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This paper documents the predictive capability of rotating blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and Improved Delayed Detached Eddy Simulation (IDDES) computations for tidal stream turbine performance and intermediate wake characteristics. Ansys/Fluent and OpenFOAM simulations are performed using mixed-cell, unstructured grids consisting of up to 11 million cells. The thrust, power and intermediate wake predictions compare reasonably well within 10% of the experimental data. For the wake predictions, OpenFOAM performs better than Ansys/Fluent, and IDDES better than URANS when the resolved turbulence is triggered. The primary limitation of the simulations is under prediction of the wake diffusion towards the turbine axis, which in return is related to the prediction of turbulence in the tip-vortex shear layer. The shear-layer involves anisotropic turbulent structures; thus, hybrid RANS/LES models, such as IDDES, are preferred over URANS. Unfortunately, IDDES fails to accurately predict the resolved turbulence in the near-wake region due to the modeled stress depletion issue. [ABSTRACT FROM AUTHOR]

The flow developed on a tidal site can be characterized by combinations of turbulence, shear flows, and waves. Horizontal-axis tidal turbines are therefore subjected to dynamic loadings that may compromise the working life of the rotor and drive train components. To this end, a series of experiments were carried out using a 0.9 m horizontal-axis tidal turbine in a tow tank facility. The experiments included two types of regular waveforms, one of them simulating an extreme wave case, the other simulating a more moderate wave case. The second regular wave was designed to match the peak period and significant wave height of an irregular wave which was also tested. Measurements of torque, thrust, and blade-bending moments were taken during the testing campaign. Speed and torque control strategies were implemented for a range of operational points to investigate the influence that a control mode had in the performance of a tidal stream turbine. The results showed similar average power and thrust values were not affected by the control strategy, nor the influence of either the regular or irregular wave cases. However, it was observed that using torque control resulted in an increase of thrust and blade root bending moment fluctuations per wave period. The increase in fluctuations was in the order of 40% when compared to the speed control cases. [ABSTRACT FROM AUTHOR]

The predictive capabilities of blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and detached eddy simulation (DES), the most commonly used hybrid RANS/large eddy simulation (LES) model, are assessed for hydrokinetic turbine performance and mean and turbulent flows in the intermediate-wake region, and results for a range of tip-speed ratio encompassing design and off-design conditions are analyzed to understand the wake recovery mechanism. The performance predictions compared within 5% of the experimental data. Both URANS and DES models performed reasonably well for the near wake predictions, where the errors were <15%. DES outperformed URANS for both mean wake deficit and turbulence predictions in the intermediate-wake region and both quantities compared within 10% of the experiments. The improved prediction by DES is because of its ability to predict the tip vortex breakdown, which plays a critical role in the wake recovery, especially for higher tip speed ratios (λ). However, DES significantly underpredicted the turbulence predictions in the near-wake region, which could be partly due to the negligence of free-surface effects and partly due to modeling issues, namely modeled stress depletion. The study reveals that the tip vortex breakdown mechanism depends on λ. For lower values of λ, instabilities generated in the root vortex core are identified to be the cause of breakdown. For higher values, the breakdown occurred because of the instabilities generated during the vortex filament entanglement. Future work should focus on investigation of other hybrid RANS/LES models to address the limitations of the DES models, and extension of the study to include free-surface effects. [ABSTRACT FROM AUTHOR]

The paper presents an experimental campaign developed to contribute to the current research considering the operation of Horizontal Axis Tidal Turbines within stochastic flow conditions, namely turbulent and wake induced flows. The campaign was conducted at approximately a 1 / 20 t h -scale within a recirculating flume. Experiments were conducted over five differing setups, yielding a baseline low Turbulence Intensity case, two high turbulence cases and two upstream device generated wake cases. The experiments were conducted at a range of differing rotor velocities established, in a novel way, by utilising both fixed speed and fixed braking torque control. The paper presents analysis of flow measurements to statistically quantify the stochastic flow conditions impinging on the model-scale tidal turbine. The power, thrust, torque and blade root bending moment of single blade were recorded and analysed against the flow conditions generated under the five cases. The analysis showed that it may well be possible to exploit the accelerated region around an upstream turbine to capture marginally higher power (6 % increase) from downstream turbines. Lastly, it was found that the control scheme adopted has a significant impact on power and load fluctuations observed at differing rotor velocities. • Presents novel research into the combined effects of turbulence and devices.. • Two differing control schemes are used fixed torque and speed control. • The paper shows further evidence of the array 'packing' effect. • The paper presents the effects of differing levels of TI on device loading. [ABSTRACT FROM AUTHOR]

Before initiating a study on the interaction of multiple wakes, it is imperative that turbine wake hydrodynamics are studied in isolation. In this paper CFD computer simulations of downstream turbine wakes have been run using a scale-resolving hybrid turbulence model known as a detached eddy simulation. To allow validation of the CFD simulations the computer models were supported by flume measurements with a lab scale tidal stream turbine run at three tip-speed ratios and three turbulence conditions, varying both turbulence intensity and length-scale. From the study it was demonstrated that turbulence intensity has a significant impact on the wake development for both recovery and width. The turbulence length scales of between 0.25 and 1.0 rotor diameter did not have a significant impact on the wake. The turbine operating condition also had an impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to flow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust). • Wake widths and lengths for a tidal turbine using three different analysis metrics are presented. • Three levels of turbulent conditions are presented using upstream grids. • The impact of the turbine operating conditions, on the wake characteristics, are analysed. • The impact of turbulence intensity and Integral length scales on the wake characteristics are analysed. [ABSTRACT FROM AUTHOR]

To understand the influence of complex hydrodynamic loads on tidal turbines, laboratory testing is necessary as a first approach. Previous laboratory work undertaken gave an indication that the use of speed control strategies may disguise the associated loading range that a turbine may be subjected to when this is operated with a variable speed control strategy. However, the preceding work was undertaken in a highly controlled environment without the influence of turbulent flows. The focus of this paper is directed towards the study of wave-induced loads on tidal turbines when these are controlled using two strategies and the impact that these parameters have on the turbine's performance when this is operated in a recirculating flume. Laboratory tests were undertaken with a 0.9 m diameter horizontal axis tidal turbine subjected to combined wave and current conditions with both regular and irregular waves. Constant speed and constant torque control strategies have been considered, for which rotor thrust, torque and blade root bending moment have been measured. Results show that similar to previous studies, average loads and power capture values remain unchanged between control strategies and the superposition of waves to the current. However, signal fluctuations are 2 to 3 times higher for torque control than for constant speed control strategy. A phase difference between the periodic signals of the turbine thrust and the incoming waves was also identified, in this case, the phase variation was lower when using torque than speed control. This work thus demonstrates the implication of studying strategies to control a marine converter from early stages of development. • Recirculating tank testing demonstrated the loading and performance effects when using two control methodologies. • Thrust, torque and root bending moment fluctuations are higher when using torque control than speed control. • Phase differences between the periodic signals of the turbine thrust and the incoming waves is analysed. • The effect of combined turbulent flows and regular and irregular wave interaction is presented. [ABSTRACT FROM AUTHOR]