A laboratory study of nonlinear changes in the directionality of extreme seas

This paper concerns the description of surface water waves, specifically nonlinear changes in the directionality. Supporting calculations are provided to establish the best method of directional wave generation, the preferred method of directional analysis and the inputs on which such a method should be based. These calculations show that a random directional method, in which the phasing, amplitude and direction of propagation of individual wave components are chosen randomly, has benefits in achieving the required ergodicity. In terms of analysis procedures, the extended maximum entropy principle, with inputs based upon vector quantities, produces the best description of directionality. With laboratory data describing the water surface elevation and the two horizontal velocity components at a single point, several steep sea states are considered. The results confirm that, as the steepness of a sea state increases, the overall directionality of the sea state reduces. More importantly, it is also shown that the largest waves become less spread or more unidirectional than the sea state as a whole. This provides an important link to earlier descriptions of deterministic wave groups produced by frequency focusing, helps to explain recent field observations and has important practical implications for the design of marine structures and vessels

Irregular wave runup statistics on plane beaches: application of a Boussinesq-type model incorporating a generating-absorbing sponge layer and second-order wave generation

Efficient absorption of reflected waves at the offshore boundary is a prerequisite for the accurate physical or theoretical modelling of long-duration irregular wave runup statistics at uniform, gently sloped beaches. This paper presents an implementation of the method suggested by Zhang et al. (2014) to achieve reflected wave absorption and simultaneous generation and propagation of incident waves in an existing numerical wave flume incorporating a moving boundary wavemaker. A generating–absorbing layer is incorporated within this 1DH hybrid Boussinesq-nonlinear shallow water equation model such that inshore-travelling incident waves, encompassing bound-wave structure approximately correct to second order, propagate unhindered while offshore-travelling reflected waves are absorbed. Once validated, the method is used to compile random wave runup statistics on uniform beach slopes broadly representative of dissipative, intermediate, and reflective beaches. Analyses of the individual runup time series, ensemble statistics and comparison to an empirical formula based on experimental runup data suggest that the main aspects of runup observed in the field are properly represented by the model. Existence of an upper limit on maximum runup is investigated using a simple extreme-value statistical analysis. Spectral saturation is examined by considering ensemble-averaged swash spectra for three representative beach slopes subject to incident waves with two different offshore significant wave heights. All spectra show f^−4 roll-off at high frequencies in agreement with many previous field studies. The effect is also investigated of the swash motions preceding one particular extreme runup event on the eventual maximum runup elevation

Optimisation of focused wave group runup on a plane beach

publisher: Elsevier articletitle: Optimisation of focused wave group runup on a plane beach journaltitle: Coastal Engineering articlelink: http://dx.doi.org/10.1016/j.coastaleng.2016.12.001 content_type: article copyright: © 2017 Elsevier B.V. All rights reserved

Towards long random sea simulations in numerical wave tanks

The present investigation concerns optimum wave absorption in numerical wave tanks. Recent developments have now established that (i) absorption controllers based on Infinite Impulse Response (IIR) filters are highly effective and (ii) cosh shaped wave board geometries offer significant potential in terms of active wave absorption due to their favourable added mass behaviour. While (i) and (ii) have been shown individually, their combination has never been demonstrated. To address this, a cosh shaped wavemaker is implemented in a time-domain numerical wave tank. Comparisons are presented between simple proportional controllers and the IIR approach, where the latter is demonstrated to offer excellent absorption performance over a very broad range of incident wave conditions. In excess of 90% amplitude (or equivalently 99% energy) absorption is demonstrated for the range 1 ≤ kh ≤ 8, where k is the wavenumber and h is the water depth. A broad-banded absorption performance of this type covers the vast majority of wave components present in practical offshore wave spectra. Test cases are presented for both regular and irregular seas, paving the way towards numerical simulations of long random sea states. This paper focuses on a two-dimensional description of the problem. The approach adopted can also be extended to three dimensions, where reduced domain sizes (no sponge layer requirements) offer orders of magnitude improvement in terms of computational cost

A laboratory study on the loading and motion of a heaving box

This paper concerns the nonlinear loading and dynamic response of a heaving rectangular box in two dimensions, using a series of experimental tests in regular and irregular wave conditions. Nonlinear forcing components are found to make major contributions to both the excitation problem and the motion response. Two main sources of nonlinearity are established: the first associated with higher-order wave–structure interactions, and the second associated with viscous dissipation. The present work quantifies the relative influence of these two sources. Adopting a series of regular wave cases, the first source, prevalent in steep wave conditions, is shown to be particularly significant in the diffraction regime, leading to significant excitation force amplifications. In deep water, these nonlinearities are primarily driven by interactions between incident and reflected wave components. The second source, due to vortex shedding, plays a minor role in the excitation problem, but has a major influence on the motion response. Vortex-induced effects are particularly important when the structure exhibits large motions, for example at resonance. To characterise the response in irregular waves, experimental data are provided comprising in excess of 100,000 individual waves, presenting one of the most substantial data sets of this kind to date. In considering these irregular sea states, the two aforementioned sources of nonlinearity are again found to be of critical importance. While wave-induced load amplifications of up to 60% may be observed in the excitation problem, the motion response is primarily governed by vortex-induced attenuations. In order to provide practical engineering solutions, two approaches are offered. For nonlinear forcing predictions, a two parameter Weibull fit is found to be both simple and accurate. In terms of the heave motion, a computationally efficient time-domain simulation, building upon a linear hydrodynamic description and a quadratic MOJS type drag term, leads to good agreement with experimental data

A laboratory investigation concerning the superharmonic free wave suppression in shallow and intermediate water conditions

This paper concerns laboratory wavemaking in shallow and in termediate water conditions. A comparison is made between two wave generatio n techniques, a first based on controlling the wavemaker displacement, and a seco nd based on controlling the wavemaker force. Nonlinear wave generation in position control is well under- stood, and many laboratories rely on established second-or der or Stream-function inputs. In deep water, using flap-type wavemakers, a force-c ontrol approach based on a linear demand signal was recently shown to offer benefits in terms of wave quality. The shallow water operation of such force-control strategies is less certain, which motivates the present study. To investigate the influence of the water depth on this type of control, a range of generation scenarios is considered, including small ampli tude and large amplitude regular waves. Adopting both supporting calculations and e xperimental evidence, the work demonstrates that first-order force-based wave gen eration in shallow water suffers from similar limitations as first-order position cont rol. This principally con- cerns the contamination of the testing area due to unwanted f ree waves, where the present focus is placed on the superharmonic range. The main advance of the work lies in the solutions it offers to ov ercome this free wave contamination. A number of nonlinear wave solutions up on which force-based generation can be based are discussed, and a suitable method ology is proposed and validated for each case. The developed methodology allows f or high quality wave generation, whilst maintaining the benefit of active wave ab sorption. The work is timely in the sense that is responds to two recent developmen ts. First, the majority of wavemaking facilities commissioned over the past two dec ades are computer con- trolled, and active absorption has become commonplace. The work presented offers solutions highly relevant to such installations. Second, d evelopments particularly in offshore wind, have seen many new structures placed in relat ively shallow-water depth. It is essential that the model testing of such structu res adequately accounts for the issues and solutions presented herein