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Commercial scale tidal stream turbines (TST) are expected to be deployed in shallow water where the depth varies from 1.5 to 3 turbine diameters. In this study, numerical simulation is conducted at realistic hydrodynamic conditions of potential tidal sites using the stationary actuator disc method at ambient turbulence varying from 5% to 20%, a range of rotor realistic rotor thrust coefficient from 0.64 to 0.98 and a rotor-diameter-to-depth ratio of 20% to 60%. The result shows that the TST wake is affected by the rotor-diameter-to-depth ratio, ambient turbulence, and thrust coefficient. The new empirical model is in accordance with the numerical simulation of a full-scale turbine and is validated with the TST experiment at different rotor-diameter-to-depth ratios with reasonable results in the far wake. This low computational model can benefit the investigation of tidal turbine parks at different configurations where the far wake is pertinent. [ABSTRACT FROM AUTHOR]

Static axial induction control and tilt control are two strategies that have the potential to increase power production in wind farms, mitigating wake effects and increasing the available power for downstream turbines. In this study, wind tunnel experiments are performed to evaluate the efficiency of these two techniques. First, the axial induction of upstream turbines in wind farms comprising two, three, and five turbines is modified through the tip-speed ratio. This strategy is found to be ineffective in increasing power extraction. Next, the power extraction and flow through a two-turbine wind farm are evaluated, considering different tilt angles for the upstream turbine, under two levels of incoming flow turbulence intensities and turbine spacing distances. It is shown that forward tilting increases the overall power extraction by deflecting the wake downwards and promoting the entrainment of high-speed fluid in the upper shear layer, regardless of the turbine spacing distance and turbulence intensity level. Also, the wake is seen to recover faster due to the increased shear between the wake and the outer flow. Tilting a turbine backward deflects the wake upwards and pulls low-speed flow from under the turbine into the wake space, increasing the available power for downstream turbines, but it is not enough to increase global power extraction. Moreover, since the wake deflection under backward tilting is not limited by ground blockage, it leads to larger secondary steering compared with forward tilting. Finally, it is demonstrated that the secondary steering of the downstream turbine's wake influences the flow encountered by a turbine positioned farther downstream. [ABSTRACT FROM AUTHOR]

Recently, wind turbine research has switched focus to vertical axis wind turbines due to the extensive research that has been performed on horizontal axis wind turbines and the potential of vertical axis wind turbines in built-up areas. This study aims to analyse the performance of a small-scale hybrid vertical axis wind turbine that can switch from functioning as a Darrieus (lift) turbine to a Savonius (drag) turbine by rotating the blades. The turbine was analysed using 3D computational fluid dynamics (CFD) simulations in ANSYS Fluent as the primary method, and the findings were verified using wind tunnel experiments. During the analysis, design parameters such as the blade length, diameter, and number of blades were varied to determine if the design had room for improvement. It was found that the current design of the turbine has an optimal efficiency of 12.5% in the Darrieus configuration, which was found to increase when the diameter or blade length was increased. The Savonius configuration was found to be more efficient at low tip-speed ratios (<0.14), and its efficiency could be increased by adding more blades. The experiments found similar trends to the simulations; however, the efficiencies obtained were on average a tenfold increase from the simulation. Implementing the changes that increased efficiency leads to an increased wake recovery distance, making it less suitable for use in a wind farm. [ABSTRACT FROM AUTHOR]

Ducted turbines have the potential to generate large-scale electricity when placed in tidal farms. Understanding the fluid-duct interaction would provide vital information for tidal farm projects. This paper describes flume experiments to investigate the performance and far wake impact on flanged duct turbines in bi-directional tidal flows. Results showed that ducted turbines generate 40% more power per rotor unit area than bare turbines. The turbines underperformed for each device in tandem configurations. However, ducted turbines outperformed bare turbines by 15%. The decreasing range of wake velocities and increasing turbulence intensities of the devices were shown to be greater in the ducted turbines. The drop in wake velocity is caused by kinetic energy extraction, and duct and rotor blockage effects. For the isolated bare and ducted turbines, full flow recovery occurs at around 13 and 20 rotor diameters, respectively. In tandem arrangements, while the bare turbine operated as if it was in isolation from 24 rotor diameters downstream, the wake flow in the ducted turbines persisted beyond. However, in terms of power output per rotor unit area, the deployment of additional ducted turbines may be advantageous. Though in the present state of development, precise economic estimates of energy costs are unavailable. • Performance and far wake impact on flanged duct turbine are experimentally studied. • Flanged duct turbines outperform the bare turbine both in isolation and within an array. • Decreasing range of wake velocities is greater in ducted turbines than in bare turbines. • Increasing range of turbulence intensities is greater in ducted turbines than in bare turbines. • The proposed flanged duct shape may well perform in both flood and ebb current flows. [ABSTRACT FROM AUTHOR]

With tidal energy demonstration projects with one or a small number of turbines having provided very positive results, the technology is moving to the commissioning and operation of tidal energy farms. For this next step, the understanding of the wakes generated downstream of the turbines is crucial to optimize the array performance. To date, the analysis of wakes of MW-scale tidal turbines has been made by numerical methods or experimentally with downscaled rotors. However, no consensus on a methodology to characterize wakes generated by full-scale turbines based on data measured on-site has yet been reached. The present paper introduces a new method to compare current velocity data measured before and during turbine operation that minimizes the impact of the spatial and temporal variability of tidal currents, thus enabling the estimation of the velocity deficit caused downstream of the turbine. Through this method, a characterization of the near wake was possible, with velocity deficits of 0.537, 0.463, 0.469 and 0.431 at 2D, 3D, 4D and 5D from the turbine. Results from this paper present a very valuable tool for the validation of numerical models aiming to estimate the wake losses in tidal energy farms. • Wake from a 0.5 MW bottom-fixed tidal turbine in a tidal site is characterized. • A new method to calculate velocity deficit with vessel-mounted ADCP data is presented. • Velocity deficit at the rotor hub height decreases from the turbine to a 5D distance. • Farther from 5D, the turbine impact on current velocity dissipates. [ABSTRACT FROM AUTHOR]

Data driven approaches are utilized for optimal sensor placement as well as for velocity prediction of wind turbine wakes. In this work, several methods are investigated for suitability in the clustering analysis and for predicting the time history of the flow field. The studies start by applying a proper orthogonal decomposition (POD) technique to extract the dynamics of the flow. This is followed by evaluations of different hyperparameters of the clustering and machine learning algorithms as well as their impacts on the prediction accuracy. Two test cases are considered: (1) the wake of a cylinder and (2) the wake of a rotating wind turbine rotor exposed to complex flow conditions. The training and test data for both cases are obtained from high fidelity CFD approaches. The studies reveal that the combination of a classification-based machine learning algorithm for optimal sensor placement and Bi-LSTM is sufficient for predicting periodic signals, but a more advanced technique is required for the highly complex data of the turbine near wake. This is done by exploiting the dynamics of the wake from the set of POD modes for flow field reconstruction. A satisfactory accuracy is achieved for an appropriately chosen prediction horizon of the Bi-LSTM networks. The obtained results show that data-driven approaches for wind turbine wake prediction can offer an alternative to conventional prediction approaches. [ABSTRACT FROM AUTHOR]

The lattice Boltzmann method is used to model a horizontal axis tidal turbine. Because tidal turbines generally operate in highly turbulent flows, a synthetic eddy method is implemented to generate realistic turbulent inflow condition. The approach makes use of the open-source code Palabos. Large eddy simulation is employed. A coupling between an immersed boundary method and a wall model is realized to model the turbine. Calculations are performed at two different turbulence rates. The upstream flow condition is first set up to match with experimental results. Numerical simulations of a tidal turbine with realistic turbulent inflow conditions are then realized with the lattice Boltzmann method. The approach is found to be in good agreement with experimental data. Cases with three different inflow turbulence rates are simulated. An almost linear evolution with the turbulence rate is observed for the axial velocity deficit. An analysis of the propagation of tip-vortices in the close wake is carried out. It is found that turbulence has a great impact on the tip-vortices propagation envelope. [ABSTRACT FROM AUTHOR]

Fast and effective numerical models describing the effect of platform motion on the performance of floating offshore wind turbines (FOWTs) are fundamental to assess energy harvesting potential in large offshore wind farms. The purpose of this paper is to implement a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) for FOWTs aerodynamics. Such a tool aims at complementing reasonable accuracy and affordable computational effort while being able to investigate the effects of the platform motions on the wake evolution. The actuator line model for FOWTs is developed by implementing a dedicated C++ library in the OpenFOAM toolbox. In addition, a tip treatment is applied to involve the tip effects induced by the pressure equalization from the suction and pressure sides. Results show that employing ALM decreases computational cost and preprocessing time for producing appropriate computational grids, as just about 400k and 600k grids are necessary for solving two representative test cases of fixed-bottom turbines (NREL Phase VI and NREL 5-MW) with reasonable accuracy. The inclusion of platform motion is then introduced, and the results showed that ALM is capable of capturing vortices trajectory, potential blade-vortex interactions, and vortex pairing and vortex ring state phenomenon in FOWTs. [ABSTRACT FROM AUTHOR]

Summary: The progression of wind turbine technology has led to wind turbines being incredibly optimized machines often approaching their theoretical maximum production capabilities. When placed together in arrays to make wind farms, however, they are subject to wake interference that greatly reduces downstream turbines' power production, increases structural loading and maintenance, reduces their lifetimes, and ultimately increases the levelized cost of energy. Development of techniques to manage wakes and operate larger and larger arrays of turbines more efficiently is now a crucial field of research. Herein, four wake management techniques in various states of development are reviewed. These include axial induction control, wake steering, the latter two combined, and active wake control. Each of these is reviewed in terms of its control strategies and use for power maximization, load reduction, and ancillary services. By evaluating existing research, several directions for future research are suggested. [ABSTRACT FROM AUTHOR]

This paper presents the results of hydrodynamic performance and wake assessment for a horizontal axis tidal current turbine. The three-dimensional instantaneous flow field is resolved utilizing the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model and polyhedral mesh. The rotor rotation is simulated by employing the sliding mesh approach. The simulation results using turbulence model of k-ω Shear Stress Transport are also presented for comparison. The simulated thrust and power coefficients and wake characteristics are validated against published experimental results. It is shown that the performance and wake velocity predicted by IDDES are closer to the experimental values than those predicted by k-ω Shear Stress Transport model. In determining the pressure coefficient, only IDDES captures the time-varying fluctuations pertaining to blade generated turbulence. The oscillations in phase-averaged performance coefficients are substantially larger for IDDES because the inflow turbulence in this model is more realistically resolved using Synthetic Eddy Method. The IDDES predicts more detailed vortex structures which has impact on accurate determination of blade pressure distribution, energy dissipation rate and downstream flow field. It is demonstrated that the IDDES model is capable to satisfactorily simulate the hydrodynamics of a horizontal axis tidal current turbine. [ABSTRACT FROM AUTHOR]

The tidal flow between bridge pillars and through open barriers is a promising source of ocean energy which can be exploited using tidal stream turbines, as proven recently by operational demonstration plants. The aim of this study is to clarify the consequences for the power output of tidal turbines when placing them in a hydraulic structure. To this end, experimental measurements of turbine power and wakes are performed, using a down-scaled turbine mounted at a submerged weir. The results are compared to an analytical model, validating its range of application for optimising turbine-weir geometries. The experimental data show that the power coefficient of the turbine can be increased by optimising the blockage of the channel and the distance between the turbine and the structure, which is related to the wake configuration. In this way, the power coefficient increased by 40% when the turbine was re-positioned from the upstream to the downstream end of the structure. The theoretical model could reproduce the measured power within 10% accuracy, proving its value as a rapid assessment tool. As such, this work advances the knowledge needed to meet targets on the transition towards renewable energy. • We present unique experimental data of turbine power and wakes. • Power output can be increased by 40% by optimising turbine position relative to a weir. • A theoretical performance model is validated with the data. • Schematising the wakes correctly is crucial to improve the model predictions. • The data reveal a geometry with high power output and small hydraulic resistance. [ABSTRACT FROM AUTHOR]

The development of marine current turbine arrays depends on the understanding of the interaction effects that exist between turbines in close proximity. Moreover, the ambient turbulence intensity also plays a major role in the behaviour of tidal turbines. Thus it is necessary to take ambient turbulence into account when studying interaction effects between several turbines. In order to highlight these interaction effects, experiments have been carried out in the IFREMER flume tank. These experiments focus on interactions between three horizontal axis turbines. This paper presents the experimental results obtained for three configurations with two ambient turbulence intensity rates. The results are presented in terms of turbine wakes and performance. The wake characterisation presents complex features for the three configurations and the lowest ambient turbulence rate: upstream turbines wakes are still present at the location of the downstream turbine and their wakes can interact or merge, depending on the tested configurations. On the contrary, for the highest turbulence rate, the downstream turbine wake is not affected in his shape by the two upstream ones which are not visible any more. In fact, as already observed in the previous studies of Mycek et al. [1, 2], the wake shape rapidly spreads out in the stream-wise direction behind the turbines. However, the velocity deficit and the turbulence intensity are higher for the downstream turbine comparing to the upstream ones. In terms of performance, one tested case presents an increase of the downstream turbine power production: when this turbine is exactly in the centre of the two upstream turbines and for the lowest turbulence rate only. A small misalignment of the layout axis with respect to the tidal current may result in a decrease of performance at the end. An analysis of the power spectral density functions of the downstream turbine torque and thrust shows that no signature of the upstream turbines can be found in these answers. Furthermore, the same spectral analysis carried out on the velocity measurements shows no signature of the upstream turbines either, from 3 diameters distance. This result is noticeable for the highest and the lowest tested turbulence cases and whatever the turbines configuration is. • Experimental results of 3 turbines in interaction: wake, power and thrust. • 3 geometrical configurations with 2 turbulence intensities are considered. • Ambient turbulence influence assessment on interaction mechanisms. • Downstream turbine large power losses can be observed. • Power spectral density analysis does not show any upstream turbine signature. [ABSTRACT FROM AUTHOR]

Harbour porpoise echolocation activity within and outside of the FORCE tidal turbine demonstration area in Minas Passage of the upper Bay of Fundy, Nova Scotia, has been monitored by up to eight bottom-moored (30-80 m depth) C-POD click train detectors across 1,210 days between May 2011 and May 2018. Detections occurred on almost every day (98.9% of days), averaging 86.5% across C-PODs, at a median of seven detection positive minutes per day and for a maximum of 44 minutes per day, noting that porpoise detection probabilities using buoy-mounted C-PODs should be considered minimum estimates. No confirmed dolphin click detections were documented. A statistical (Generalized Additive Model using GEE, or GAM- GEE) approach with a logit link function confirmed porpoise presence varied significantly by time of year (peak period mid-June and a lower secondary peak early November), by tidal current speed and tidal height (preference for 0-2.5 m/s ebb tides), by time of day (higher click activity at night), and across the lunar cycle. Seasonal and diel variability may be partly driven by prey accessibility. C-POD monitoring performance (termed Percent Time Lost) varied spatially and temporally. The Percent Time Lost was associated with non-biological sediment movement-related clicks causing the C-POD's maximum click detection memory buffer to be exceeded, notably during very high (e.g., spring tide) tidal speeds. The effects of these covariates were controlled for within the subsequent GAM-GEE modelling process to assess the effects of the installation and 130 days of operations of a 16 m diameter 2 MW OpenHydro tidal turbine. Importantly, despite the winter turbine deployment period, the C-POD data highlighted that porpoises were not excluded by turbine installation and operations over the mid-field study area, with click detections occurring on 98.5% of days, averaging 82% across C-PODs, with a median of three minutes per day. However, GAM-GEE modelling identified a significant decrease in porpoise click activity when the turbine was operational; this effect varied spatially, with a significant decrease in porpoise activity at the two sites closest (200-230 m) to the operating turbine berth of the five C-POD monitoring sites assessed. C-PODs at two sites that were more than a kilometre away did not show a significant turbine effect, while a third site, at 1,690 m from the turbine and in deeper water to the south, showed an increase in porpoise click detections. A longer time series of turbine deployment is believed required before robust conclusions can be drawn on turbine effects. Nevertheless, it was clear that after turbine removal, porpoise click activity rates returned to pre-installation baseline rates. [ABSTRACT FROM AUTHOR]

Hydrokinetic turbines, which convert the kinetic energy of flowing water into electrical energy, are increasingly being deployed in arrays, much like multiple wind turbines are commonly organized into a wind farm. Deploying multiple devices in an array can increase the amount of energy produced from a site and tends to make project economics more favourable. Despite growing interest in hydrokinetic turbine arrays, relatively few researchers have studied arrays and limited information exists concerning the character of the turbulent flows within arrays and methods for optimizing array layout, particularly for cross-flow turbines. This paper presents results from a set of experiments conducted in a current flume in which the wakes produced by a single cross-flow turbine and pairs of identical cross-flow turbines were measured in detail. Two different ambient turbulence intensities were studied so that the effect of turbulence level on wake recovery could be quantified. The velocity data has been analyzed to identify the mean velocity and the turbulence intensity in the turbine wake(s). These experiments form one component of a larger study that aims to develop numerical methods and guidelines to help project developers optimize the design of turbine arrays. [ABSTRACT FROM AUTHOR]

The efflux velocity is the basis for the prediction of turbine wake. A novel energy coefficient is defined to propose a new theoretical equation to predict the efflux velocity of tidal current turbine in this paper. Several CFD cases with different tip speed ratio and solidity is conducted using the DES-SA model. In order to overcome the limitations of the axial momentum theory, the effects of tip speed ratio and solidity on the efflux velocity are studied and the energy coefficients with different tip speed ratio and solidity are determined using the proposed equation based on the CFD results. Several semi-empirical efflux velocity equations are finally proposed by fitting the equation of the energy coefficient with tip speed ratio. The application of these equations in the prediction of wake flow and the power calculation of tidal turbine are also introduced in this paper. [ABSTRACT FROM AUTHOR]

Understanding the wake interactions among turbines is important for developing efficient tidal farm configurations. The present study compares the power and wake generated by a 2-row by 1-column array separated at three lateral spacings with those of a single tidal stream turbine. According to the results, power fluctuations increase with the tip speed ratio, with mean values corresponding to the inverse-u shape prediction of the blade element methods. For a single turbine, the cloud diagram of the lateral velocity deficit follows a half-elliptical contour with its center at the hub and initial covertices at the tip. The twin turbines largely exhibit these symmetric transversal distributions; however, they are doubled, with wake fusion occurring at the mid-width and closer with turbines approximation. The superposition of Gaussian-type velocity distributions matches the experimental bimodal functions with horizontally decreasing heights only after x > 6D. Vertically, the TIx in all conditions displays two strong areas, just above and below the turbine, owing to the interaction of the tip with tower vortices and the free and bed surface. In the two-unit arrangement, the blockage effect and the velocity of the mid-passage flow increase with the lateral separation of the turbines. This has a direct effect on wake combination, width, and recovery. • The wake interaction among two adjacent turbines is experimentally investigated under three lateral spacings. • In the twin-concept, the blockage and mid-passage velocity augments with turbines lateral farness. • Under the studied arrangements, the mean performance of the turbines agrees well with the steady-based predictions. • The unsteady power response relates more with the turbulence intensity and rotational speed, rather than lateral spacing. [ABSTRACT FROM AUTHOR]

A detailed wake analysis of two different turbine concepts is conducted to gain a fundamental understanding of the main energy recovery processes at play in each case. An axial-flow turbine and a cross-flow turbine are considered. Both operate near their respective optimal efficiency conditions in a uniform oncoming flow and at a Reynolds number of 10 7 . Three-dimensional Delayed Detached-Eddy Simulations (DDES) are carried out and the time-averaged Unsteady Reynolds-averaged Navier–Stokes (URANS) equations are used as a post-processing tool in order to assess the importance of the various contributions affecting the wake recovery quantitatively. It is found that the dominant mechanism is fundamentally different between the two turbine technologies. Indeed, while the axial-flow turbine's wake is strongly influenced by an instability phenomenon leading to a significant turbulent transport, the cross-flow turbine's wake recovery is found to be much more related to the mean spanwise velocity field. As a result, unlike the axial-flow turbine's wake dynamics which is highly dependent on the turbulent characteristics of the oncoming flow, the cross-flow turbine's wake is expected to be less sensitive to these turbulent characteristics but highly dependent on the geometric characteristics of the turbine such as the turbine's aspect ratio. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a semi-analytic model designed to optimize tidal farm layouts by maximizing the mechanical power production. A meta-heuristics method is used to find the turbine placement which minimizes the flow interaction between the turbines. The velocities in the wakes of turbines are simulated with an analytic model. The methodology is first applied to idealized cases: constant current magnitude and direction, and flow aligned with the turbines. Those preliminary tests permit to test the consistency of the results. In particular, they show that the optimal density of the devices grows with increasing turbulent intensities or increasing upstream velocity magnitude. The methodology is then applied to a site located in the Alderney Race (Raz Blanchard in French), situated between the Alderney Island and La Hague Cape (France). The results show that the optimal placement is influenced by the asymmetry of the tidal current and that the best layout is strongly dependent on the directional spreading of the current with respect to the predominant direction. [ABSTRACT FROM AUTHOR]

The increased turbulence behind the turbine induces a fatigue load on the downstream turbine blades. An accurate estimate of turbulent intensity in turbine wake is paramount to optimizing the placement of turbines in tidal turbine parks. A simple empirical model is developed using data fitting from numerical simulation of a non-rotational actuator disc model to estimate the added turbulence in realistic tidal stream conditions similar to the Alderney Race, with a rotor diameter to depth ratio of 40%. The study shows a self-similar Gaussian shape streamwise turbulent intensity in a lateral direction similar to the velocity deficit profile. The turbulent wake radius expands according to a power-law depending on the ambient turbulent intensity. The added turbulence model justifies that the major turbulence source in the near wake is attributed to the rotor as it is weakly dependent on the ambient turbulence in the flow. As no known existing empirical model for tidal turbine wake added turbulence, the model is compared with existing models of unbounded wind turbines. The proposed model estimates the average turbulence in the far wake can assist turbine placement in a tidal farm. • Turbulent wake expands according to power law and depends on the turbulent intensity. • High turbulent intensity facilitates faster recovery to ambient condition. • Added turbulence is weakly affected by the ambient turbulence in the flow. • The rotor is the major source of turbulence in the near wake. • The proposed model estimates the average turbulence in the far wake. [ABSTRACT FROM AUTHOR]

To decrease the need for fossil fuels, the alternative energy resources must be not only economically viable but also sustainable in the long term. One of the most promising alternatives is the marine renewable energy resource. The relatively young marine energy industry is presented with two challenges: first, to deliver a continuous reliable power supply, and second, to minimize potentially harmful effects of the power extraction on the marine environment. The requirement to understand the interactions between a tidal turbine and the surrounding flow environment motivated this work. A tidal turbine mounted on the seabed induces a wake that extends far downstream of the device. As the direction of tidal flow changes, so does the position of the wake with respect to the device. The detailed study of the turbine wake has been conducted by means of computer simulations. An existing finite-volume computer model called the vorticity transport model has been modified to suit the purpose of simulating the wake of a horizontal axis tidal turbine subjected to a nonuniform flow typical of that close to the seabed. High-resolution computer simulations suggest that a progressive fragmentation of the vortical structure occurs during the development of the wake of a tidal turbine. The predicted fragmentation generates small-scale unsteady flow phenomena beyond five rotor diameters downstream of the device in the area previously thought unaffected by the presence of a tidal turbine. The effects of nonuniform flow on the vorticity structure downstream of a tidal turbine and the fragmentation process are analyzed in this work. [ABSTRACT FROM PUBLISHER]

An experimental study measuring the performance and wake characteristics of a 1:10th scale horizontal axis turbine in steady uniform flow conditions is presented in this paper. Large scale towing tests conducted in a lake were devised to model the performance of the tidal turbine and measure the wake produced. As a simplification of the marine environment, towing the turbine in a lake provides approximately steady, uniform inflow conditions. A 16 m long×6 m wide catamaran was constructed for the test programme. This doubled as a towing rig and flow measurement platform, providing a fixed frame of reference for measurements in the wake of a horizontal axis tidal turbine. Velocity mapping was conducted using Acoustic Doppler Velocimeters. The results indicate varying the inflow speed yielded little difference in the efficiency of the turbine or the wake velocity deficit characteristics provided the same tip speed ratio is used. Increasing the inflow velocity from 0.9 m/s to 1.2 m/s influenced the turbulent wake characteristics more markedly. The results also demonstrate that the flow field in the wake of a horizontal axis tidal turbine is strongly affected by the turbine support structure. [ABSTRACT FROM AUTHOR]

Wind energy has become one of the most commercially prospective renewable energies. However, the wake effect of wind turbine can reduce the power generation efficiency and increase the fatigue loading of downstream turbines. Hence, the wake effect study has attracted increasing interests. Compared with the extensive study on the single turbine wake, that on the superposition effect of multiple turbine (multi-turbine) wakes is limited. In this study, the characteristics of the wake velocity and turbulence intensity are studied and the exponential superposition model is proposed for the aligned multi-turbine wakes. Firstly, Simulator for Offshore Wind Farm Applications (SOWFA), a high-fidelity simulator for the interaction between wind turbine dynamics and the flow in a wind farm, is used to analyze the distribution of aligned multi-turbine wakes. It is observed that wakes reach the steady state from second turbine in the aligned turbines. Then influences of different factors on the accuracy of existing superposition models are studied. It is found spacing has significant effect on the performance of superposition models. Furthermore, the exponential superposition model with higher applicability is proposed for the wake velocity and turbulence intensity. Finally, this model is validated by the benchmark data of real wind farms. • Characteristics of the wake velocity and turbulence intensity of aligned multiple turbines are addressed. • Influences of different factors on the accuracy of existing superposition models are addressed. • Exponential superposition model is proposed for the wake velocity and turbulence intensity. • Validations in real wind farms show exponential superposition model has good performance. [ABSTRACT FROM AUTHOR]

The increasing demand for renewable energy resources has promoted the construction of offshore wind farms e.g. in the North Sea. While the wind farm layout consists of an array of large turbines, the interrelation of wind turbine wakes with the remaining array is of substantial interest. The downstream spatial evolution of turbulent wind turbine wakes is very complex and depends on manifold parameters such as wind speed, wind direction and ambient atmospheric stability conditions. To complement and validate existing numerical models, corresponding observations are needed. While in-situ measurements with e.g. anemometers provide a time-series at the given location, the merits of ground-based and space- or airborne remote sensing techniques are indisputable in terms of spatial coverage. Active microwave devices, such as Scatterometer and Synthetic Aperture Radar (SAR), have proven their capabilities of providing sea surface wind measurements and particularly SAR images reveal wind variations at a high spatial resolution while retaining the large coverage area. Platform-based Doppler LiDAR can resolve wind fields with a high spatial coverage and repetition rates of seconds to minutes. In order to study the capabilities of both methods for the investigation of small scale wind field structures, we present a direct comparison of observations obtained by high resolution TerraSAR-X (TS-X) X-band SAR data and platform-based LiDAR devices at the North Sea wind farm alpha ventus. We furthermore compare the results with meteorological data from the COSMO-DE model run by the German Weather Service DWD. Our study indicates that the overall agreement between SAR and LiDAR wind fields is good and that under appropriate conditions small scale wind field variations compare significantly well. [ABSTRACT FROM AUTHOR]

Recent studies have coupled blade element momentum (BEM) theory with the Reynolds Averaged Navier–Stokes equations in computational fluid dynamics (CFD) software, as the BEM-CFD method to analyse the flows in marine current turbines is with much less computational resources. The accuracy of the BEM-CFD calculation was evaluated by analysing the performance and flow field characteristics of an isolated horizontal axis marine current turbine with comparisons to a full rotor geometry simulation and experimental data. The comparisons show that the full rotor geometry simulation gives good predictions near the optimal conditions (TSR = 5–7), but is less accurate for off-design conditions. The BEM-CFD results, which are based on two-dimensional hydrofoil theory, are evaluated using the experimental and numerical lift and drag coefficients. It shows that the two-dimensional lift and drag coefficients had significant effects on the BEM-CFD predictions. Overall, the BEM-CFD based on the numerical hydrofoil data can accurately predict the thrust, but generally overestimates the power. The influence of the lift and drag terms on the BEM-CFD predictions suggest that more reasonable 2D predictions for hydrofoils and the 3D effects should be considered to improve the BEM-CFD accuracy. BEM-CFD can reasonably reflect the circumferential averaged velocity characteristics near the rotor for the optimal condition (TSR = 6) and gets symmetrical features in the wake, but it cannot predict the detailed flow features caused by the finite number of blades due to the limitations of the BEM-CFD method. [ABSTRACT FROM AUTHOR]

Tidal energy is one of the major sources of renewable energy. To accelerate the development of tidal energy, improved designs of Tidal Current Turbine (TCT) are necessary. The effect of tower on performance and wake of TCT is investigated using Computational Fluid Dynamics (CFD) simulations. Transient analysis with transient rotor stator frame change model and shear stress transport turbulence model are utilized in ANSYS CFX. An experimentally validated numerical model with full scale tidal turbine with a blockage ratio of 14.27% and Tip Speed Ratio (TSR) 4.87 is used to simulate the effect of different tower diameters on performance and wake. The effect of different tower diameters is quantified in terms of coefficient of performance (CP). Coefficient of performance for a 3.5 m tower diameter is 0.472 which is followed by 3, 2.5 and 2 m with coefficients of performance of 0.476, 0.478 and 0.476 respectively. Similarly, the coefficient of thrust (CT) on the rotor for 3.5 m tower diameter is 0.902, for 3 m diameter 0.906 and for 2.5 and 2 m diameters are 0.908 and 0.906 respectively. [ABSTRACT FROM AUTHOR]

In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An analysis of the wake characteristics of twin turbines forms the basis of array optimisation. This study aimed to examine the instability characteristics of a twin-turbine wake with two rotational configurations. The dynamic evolution characteristics of coherent structures with different scales in the wake were analysed via wavelet analysis. The results show that an inverse energy cascade process occurs after the high-frequency small-scale coherent structures induced by rotation lose their coherence. This self-organising characteristic is more apparent in the quasi two-dimensional wake of a forward-moving counter-rotating turbine (Array 1) than in that of a backward-moving counter-rotating turbine (Array 2). With greater organisation and coherence, the wake of Array 1 exhibits low-frequency instability characteristics dominated by a large-scale coherent structure. In addition, the signals reconstructed using wavelet transform show that asymmetric modes exist between low-frequency large-scale coherent structures. The experimental results provide a new perspective on the instability mechanism of twin-turbine wakes, as well as important data for numerical modelling. [ABSTRACT FROM AUTHOR]

The wake characteristics of an axial flow hydrokinetic turbine were investigated for various tip speed ratios using velocity measurements and a flow visualization technique. The experimental results showed that the structures of the wake within six rotor diameters downstream of the turbine are significantly affected by a change in the tip speed ratio. As a tip speed ratio decreases, the core region of the wake that is featured by low axial velocity near the turbine rotor hub became more unstable, producing higher turbulence levels in the radial and azimuthal directions. It resulted in different growth rates of the shear layer that expands from the core region toward the outer part of the wake and that interacts with the tip vortices to trigger wake meandering. It turns out that the swirl number of the wake is a key factor that determines the stability of the core region in the near wake region. At locations more than six rotor diameters away from the turbines, the mean and turbulence characteristics became nearly independent of the change in the tip speed ratio. This was due to large-scale turbulent mixing whose size was in the order of the rotor diameter. • Effects of tip speed ratio on the mean flow and turbulence are investigated.. • Significant changes in the mean flow and turbulence observed up to x=6D. • Different instabilities in the core region was explained using the swirl number. • The frequency of the near wake is nearly the same as that in the far wake. • At x¿6D, the mean velocity and turbulence showed little dependence on the TSR. [ABSTRACT FROM AUTHOR]

• Super-large-scale flow imaging of utility-scale wind turbine near wake behavior. • Dynamic wake modulation occurs under fluctuating inflow and turbine operation. • Dynamic wake modulation can lead to significantly faster wake recovery. • Readily-available turbine parameters can predict dynamic wake modulation. • Guidelines for improving turbine wake and farm models proposed. Understanding wind turbine wake mixing and recovery is critical for improving the power generation and structural stability of downwind turbines in a wind farm. In the field, where incoming flow and turbine operation are constantly changing, the rate of wake recovery can be significantly influenced by dynamic wake modulation, which has not yet been explored. Here we present the first investigation of dynamic wake modulation in the near wake of an operational utility-scale wind turbine, and quantify its relationship with changing conditions. This experimental investigation is enabled using novel super-large-scale flow visualization with natural snowfall, providing unprecedented spatiotemporal resolution to resolve instantaneous changes of the wake envelope in the field. These measurements reveal the significant influence of dynamic wake modulation, which causes an increase in flux across the wake boundary of 11% on average, on wake recovery, providing insights into necessary modifications to traditional wake and farm models. Further, our study uncovers the direct connection between dynamic wake modulation and operational parameters readily available to the turbine controller such as yaw error, blade pitch, and tip speed ratio. These connections pave the way for more precise wake prediction and control algorithms under field conditions for wind farm optimization. [ABSTRACT FROM AUTHOR]

Vertical axis tidal turbines are devices that extract the kinetic energy from tidal currents. Tidal currents can be highly turbulent. Since ambient turbulence affects the turbine hydrodynamic, it is critical to understand its influence in order to optimize tidal farms. Actuator Line Model (ALM) combined with Large Eddy Simulation (LES) is a promising way to comprehend this phenomenon. In this article, an ALM was implemented into a Lattice Boltzmann Method (LBM) LES solver. This implementation gives good results for predicting the wake of a vertical axis tidal turbine placed into a turbulent boundary layer. The validated numerical configuration was then used to compute the wake of a real size ducted vertical axis tidal turbine. Several upstream turbulence rates were simulated. It was found that the shape of the wake is strongly influenced by the ambient turbulence. The cost-to-precision ratio of ALM-LBM-LES compared to fully resolved LBM-LES makes it a promising way of modeling tidal farms. [ABSTRACT FROM AUTHOR]

Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity. [ABSTRACT FROM AUTHOR]