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As previously experienced by the wind industry, it is envisaged that tidal stream turbine blades will present misalignments or blade deformations over time as they are constantly working under harsh and highly unsteady environments. Blade misalignment will affect the power capture of a tidal stream turbine and if not detected in time could affect other components of the drive train. Therefore, the aim of this paper is to compare the use of two numerical modelling techniques to predict the performance of a tidal stream turbine working under off-design conditions, in this case, the misalignment of one or more blades. The techniques used in this study are Blade Element Momentum Theory and Computational Fluid Dynamics. The numerical models simulate the performance of a three-bladed horizontal axis tidal stream turbine with one or two blades offset from the optimum pitch setting. The simulations were undertaken at 1.0 m/s flow speeds. The results demonstrated that both unsteady BEMT and steady or transient CFD are able to predict power coefficients when there is a certain level of misalignment in one or even two blades. However, both techniques failed to accurately predict a loss of power performance at high rotational speeds. [ABSTRACT FROM AUTHOR]

The purpose of the present work is to evaluate the capability of the Actuator Line Method (ALM) to simulate flow through a horizontal axis tidal stream turbine. A numerical model combining the ALM with large eddy simulation technique is developed and applied to compute the flow past a laboratory-scale tidal stream turbine. The flow field is analysed in terms of streamwise mean velocity, turbulence intensity, turbulent kinetic energy and the decay rate of the maximum turbulent kinetic energy behind the turbine. It is found that the ALM performs well in predicting the mean flow and turbulence characteristics behind the turbine. The flow field predicted show a clear transition from an organised vorticity region near the turbine to a highly turbulent flow downstream. The location of this transition and the controlling parameters are discussed but further investigation, both numerical and experimental is required in order to clarify its effects on the flow structure and the performance of downstream turbines in tidal turbine arrays. [ABSTRACT FROM AUTHOR]

It is important to understand tidal stream turbine performance and flow field, if tidal energy is to advance. The operating condition of a tidal stream turbine with a supporting structure has a significant impact on its performance and wake recovery. The aim of this work is to provide an understanding of turbine submerged depth that governs the downstream wake structure and its recovery to the free-stream velocity profile. An experimentally validated numerical model, based on a computational fluid dynamics (CFD) tool, was present to obtain longitudinal, transverse and vertical velocity profiles. Wake characteristics measurements have been carried out in an open channel at Hohai University. The results indicate that varying the turbine proximity to the water surface introduces differential mass flow rate around the rotor that could make the wake persist differently downstream. CFD shows the same predicted wake recovery tendency with the experiments, and an agreement from CFD and experiments is good in the far-wake region. The results presented demonstrate that CFD is a good tool to simulate the performance of tidal turbines particularly in the far-wake region and that the turbine proximity to the water surface has an effect on the wake recovery. [ABSTRACT FROM AUTHOR]

In order to better manage and maintain deployed Tidal Stream Turbine (TST) devices their response to complicated and severe loading mechanisms must be established. To aid this process the research presented details a methodology for mapping TST operational data, taken under a variety of operating conditions, to a set of model parameters. The parameter sets were developed based on a TST rotor torque model which, as well as providing means of characterising turbine behaviour, can be used to create TST simulations with minimal computation expense. The use of the model in facilitating parameter surface mapping is demonstrated via its application to a set of rotor torque measurements made of a 1/20th scale TST during flume testing. This model is then deployed to recreate the known rotor behaviour which is compared with the original flume based measurements. This is a flexible tool that can be applied to investigate turbine performance under conditions that cannot be readily replicated using tank-based experiments. Furthermore, Computational Fluid Dynamics simulations of such conditions could be time consuming and computationally expensive. To this end, the use of the model in creating drivetrain test bed based simulations is demonstrated. The model, which can be calculated in real-time, is used to develop representative turbine simulations at high turbulence intensity levels which were not achievable during flume experimentation. The intention is to provide a test-bed for future turbine performance monitoring under more realistic, site specific conditions. The work will also support the deployment of performance surfaces in real-life turbine applications. [ABSTRACT FROM AUTHOR]

This paper analyses two different configurations of horizontal axis Tidal Stream Turbines (TSTs) using a Blade Element Momentum Theory (BEMT) model. Initially, a ‘conventional’ three bladed and bare turbine is assessed, comparing against experimental measurements and existing literature. Excellent agreement is seen, increasing confidence in both the implementation of the theory and the applicability of the method. The focus of the paper lies on the analysis of a ducted and open centre turbine. An analytical adjustment to the BEMT model is applied, using empirical expressions detailed in the literature which are devised from Computational Fluid Dynamics (CFD) studies. This is modified to a symmetrical duct profile, calibrating certain geometrical parameters against blade resolved CFD studies of a bi-directional device. The results are validated with a coupled CFD blade element model (RANS BEM), where both models align very closely (within 2%) for most tip speed ratios (TSRs), including the peak power condition. Over predictions are seen at higher TSRs of up to 25% in power and 13% in thrust at TSR = 5, due to model limitations in replicating fully the complex flow interactions around the hub and the open centre. The presented approach benefits from significantly lower computational requirements, several orders of magnitude lower than reported in the RANS-BEM case, allowing practicable engineering assessments of turbine performance and reliability. [ABSTRACT FROM AUTHOR]

The presence of Tidal Stream Turbines (TST) for tidal power production, leads to changes in the local physical environment that could affect fish. While other work has considered the implications with respect to conventional hydroelectric devices (i.e. hydroelectric dams), including studies such as physical impact with the rotors and pressure variation effects, this research considers the effects of sudden changes in pressure and turbulence on the hypothetical fish with respect to TSTs. Computational fluid dynamics (CFD) is used to investigate changes to the environment, and thus study the implications for fish. Two CFD methods are employed, an embedded Blade Element representation of the rotor in a RANS CFD model, and a blade resolved geometry using a moving reference frame. A new data interpretation approach is proposed as the primary source of environmental impact data; ‘rate of change of pressure’ with time along a streamtrace. This work also presents results for pressure, pressure gradients, shear rates and turbulence to draw conclusions about changes to the local physical environment. The assessment of the local impact is discussed in terms of the implications to individual fish passing a single or array of TST devices. [ABSTRACT FROM AUTHOR]

• A CFD code is established for the surge motion of the HATT under free surface condition. • The Fourier series expansion and the least square method are employed to analyze the hydrodynamic coefficients. • Surge effect increases turbine performance unsteadiness and reduces the length of the wake. • Surge-induced toroidal vortex results more sensitive with amplitude rather than period. • Wake proximity to the free surface, peak and cavitation effects rise with surge. In this paper, a Computational Fluid Dynamics (CFD) code is used to investigate the tidal stream turbine performance under free surface condition and with surge motion: amplitudes of 1/24–1/4 rotor diameter and period of 3–12 s. The CFD model is evaluated against experiments of a piled turbine in a circulating flume, providing a difference of only 1.46% at rated TSR. The unsteady power and thrust follow the sum of a constant (similar to steady), velocity-induced, and acceleration-induced terms. In all tests, the damping term for the power response is approximately 3 times the steady power coefficient (Cp ∼ 0.33), whilst for the thrust, 1.6 times the steady thrust coefficient (Cz ∼ 0.77). Ignoring the small acceleration-induced coefficient leads to negligible simulation errors. Taken together, augmenting the surge amplitude and frequency increases the time-averaged and fluctuation of the power and thrust coefficient. Significant high- and low-pressure areas form around the blade edges, in function of the resultant velocity (sum of induced and inflow term), promoting peak and cavitation effects. From the ecological perspective, the induced-velocity develops a toroidal vortex near the wake, then mixes with the hub and tip vortexes, and propagates streamwisely toward the free surface. This effect is more pronounced with the surge amplitude rather than period, thereby exacerbating the wake deficit, and the wake structure is more sensitive to the variation of surge period so the restriction on the oscillation frequency should be considered as a priority in the designing phase. In the future, it will be important to explore the effects of blockage and depth immersion to assess extreme functioning and cavitation effects. [ABSTRACT FROM AUTHOR]

Abstract: Studies of tidal stream turbine performance and of wake development are often conducted in tow-tanks or in regulated flumes with uniform flows across the turbine. Whilst such studies can be very useful, it is questionable as to what extent the results would differ if the flows were more complex in nature, for instance if the flows were unsteady or non-uniform or even both. This study aims to explore whether the results would be affected once we move away from the uniform flow scenario. A numerical modelling study is presented in which tidal stream turbine performance and wake development in non-uniform flow conditions are assessed. The model implements the Blade Element Momentum method for characterising turbine rotor source terms which are used within a computational fluid dynamics model for predicting the interaction between the turbines and the surrounding flow. The model is applied to a rectangular domain and a range of slopes are implemented for the water surface to instigate an increase in flow velocity along the domain. Within an accelerated flow domain wake recovery occurred more rapidly although rotor performance was not affected. [Copyright &y& Elsevier]

A comparison of a tidal turbine's performance and structural loads is conducted using lab-scale numerical models and experimental testing under multiple current-only and wave-current conditions at the IFREMER wave-current flume. Experimental testing, used to validate CFD models, was accomplished using a 0.9 m diameter, 3-bladed tidal turbine and had a blockage ratio of 8% while the turbine was submerged. Initial investigations analysed the performance and loads on the turbine under uniform and profiled current-only conditions. The presence of a profiled velocity gradient was found to have a negligible effect on the average performance characteristics; however, transient thrust, torque and out of plane bending moment loads experienced much greater variations. These load fluctuations were further increased with increasing levels of shear in the velocity profile, while peaks in the turbine loads coincided with its rotational frequency. The addition of regular, Stokes 2nd Order Theory waves added to the complexity of the flow conditions experienced by the turbine. The effect on the average performance characteristics were negligible while the total turbine thrust and torque fluctuations increased by 35 times that of the current-only cases. Peaks in the loads aligned with the wave surface elevation, indicating the importance of transient analyses of dynamic loads. • The mode of the streamwise velocity accurately represents predominant flow conditions. • Mean turbine performance was unaffected by profiled velocity gradients and waves. • The turbine blade loading fluctuations increased with increasing velocity shear. • Blade loading fluctuations, due to waves, were the same as the highest shearing velocity. • Optimised free surface, multiphase, CFD models were successfully validated experimentally. [ABSTRACT FROM AUTHOR]

This study aims to investigate the wake characteristics of a horizontal axis tidal stream turbine supported by a monopile using a numerical approach. Computational fluid dynamics (CFD) simulations based on the open source software OpenFOAM have been performed to enhance understanding of a turbine's wake. The numerical simulations adopt both the actuator line method and the full rotor geometry method. The numerical results are found to be consistent with experimental data, although some discrepancies are observed at a distance of one rotor diameter downstream. Comparison of numerical results from both methods is performed. The results show that both methods can obtain important flow features and provide similar simulation in the wake of the turbine model. The actuator line method is able to give a better prediction in stream-wise velocity distribution, although it underestimates the turbulence intensity, circumferential velocity and vorticity magnitude slightly, compared with the full rotor geometry method. It is also found that the wake of the monopile and the rotor interact strongly in the downstream field, especially in the region immediately behind the structure. A strong interaction occurs within approximately two rotor diameters downstream. [ABSTRACT FROM AUTHOR]

This paper explores the influence of blockage and free-surface deformation on the hydrodynamic performance of a generic marine cross-flow turbine. Flows through a three-bladed turbine with solidity 0.125 are simulated at field-test blade Reynolds numbers, O (105-106), for three different cross-stream blockages: 12.5, 25 and 50 per cent. Two representations of the free-surface boundary are considered: rigid lid and deformable free surface. Increasing the blockage is observed to lead to substantial increases in the power coefficient; the highest power coefficient computed is 1.23. Only small differences are observed between the two free-surface representations, with the deforming free-surface turbine out-performing the rigid lid turbine by 6.7 per cent in power at the highest blockage considered. This difference is attributed to the increase in effective blockage owing to the deformation of the free surface. Hydrodynamic efficiency, the ratio of useful power generated to overall power removed from the flow, is found to increase with blockage, which is consistent with the presence of a higher flow velocity through the core of the turbine at higher blockage ratios. Froude number is found to have little effect on thrust and power coefficients, but significant influence on surface elevation drop across the turbine. [ABSTRACT FROM AUTHOR]