Monthly Archives: April 2013

Developing and Validating a Forecast/Hindcast System for the Mediterranean Sea


L. Mentaschi, G. Besio, F. Cassola, and A. Mazzino – Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, April, 2013

Abstract

In this work a study of the performances of the WAVEWATCH III (WWIII) model in the Mediterranean basin is presented. Analysis is carried out referring to seventeen case studies corresponding to heavy storms in northern Tyrrhenian basin (Ligurian sea) in the last twenty five years. Simulation results are validated using buoy data provided by the Rete Ondametrica Nazionale (RON). An analysis of the usage and performances of different statistical error indicators is provided, showing that widespread NRMSE indicator is biased towards models that under estimate prediction. The well established source terms parameterization by Tolman and Chalikov (1996) has been compared with the one proposed by Ardhuin et al. (2010), set up with the parameterization by Bidlot et al., (2005), and with the set known as ACC350. The obtained results reveal that the ACC350 parameterization works better in severe conditions, though tends to overestimate wave height and underestimate period. A further sensitivity analysis in the parameters space is carried out around ACC350 parameterization, finding that variations in the default set of parameters, involving small wave growth reduction or an increase of energy dissipation, lead to a slight improvement of the overall simulation performances.

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Filed under Modeling, Resource Characterization

Numerical and Physical Modeling of Extreme Waves at Wave Hub


E. Ransley, M. Hann, D. Greaves, A. Raby, D. Simmonds – Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, April, 2013

Abstract

With a history of international failures, the survivability of coupled systems of wave energy devices and their moorings, particularly those to be installed at development sites like Wave Hub, is surrounded by uncertainty. Potential design solutions require a better understanding of the hydrodynamics and structural loading experienced during extreme events, like rogue wave impact, in order to mitigate the risk of device and mooring failure. Rogue waves are waves with amplitudes far greater than those expected, given the surrounding sea conditions. Intense study into these events stems from their potential for catastrophic impact on ocean engineering structures. However, little is known about their physical origins and, currently, there is no consensus on their definition or explanation of the mechanism which drives them. This paper concerns the numerical modeling and experimental validation of extreme rogue wave examples at the Wave Hub site. Using hindcast data, the 100 year extreme wave at the Wave Hub site is determined. This extreme wave is replicated in Plymouth University’s new COAST Lab using a NewWave, dispersive focusing input. To simulate and analyse these events, we duplicate these conditions in a numerical wave tank (NWT), solving the fully nonlinear Navier-Stokes equations, with a free surface, using the Volume of Fluid (VoF) method and open source CFD library OpenFOAM®. The comparison shows that the CFD software is capable of simulating focused waves similar to those produced in the physical tank but tends to overestimate the crest heights. It is also noted that nonlinear effects are important when considering the shape and location of focused wave events.

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On the park effect in arrays of oscillating wave energy converters


A. Babarit – Renewable Energy, October, 2013

Abstract

This paper aims to provide guidelines for designing the layout of arrays of oscillating Wave Energy Converters (WECs) based on a review of the literature of wave interactions and park effect in WEC arrays that has been published over the past 30 years.

First, the fundamentals of wave energy absorption by oscillating bodies are summarised, and the principal differences between the park effect in arrays of wave energy converters and wind turbines are highlighted. Then, the numerical approaches commonly used to deal with WEC arrays are outlined briefly and their limitations are discussed. It is argued that, at present, only Boundary Element Methods (BEM) are capable of the appropriate analysis. Finally, previous work on wave interactions and park effect in WEC arrays is reviewed. Similar trends are found in these studies, which allow conclusions to be drawn regarding the significance of the park effect as a function of the number of WECs in the array and their spacing. Based on these conclusions, the following tentative guidelines are proposed:

For small arrays of conventional devices (fewer than 10 devices of typical dimension 10–20 m) with usual layouts (regular or shifted grids with separating distance of order 100–200 m), the park effect appears to be negligible. For larger arrays (more than 10 devices), a negative park effect seems to be increasingly important with increasing number of rows (the lines of WECs perpendicular to the incident wave direction). Therefore, the number of rows should remain as small as possible, with a separating distance as large as possible. For arrays of non-conventional WECs (WECs of typical dimensions much larger than 10–20 m), no information has been found. However, trends similar to the previous cases could be expected, provided that aspect ratios are maintained.

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Wave resource assessment along the Cornish coast (UK) from a 23-year hindcast dataset validated against buoy measurements


J.C.C. van Nieuwkoop, H.C.M. Smith, G.H. Smith, and L. Johanning – Renewable Energy, October, 2013

Abstract

Long-term knowledge of the wave climate of a potential wave energy site is essential for project planning and design, not only for an understanding of the resource variability, but also for the prediction of design wave conditions. The southwest region of the UK is at the forefront of the country’s wave energy development, with two operational test sites. However, no detailed long-term resource assessment has yet been performed. This paper presents a long-term wave hindcast for southwest England, performed using the numerical wave model SWAN, with a particular focus on two energy device test facilities: ‘Wave Hub’ on the energetic and exposed north Cornwall coast, and ‘FaB Test’ on the more sheltered south coast. A high-resolution wave model suite, aimed at establishing nearshore wave hindcasts, is described and evaluated. The suite is run for a 23-year period, starting in 1989 and continuing to 2011. The hindcast is compared with measurement data and the results are analysed for the two test sites. Special attention is given to the implications of present hindcast errors and how the hindcast errors can be minimized.

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Filed under Field Measurements, Modeling, Resource Characterization

Seaweaver: A new surge-resonant wave energy converter


R. Chaplin – Renewable Energy, September, 2013

Abstract

This paper presents an experimental study of a new resonant-in-surge wave energy converter concept that is a successor to the “Frond” and “Wraspa” WECs investigated at Lancaster University and elsewhere. The concept is of a buoyant, rigid, surface-piercing diaphragm, the edges of which are held still but whose central area moves in resonance with the swell component of an ocean wave system. The two bodies forming this diaphragm are pivoted on the arms of a Y-shaped tubular frame that is held in position by four taut mooring lines. It is proposed to move the frame compensate for tidal changes and to maintain the WECs bearing so that it always faces the principal swell direction.

Capture-widths found in the tank tests, in regular waves of 0.5 W/m, were generally around 20 cm at 1/100 scale and were greater than this in smaller waves. A method of slow tuning was demonstrated and a method of fast-tuning, previously trialled with Wraspa, is also described. In view of the likely low structural and installation costs, combined with good power, proposed future work includes mixed-wave trials on one “best” design at scales ranging from 1/170 to 1/3, the latter being a marine trial over 1 year in the Irish Sea at a point where conditions are known to closely model North Atlantic conditions. Such a trial would include measurement of structural loads in extreme seas: these are discussed below.

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Filed under Experiments, System Development