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Environmental flows are generally characterized by complex bed morphology and high current speeds. Such configurations favor the formation of vortex structures that strongly affect hydrody-namics and sediment transport. Large-Eddy Simulation (LES) enables investigation of the dynam-ics of the largest turbulence scales and, thanks to enhanced calculation resources, has now become applicable for simulating environmental flows. In this paper, a LES approach is developed in a CFD code (TELEMAC-3D), which was originally developed to simulate free surface flows using RANS methods. The present developments involve implementing subgrid models, boundary con-ditions and numerical schemes suitable for LES. The LES version of TELEMAC-3D was validated by comparing results on the model with experimental data for flow past a cylinder. Then, the model was applied to a test case representing flow over dunes. After validating the hydrodynamics, the model was used to assess the bottom shear stress, using both a RANS and a LES approach. Com-parison highlighted the potential contribution of LES to investigating the hydrodynamic forces acting on the bottom.

The deployment of tidal turbines requires a precise hydrodynamic characterisation of the production site. Acoustic Doppler Current Profilers (ADCP), usually employed for measuring the time-mean characteristics of environmental flows, could also be used for assessing the main features of turbulence. ADCP measurements are sensitive to many sources of uncertainties associated mainly with the spreading of the beams or the assumptions made on flow homogeneity. The ability of ADCPs to accurately measure the hydrodynamic parameters of a given flow can be tested on a synthetic dataset. However, it is difficult to generate a dataset representative of a real environmental flow. In this work, large-eddy simulation of a high Reynolds flow over a rough seabed is performed and used to assess the accuracy of two, coupled, 4-beam ADCP systems forming an 8-beam arrangement. The study confirms the relevance and efficiency of the tested 8-beam configuration for the characterisation of turbulence. The results near the seabed are of a lower quality, with up to 50 % error on the Reynolds stresses for elevations under twice the roughness height, which questions the interpretation of ADCP measurements in the lower part of the water column. Also, the spatial averaging over ADCP cells leads to an underestimation of the turbulence intensity of 10 % to 20 %.

A system of two coupled four-beam acoustic Doppler current profilers was used to collect turbulence measurements over a 36-h period at a highly energetic tidal energy site in Alderney Race. This system enables the evaluation of the six components of the Reynolds stress tensor throughout a large proportion of the water column. The present study provides mean vertical profiles of the velocity, the turbulence intensity and the integral lengthscale along the streamwise, spanwise and vertical direction of the tidal current. Based on our results and considering a tidal-stream energy convertor (TEC) aligned with the current main direction, the main elements of turbulence prone to affect the structure (material fatigue) and to alter power generation would likely be: (i) the streamwise turbulence intensity ( I x ), (ii) the shear stress, v ′ w ′ ¯ , (iii) the normal stress, u ′ 2 ¯ and (iv) the vertical integral lengthscale ( L z ). The streamwise turbulence intensity, ( I x ), was found to be higher than that estimated at other tidal energy sites across the world for similar height above bottom. Along the vertical direction, the length ( L z ) of the large-scale turbulence eddies was found to be equivalent to the rotor diameter of the TEC Sabella D10. It is considered that the turbulence metrics presented in this paper will be valuable for TECs designers, helping them optimize their designs as well as improve loading prediction through the lifetime of the machines. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Sites suitable for the deployment of tidal turbines generally show a combination of complex seabed morphologies and extreme current magnitudes. Such configurations favour the formation of vortices, which can be very powerful. Anticipating the vortex effect on the turbine performance and/or lifespan requires refined description of the turbulence. Thanks to increased calculation resources, large-eddy simulation (LES) can now be applied to natural flow. An LES approach developed within the TELEMAC-3D open-source software is presented here. After validating the model with in-situ measurements, the model is applied to characterize the flow statistics of the Alderney Race. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Numerical modelling of the flow interactions between tidal turbines in arrays is a prerequisite to assess the energy production potential and to optimise the layout of tidal stream energy farms. It requires (i) a refined representation of the tide propagation and (ii) a reliable estimate of the flow characteristics around the turbines. The Actuator Disk (AD) theory is recognised as a reliable parameterisation to approach the far wake of fixed horizontal-axis turbines and was therefore implemented to provide recommendations as regards to the design of arrays, especially the optimal spacing between devices. However, analysis of the arrangement of tidal stream turbines are typically restricted to schematic test cases with idealised bathymetry and inflow conditions. The present study aims to investigate the flow interactions between turbines in a stream energy site with spatially- and time-varying tidal conditions. The site of application is the Alderney Race (English Channel), an area with strong potential for the exploitation of tidal energy in European shelf seas. The three-dimensional numerical model Telemac3D is used to capture the flow at both the regional and the array scales. Predictions are first assessed against ADCP measurements acquired around the island of Alderney. A series of simulations, representing turbines with the AD theory, are then performed to investigate the influence of the wake interactions on the energy production of individual devices. In those simulations, the turbines are represented with the AD theory. The zone occupied by the turbines is 20D-long and 15D-wide (D is the turbine diameter). Different layouts (isolated, aligned and staggered turbines) and different longitudinal and lateral spacings are considered to investigate the effect of the turbines’ arrangement and density on the array energy production. The results show that, for a given turbines density within the array, the staggered layout produces more than the aligned layout (+16%). When the turbines are staggered, a minimal lateral spacing of 5D is required to avoid wake overlapping. However, when the turbines are aligned, the lateral spacing has a small influence on the turbines production. It is therefore possible to pack the turbines close to each other in the lateral direction. The simulations also outline that the turbulence develops much faster within the array when the turbines are aligned.

Strong tidal power flows are highly turbulent. The role of the seabed roughness on the generation of turbulence is not yet fully understood. Numerical simulation is a promising observation method of turbulent events in three dimensions. In this work, the lattice Boltzmann method is used to simulate turbulent tidal flow at a local scale. The capacity of the method to treat large simulation meshes is exploited to attenuate computational costs. Vertical profiles of velocity and velocity variance are generated from the simulation of a 0.23 km2 domain. The simulation is performed with realistic tidal conditions and seabed morphology. Numerical results are in good agreement with in situ acoustic Doppler current profiler measurements. Large turbulent flow structures generated by seabed roughness are observed in the water column through λ2 criterion visualisation.

Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at mid-depth during peak flooding and ebbing tidal conditions. The dissipation rate e and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided e and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35 % and 20 % respectively. The integral lengthscale was found to be 2–3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines.

International audience; Marine renewable energies (MRE) allow the diversification of the energy mix and are thus developing rapidly. The extraction of tidal energy is one of the most promising MRE and is nowadays in a preindustrial phase. The Alderney Race (Raz Blanchard in French), situated between the island of Alderney (Channel Islands) and the Cap de la Hague in France, capitalizes a resource estimated between 5 and 14 TWh/year. This great tidal resource attracts tidal energy developers. Harnessing the high potential of the Alderney Race requires deploying numerous turbines in arrays. The purpose of this paper is to investigate the influence of the wake effects on the electrical dynamics of commercial tidal farms. The proposed methodology is applied to a hypothetical tidal farm in the Alderney Race. The influence of the wake effects on the power produced by a tidal farm is assessed by comparing simulations where the effect is neglected to simulations with wake effects. Two operational conditions are analyzed: Normal operation (one-day simulation with varying tidal speed); grid disturbances (voltage dip at the tidal farm point of common coupling). Simulations show that neglecting the wake effect implies significant errors in active power estimation at the point of common coupling during normal operation. Moreover, differences in the behavior of the system during voltage dip occur if the wake effect is not taken into account.

International audience; A tidal turbine is a device converting hydrodynamic power into electrical power. Lately, more and more projects have been developed in order to optimize the productivity of this kind of energy. In such research with industrial interest, under the impact of the wake effect on the output power, the analysis of a tidal farm layout is regarded as the first priority. Simple approaches such as those developed for wind farms could be used in tidal turbine arrangement optimization. These methodologies can be improved by taking into account the turbulence in tidal farms and tidal turbines' mechanical characteristics. The goal of this work is to propose a predictive analytical model to estimate the tidal speed in the far wake of tidal turbines with small diameter to depth ratio (20% here). It is a first step prior to integrate the wake model in a tidal farm layout optimization algorithm. The wake model development is achieved reanalyzing the far wake's equations used in wind farm applications. A turbine represented by an Actuator Disc (AD) in conjunction with a Computational Fluid Dynamics (CFD) numerical model is used as a reference for this purpose. The CFD-AD model has been validated with experimental results from literature. The novelty of the present work consists in expressing the far wake's radius expansion as a function of the ambient turbulence and the thrust coefficient. The proposed equation is used in conjunction with the Jensen's model in a manner that the velocities downstream a tidal turbine can be estimated. The velocity distribution in the far wake of a single turbine obtained by the proposed model is in good agreement with the CFD numerical model. As a matter of fact, the model provides satisfactory accuracy in the cases of two parks: one with five aligned turbines and one with ten staggered turbines.

Highly energetic turbulent flow structures are observed in strong tidal flows. If they are suspected to result from interactions between the flow and the seabed morphology, the physical processes involved in their generation, as well as their impact on the structure of the flow, are not yet fully understood. Here, the lattice-Boltzmann method is used to simulate a strong tidal flow in large-eddy simulation. The effect of the seabed morphology on the hydrodynamic characteristics of the flow is studied. A high spatial variability of the turbulent kinetic energy (TKE) production is observed. The flow average velocity is significantly reduced in areas of high TKE production. These areas are not necessarily associated with the largest seabed landforms. However, some seabed landforms of specific shapes are identified as turbulence generators. The areas of high turbulence are associated with trails of vortices successively released from the seabed and following similar trajectories. The generation of a large coherent flow structure is observed at the intersection of two vortex trails, suggesting that such a structure, that could be identified as the large boils commonly observed at the surface of strong tidal power flows, could result from the aggregation of smaller vortices.

The tides are a predictable, renewable, source of energy that, if harnessed, can provide significant levels of electricity generation. The Alderney Race (AR), with current speeds that exceed 5 m s −1 during spring tides, is one of the most concentrated regions of tidal energy in the world, with the upper-bound resource estimated at 5.1 GW. Owing to its significance, the AR is frequently used for model case studies of tidal energy conversion, and here we review these model applications and outcomes. We examine a range of temporal and spatial modelling scales, from regional models applied to resource assessment and characterization, to more detailed models that include energy extraction and array optimization. We also examine a range of physical processes that influence the tidal energy resource, including the role of waves and turbulence in tidal energy resource assessment and loadings on turbines. The review discusses model validation, and covers a range of numerical modelling approaches, from two-dimensional to three-dimensional tidal models, two-way coupled wave-tide models, Large Eddy Simulation (LES) models, and the application of optimization techniques. The review contains guidance on model approaches and sources of data that can be used for future studies of the AR, or translated to other tidal energy regions. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Resource assessment is an important phase of tidal energy projects. Extensive literature is nowadays available on this topic with most studies focussing on the time-scales of the tide or the spring-neap cycle. The evolution of the resource over longer time scales has yet received very little attention in the scientific literature. In the present contribution, we investigate the effect of the 18.6 year lunar nodal cycle on the depth-averaged current of the Alderney Race (France). Firstly, we identify the major constituents of the study zone with the UTide Matlab functions. Two datasets are used: a 36.5 day time-series of depth-averaged current and elevations acquired in the Alderney Race and a 24 year times-series of tidal heights acquired in a neighbouring harbour. The analysis shows that the tide is mostly controlled by the semi-diurnal constituents M2, S2 and N2. Secondly, we verify the ability of UTide functions to predict the changes in amplitude of the major components over decadal time-scale. Thirdly, we predict the tidal resource of the Alderney Race over the period 2014 – 2034 and analyse the yearly changes within this period. The results show that the change of resource is of the order of +/-10% and that the next decade (2020–2029) will be characterised by a significant reduction of the resource.

International audience; 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%.

International audience; In the next years, tidal farm will increase in size and density of devices to obtain an economically significant amount of energy. Because of the high density of the devices within the farm, the turbines will be hydro-dynamically coupled. The negative effects of this coupling could be reduced by optimizing the tidal farm layout or by optimizing the operating point of each turbine within the farm. In this paper, we investigate the second strategy. The objective is to maximize the overall production of the farm. The power of the upstream tidal turbines is then reduced, allowing the increase of power of the downstream turbines. The Binary Particle Swarm Optimization (BPSO) Algorithm is used in order to find the rotational speed for each device such that the net energy yield is optimized over a tidal cycle. The proposed methodology is firstly applied to an ideal case (i.e. tidal flow constant in magnitude and direction) to assess the influence of the machines density, the ambient turbulence rate and the speed magnitude on the power improvement rate. Secondly, the method is applied to a hypothetical tidal farm located in the Alderney Race (Raz Blanchard in French), situated between the Alderney Island and La Hague Cape (France). The results of these scenarios indicate that the proposed strategy is interesting mainly when the density of devices is high and/or when the ambient turbulence rate is lower than 10%. In this cases, it is possible to improve the rate by 2.5%.

International audience; The purpose of this paper is to present a semi-analytic model designed to optimize tidal farm layouts bymaximizing the mechanical power production. A meta-heuristics method is used to find the turbineplacement which minimizes the flow interaction between the turbines. The velocities in the wakes of tur-bines are simulated with an analytic model. The methodology is first applied to idealized cases: constantcurrent magnitude and direction, and flow aligned with the turbines. Those preliminary tests permit totest the consistency of the results. In particular, they show that the optimal density of the devices growswith increasing turbulent intensities or increasing upstream velocity magnitude. The methodology isthen applied to a site located in the Alderney Race (Raz Blanchard in French), situated between theAlderney Island and La Hague Cape (France). The results show that the optimal placement is influencedby the asymmetry of the tidal current and that the best layout is strongly dependent on the directionalspreading of the current with respect to the predominant direction.

International audience; Tidal turbine will be installed in area with high current and high turbulence level. A characterisation of this last is required. The aim of the project THYMOTE is to characterize and understand the generation of eddies from smaller to several tens of meters. Three technics are used: Numerical modelling, Physical modelling, field measurements. Physical and numerical modelling show clearly the appearance of the eddies close to the bottom in presence of dunes or rocks and their motion towards the free surface.

International audience; The present study aims to develop a model to simulate in 2 dimensions, the motion of the fluid and of the loaded Darrieus turbine without imposing the rotational speed. A resistive torque has been added to account for the load of a generator. Experimental and numerical studies were conducted and presented here. The experiment was setup in the air first, on a wind turbine for reason of convenience. The simulations were made with Fluent (v14) for which a User Defined Function was developed. Then the experimental conditions were reproduced numerically for three cases. The study focuses on the determination of a factor (f) giving the relationship between the experimental and the simulated resistive torque in 2 dimensions. The results are quite satisfying in this preliminary study as they demonstrate that the model developed works well. Some improvement has to be made to refine the value of f .

The Actuator Disk concept is suitable to simulate the flow interactions between tidal turbines. As the computational expense of AD simulations is too large for applications at regional scale, the “bed friction” method is generally preferred. It consists in applying an enhanced drag over the tidal farm area. The main drawback is that the flow around each turbine is not resolved and that the flow interactions between the turbines’ wakes are neglected. The extent to which this simplification affects the accuracy of the large scale flow is not well understood. Here, we propose a methodology for representing large arrays of tidal turbines in Shallow Water Equations solvers. It consists in representing individual turbines as small areas where a sink momentum term is applied. The sink term is calculated from the vertical integration of the force exerted on an AD. After validating the model, we apply the methodology to simulate the effect of 45 turbines placed in the Alderney Race. The results are compared to the results obtained with the “bed friction” method. The two approaches give similar results because the hydrodynamics conditions of the Alderney Race favor the mixing of the wakes which prevents the flow interactions between the turbines.

A regional 2DH hydrodynamic model is used to estimate the tidal stream resource of a site located in a macrotidal environment with extreme tidal velocities. The study site is the Alderney Race (Raz Blanchard in French) which is a straight located in the English Channel between the Alderney Island and La Hague cape (France). The estimation of the resource is used to build two realistic tidal energy extraction scenarios consisting in placing a 290 MW tidal turbine array in two different areas. Then, we analyze the impact of turbines on the hydrodynamics and the sediment transport. The hydrodynamic perturbation is restricted to the vicinity of the array where the mean current velocity reduction reaches 0.3 m/s locally (corresponding to 15% of the baseline velocity). Focusing on the variable driving the bedload (the critical erosion threshold exceedance), we confirm that tidal energy extraction tends to reduce the bedload rate and to deflect the sediment fluxes. Our simulations with a simple model for the suspended sediment transport suggest that tidal energy extraction has a significant effect on the area of deposition of the particles transiting through the tidal farm. For the baseline, the sediment particles transiting through the tidal farm deposit mostly in the eastern part of the English Channel. Depending on the location of the tidal farm, the sediment mass balance between the eastern and the western part of the English Channel changes drastically.

International audience; The commercial exploitation of kinetic energy from tidal current involves deploying multiple turbines constituting a park or a farm. The turbines located within the park suffer from the disrupted flow due to the wake of machines located further upstream. This will potentially alter their performance. The evolution of the wake is influenced by the environment and by the machine. A two‑dimensional model of a vertical axis turbine of Darrieus type was per‑ formed to investigate the flow within and downstream the turbine. The relative influences on the characteristics of the wake of two parameters were evaluated: the speed of the upstream flow and the Tip speed ratio λ. The latter seems to be dominant for the lack of speed while the level of turbulence is a function of the two parameters setting.

International audience; This study deals with the influence of static and dynamic objects on the airflow (generated by a unidirectional air supply) inside an “open” ventilated system used in an agro-food production line. Firstly, the presence of operator’s arms, considered here as objects, leads to the appearance of recirculating zones under each arm. The influence of different positions of the static arms protected by gloves on the airflow organization has been studied. Secondly, modifications induced by the motion of a conveyor system or of moving arms on the airflow have been explored.In an experimental approach, airflow organization has been studied by visualization and by Particle Image Velocimetry for static cases and moving conveyor case.Numerical simulations were carried out using Computational Fluids Dynamics codes. Results obtained for moving gloves cases highlight the contribution of dynamic meshes tools, implemented in the CFD codes to make simulations of moving objects.