Three-dimensional modelling of turbine wake interactions at a tidal stream energy site

One of the biggest uncertainties in tidal stream energy resource assessment is how tidal energy conversion, particularly at large scale, will interact with the resource. As few arrays are currently operational, data collected from these developments tends to be commercially sensitive. Therefore, array interaction with the resource is generally assessed using numerical models. A fully three-dimensional numerical approach based on Actuator Disk theory was implemented into the Regional Ocean Modelling System (ROMS) to simulate the energy extraction by tidal stream turbines. Emphasis was placed on wake interactions and cumulative effects of individual devices on energy extraction at array scale. This model was applied at the tidal stream energy site of the Fromveur Strait (western Brittany, France) considering an array of horizontal-axis turbines of 10-m diameter, matching the device technology currently operating in the Strait. Two tidal energy metrics were considered to describe asymmetries in tidal current magnitude and direction. The area with reduced asymmetry in current magnitude was selected to implement the turbine array. A nested grid technique was adopted to cascade processes from the regional scale to the high-resolution local farm domain. The computation was conducted over the inner-nested array domain covering the tidal farm with horizontal and vertical resolutions of 1 m, matching the 1/10th turbine diameter (D) recommended to resolve velocity and turbulence intensity along device wakes. The array layout initially followed recommended staggered configurations with longitudinal and lateral spacings of 10D and 5D, respectively. However, during mean spring tidal conditions, the misalignment of peak flood currents induced significant wake interactions that reduced the array output by about 15% in comparison to peak ebb. These interactions were investigated to adapt array layouts, minimise wake interactions, and optimise the energy conversion. By reducing the lateral spacing between devices to 3D (measured centre to centre rather than tip to tip), the flood ebb asymmetry in energy extraction was lowered from 15% to 2%

Assessing the impact of tidal stream energy extraction on the Lagrangian circulation

International audienceWhereas numerous numerical modelling have been conducted to gain further insights about the hydrodynamic characteristics of tidal stream energy sites, the long-term transport of water particles has been routinely ignored. However, a refined evaluation of circulation pathways is fundamental to assess water-quality changes induced by tidal energy converters. The present study investigates the tide-induced Lagrangian circulation in northwestern coastal waters of Brittany (France, western Europe), including numerous islands, shoals and straits with strong potential for turbine farm implementation. Particular attention is dedicated to the hydrodynamic impact of tidal stream power extraction from a series of horizontal-axis turbines. An original approach, based on the coupling of the Lagrangian barycentric method with predictions from a model with high spatial resolution, is considered to assess circulation pathways in a single synoptic view. Spatial results reveal a complex Lagrangian circulation highly controlled by bathymetry and islands geometry. The residual flow in the Fromveur Strait, separating Ushant island fromMoì ene archipelago, displays thus a strong asymmetry, between a prominent northeastern pathway with residual currents up to 0.45 m s −1 and a southward circulation. The positions of cyclonic and anti-cyclonic recirculations, both upstream and downstream the Strait, are furthermore precisely defined. The north eddy is found closely located to the sand bank of the Four. The extraction of tidal stream power impacts mainly the residual Lagrangian circulation along the current stream emerging from the Strait, with a tendency for surrounding eddies to get closer to the tidal stream energy site. The displacements of the south and north recirculations may thus exceed 1.5 km with possible implications on the evolution of surrounding seabed features

Tidal Turbines’ Layout in a Stream with Asymmetry and Misalignment

A refined assessment of tidal currents variability is a prerequisite for successful turbine deployment in the marine environment. However, the numerical evaluation of the tidal kinetic energy resource relies, most of the time, on integrated parameters, such as the averaged or maximum stream powers. Predictions from a high resolution three-dimensional model are exploited here to characterize the asymmetry and misalignment between the flood and ebb tidal currents in the “Raz de Sein”, a strait off western Brittany (France) with strong potential for array development. A series of parameters is considered to assess resource variability and refine the cartography of local potential tidal stream energy sites. The strait is characterized by strong tidal flow divergence with currents’ asymmetry liable to vary output power by 60% over a tidal cycle. Pronounced misalignments over 20 ∘ are furthermore identified in a great part of energetic locations, and this may account for a deficit of the monthly averaged extractable energy by more than 12%. As sea space is limited for turbines, it is finally suggested to aggregate flood and ebb-dominant stream powers on both parts of the strait to output energy with reduced asymmetry

Rôles de l'hétérogénéité des sédiments de fond et des interactions houle-courant sur l'hydrodynamique et la dynamique sédimentaire en zone subtidale (applications en Manche orientale et à la pointe de la Bretagne)

Les processus hydrodynamiques et sédimentaires subtidaux sont étudiés en Manche au niveau du détroit du Pas-de-Calais et de son littoral méridional, et de la Mer d Iroise et de la Baie de Douarnenez, à travers diverses modalités d influence de l hétérogénéité spatiale des sédiments de fond, de la houle via son interaction avec le courant en couche limite de fond et du vent. Une procédure nouvelle d interpolation spatiale des distributions granulométriques permet de respecter l hétérogénéité du substrat. L approche méthodologique combine et confronte mesures de terrain et modélisation numérique. La modélisation, couplant des codes de circulation 3D (COHERENS) et de propagation de houle (SWAN), incorpore des modules de transport sédimentaire non-cohésif multiclasse, un module spécifique d interaction houle-courant et diverses paramétrisations liées aux rides sableuses de fond (géométrie, coefficient de frottement), ainsi qu un calcul optimisé du dépôt à l interface eau-sédiment. La comparaison de prédictions numériques aux mesures recueillies le long du littoral méridional du détroit du Pas-de-Calais montre les interactions entre la bande littorale de sables fins envasés et la variabilité spatio-temporelle de l hydrodynamique en conditions de courant de marée seul et superposé à une houle de Nord. La modélisation est ensuite appliquée à l ensemble de la Manche orientale et centrale. En Baie de Douarnenez, l analyse locale débouche sur une modélisation des influences respectives des courants de marée seuls, dune houle de tempête superposée aux courants de marée et du vent. Enfin, une étude théorique des effets d un cap sur la formation des bancs en drapeau est présentée.Hydrodynamic and sedimentary processes are studied in the English channel, particularly in the southern Dover Straits, and in the Sea of Iroise and the bay of Douarnenez, with emphasis on the effects of spatial heterogeneity of bottom sediments, surface gravity waves via their interaction with current in the bottom boundary layer and wind. The heterogeneity of bottom sediments is ensured through the use of a new spatial interpolation procedure of granulometric distribution. The methodological approach combines and compares field measurements with numerical modelling. Modelling that couples 3D circulation model (COHERENS) and wave propagation model (SWAN), incorporates a set of non-cohesive polydispersive sediment transport equations, a bottom boundary layer wave-current interaction model and various parametrisations of bottom rippies (geometry, friction coefficient), and optimized computation of the near-bed deposition rate. Comparison of results with measurements in the Dover Straits highlight the interaction between the nearshore silty fine sands and the spatio-temporal variability of hydrodynamics under conditions of tide alone and in wind-generated surface gravity waves superimposed on tide. Modelling is then applied to the eastern and central English channel. In the bay of Douarnenez, the local analysis aims at the modelling of respective effects of tidal currents alone, stormy waves superimposed on tidal currents and wind. Finally, a theoretical study of headland-associated sand bank is performed.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

MULTIPHYSICS AND MULTISCALE COUPLINGS FOR WAVE HEIGHT MODELLING IN LE HAVRE HARBOUR

International audienceA sensitivity study is performed to investigate the importance of multiphysics and multiscale couplings for modelling wave height in the outer Seine estuary (English Channel), the harbour of Le Havre and its new infrastructures Port 2000. Multiphysics coupling focuses on the effects of the tide in terms of time-varying water depths and currents on wind-generated surface-gravity waves. Simulations are based on the coupling of phase-averaged wave-propagation modules with circulation modules. This coupling is evaluated by comparing numerical results of the significant wave height with field data collected in November 2008 at two wave buoys moored in the access channel to Le Havre harbour. Predictions confirm a periodic local increase reaching 30 % of the wave height induced by current refraction. Multiscale coupling focuses on the importance of nested computational domains for predicting wave heights in the inner basin of Port 2000. A phase-resolving wave module is set up at this ultimate scale. Need of coupling this module with simulations at the scale of the outer Seine estuary is evaluated by comparing wave-agitation modellings driven by spatially-uniform and spatially-varying forcings with measurements in the inner basin. This comparison highlights the sensitivity of predictions to the directions of waves approaching the harbour entrance

Effects of the coupling between TELEMAC 2D andTOMAWAC on SISYPHE modelling in the outer Seineestuary

International audienceThe present study investigates the need to consider the modifications of the waves fields by the ambient water- depths and currents for sediment transport computations with the TELEMAC modelling system. The application is dedicated to the outer Seine estuary (France, English Channel). The wave propagation module TOMAWAC is coupled with the bidimensional horizontal circulation module TELEMAC 2D. Times histories of depth-averaged mean currents and waves heights are compared with field data collected in the access channel to the harbour of Le Havre. Predictions exhibit a local increase by 30 % of the wave height induced by the current. The sensitivity study compares numerical results issued from SISYPHE integrating hydrodynamics of uncoupled and coupled TOMAWAC – TELEMAC 2D simulations. Computations are performed considering successively suspended transport of cohesive mud and bedload transport of sand. The effect of the hydrodynamic coupling is analysed through the variability of the total maximum wave and current bottom shear stress, the maximum suspended sediment concentration and bedload transport rates and the resulting seabed evolutions

Wave Energy Potential in the Sea of Iroise

International audienceThe wave energy resource in coastal areas of the Sea of Iroise (western Europe) is evaluated with an unstructured version of the phase-averaged wave model SWAN (Simulating WAves Nearshore) for a eight-year period between 2004 and 2011. Numerical predictions are calibrated and evaluated against available measurements of significant wave height and peak period at nine wave buoys in offshore and nearshore waters. In spite of strong energy dissipation in shallow water, up to 60 % of wave power, the medium-term evaluation of the resource reveals major coastal energetic patterns, between 20 and 35 kW m −1 , off the isles of Ushant and Sein and in the nearshore areas of the Crozon Peninsula, the bay of Audierne and the northern coastline. The variability of wave power production is estimated revealing, in accordance with previous numerical estimations over European shelf seas, significant inter-annual and inter-seasonal evolutions at the scale of the Sea of Iroise. Changes are particularly noticeable during the winter period with opposite situations in the distribution of monthly averaged wave energy flux. In the perspective of implementation of wave energy converters, the local distributions of energy flux against periods and directions are finally investigated in areas of maximum mean wave power. Index Terms—marine renewable energy, wave power, SWAN, unstructured grid, Brittany, western Europe

Modélisation des mises en suspension des sédiments de fond dans le détroit du Pas-de-Calais en condition de tempête – comparaison avec les observations

International audienceA three-dimensional modelling of polydispersive suspended sediment transport based on the coupling of the circulation model COHERENS and the wave propagation model SWAN is applied in the Dover Strait and its coastal adjacent areas during a storm. The model reproduces measurements of waves parameters, tidal currents, bottom tidal shear stress velocity and the total suspended sediment concentration off Merlimont beach. The wave effects on these parameters and the concentration of the different classes of suspended sedimentary particles are highlighted.Un modèle hydro-sédimentaire tridimensionnel multiclasse basé sur le couplage des codes de circulation COHERENS et de propagation de houle SWAN est appliqué à l'environnement du détroit du Pas-de-Calais et des zones côtières adjacentes au cours d'une tempête. Le modèle reproduit les mesures de houle, des courants de marée, de la vitesse de frottement liée au courant et de la concentration de sédiment en suspension totale au large de la plage de Merlimont. Les effets de la houle sur ces paramètres et les concentrations des diverses classes sédimentaires en suspension sont mis en évidence

Effets d’une houle de tempête sur les mises en suspension des sédiments de fond dans le détroit du Pas-de-Calais.: Comparaison de la modélisation avec les observations

International audienceA three-dimensional modelling of suspended multicomponent sediment transport based on the coupling of the circulation model COHERENS and the wave propagation model SWAN is applied in the Dover Strait and its coastal adjacent areas during a storm. The model reproduces measurements of waves parameters, tidal currents and the total suspended sediment concentration off Merlimont beach. The wave effects on these parameters and the concentration of the different classes of suspended sedimentary particles are highlighted.Un modèle hydro-sédimentaire tridimensionnel multiclasse basé sur le couplage des codes de circulation COHERENS et de propagation de houle SWAN est appliqué à l'environnement du détroit du Pas-de-Calais et des zones côtières adjacentes au cours d'une tempête. Le modèle reproduit les mesures de houle, des courants de marée et de la concentration de sédiment en suspension totale au large de la plage de Merlimont. Les effets de la houle sur ces paramètres et les concentrations des diverses classes sédimentaires en suspension sont mis en évidence