift of the focal location for focusing wave groups are overcome. Focused wave groups are produced with a first- and second order-paddle motion, and the propagation of free and bound waves is validated against the experimental results. A very good overall degree of accordance is reported, which denotes that the proposed methodology can produce waves breaking at a focused location

Simulation of Steep Waves Interacting with a Cylinder by Coupling CFD and Lagrangian Models

This paper presents numerical modelling results of the interaction between a group of steep waves and a fixed vertical cylinder performed with a one-way coupled hybrid model. A set of experimental data is used to benchmark the accuracy of the modelling results. The wavemaker signal generated in the physical experiments is used to reproduce the incident wave conditions without a priori knowledge of the rest of the dataset. A Lagrangian numerical wave flume propagates the wave group, producing the nonlinear free surface elevation and wave kinematics with high accuracy in the vicinity of the cylindrical structure. This data is used as the input to the olaFlow CFD model, which calculates the wave–structure interactions. One-way coupling approaches based on boundary conditions and relaxation zones are tested and compared in terms of the recorded free surface elevation and pressures at the structure. The results present an adequate degree of accordance, and turbulence effects are found to be negligible in the simulations

An experimental study on wave forces on a vertical cylinder due to spilling breaking and near-breaking wave groups

This paper highlights the importance of spilling breaking waves in the design of offshore structures. Although wave loading on offshore structures due to non-breaking waves has been extensively studied, wave loading due to breaking waves is uncertain and not very well understood. Plunging breaking waves in deep water are very unusual, whereas spilling breaking waves are very common in extreme seas. Nevertheless, no significant research efforts have been made to study the effects of spilling breaking waves. The present study addresses this, comparing the wave loading from highly nonlinear non-breaking waves and spilling breaking waves. Focused wave groups have been used to generate the non-breaking and spilling breaking wave conditions in a wave tank. The experiments have shown that spilling breaking waves generate significantly larger forces on the cylindrical model than highly non-linear non-breaking waves of equivalent size. This is shown to be due to the direct excitation at high frequency as well as the resonant and impact excitation of higher natural frequency modes. It is concluded that spilling breaking waves can generate very significant forces and should therefore be considered in the design of offshore structures

Current blockage in sheared flow: Experiments and numerical modelling of regular waves and strongly sheared current through a space-frame structure

Space-frame structures supporting marine renewable energy devices such as offshore wind turbines are exposed to complex hydrodynamic forces resulting from the coexistence of waves and currents. Previous investigations on the interaction of such structure acting as an obstacle array with regular waves and in-line uniform current reported a reduced fluid loading due to current blockage. This paper documents laboratory-scale experimental evidence for reduced fluid loading on a truss structure exposed to regular waves with in-line sheared current in shallow water. Strongly sheared current of different speeds is generated and profiled using purposely-built wire resistance arrays in a wave-current flume, and a range of regular waves are created using a piston-type wavemaker. The global hydrodynamic force time history on a truss structure is measured for a range of sheared current speeds and regular wave heights. For all test cases, two loading configurations are considered, with the truss positioned end-on and diagonal to the incident flow direction. Comparisons are made with the analytical current blockage model for steady uniform current by Taylor (1991) and Taylor et al. (2013), and with the numerical simulations conducted in OpenFOAM using a porous tower model following the approach by Santo et al. (2015). Under the same input condition, the diagonal loading configuration is observed to attract higher forces and therefore it should not be ignored when assessing the survivability of such structures. Overall, good agreement in terms of the peak forces and the shapes of force time history is achieved, all with a single and consistent value for each of Cd and Cm. On the other hand, predictions using standard Morison with no blockage and the present API recommendation with the same Cd and Cm result in force overpredictions for all cases of regular waves with in-line current. For steady sheared current flow through a porous tower, apart from the dominant lateral flow divergence, numerical flow visualisation reveals an existence of vertical flow interaction in the porous tower. This is attributed to the non-uniform loading with water depth and was not observed previously for uniform current flow. This study provides the first experimental validation and justification on the use of a simple porous block in representing a complex geometry of real space-frame structures when exposed to combined large regular waves and in-line current

Intercomparison of Three Open-Source Numerical Flumes for the Surface Dynamics of Steep Focused Wave Groups

NewWave-type focused wave groups are commonly used to simulate the design wave for a given sea state. These extreme wave events are challenging to reproduce numerically by the various Numerical Wave Tanks (NWTs), due to the high steepness of the wave group and the occurring wave-wave interactions. For such complex problems, the validation of NWTs against experimental results is vital for confirming the applicability of the models. Intercomparisons among different solvers are also important for selecting the most appropriate model in terms of balancing between accuracy and computational cost. The present study compares three open-source NWTs in OpenFOAM, SWASH and HOS-NWT, with experimental results for limiting breaking focused wave groups. The comparison is performed by analysing the propagation of steep wave groups and their extracted harmonics after employing an accurate focusing methodology. The scope is to investigate the capabilities of the solvers for simulating extreme NewWave-type groups, which can be used as the “design wave” for ocean and coastal engineering applications. The results demonstrate the very good performance of the numerical models and provide valuable insights to the design of the NWTs, while highlighting potential limitations in the reproduction of specific harmonics of the wave group.</jats:p