Impact of environmental turbulence on the performance and loadings of a tidal stream turbine

A large-eddy simulation (LES) of a laboratory-scale horizontal axis tidal stream turbine operating over an irregular bathymetry in the form of dunes is performed. The Reynolds number based on the approach velocity and the chord length of the turbine blades is approximately 60,000. The simulated turbine is a 1:30 scale model of a full-scale prototype and both turbines operate at very similar tip-speed ratio of λ ≈ 3. The simulations provide quantitative evidence of the effect of seabed-induced turbulence on the instantaneous performance and structural loadings of the turbine revealing how large-scale, energetic turbulence structures affect turbine performance and bending moments of the rotor blades. The data analysis shows that wake recovery is notably enhanced in comparison to the same turbine operating above a flat-bed and this is due to the higher turbulence levels generated by the dune. The results demonstrate the need for studying in detail the flow and turbulence characteristics at potential tidal turbine deployment sites and to incorporate observed large-scale velocity and pressure fluctuations into the structural design of the turbines

Theoretical modelling of the three-dimensional wake of vertical axis turbines

Arrays of Vertical Axis Wind Turbines (VAWTs) can achieve larger power generation per land area than horizontal axis turbines farms, due to the positive synergy between VATs in close proximity. Theoretical wake models enable the reliable design of the array layout that maximises the energy output, which need to depict the driving wake dynamics. VAWTs generate a highly complex wake that evolves according to two governing length-scales, namely the turbine rotor's diameter and height which define a rectangular shape of the wake cross-section, and feature distinct wake expansion rates. This paper presents analytical VAWT wake models that account for an asymmetric distribution of such wake expansion adopting a top-hat and Gaussian velocity deficit distribution. Our proposed analytical Gaussian model leads to an enhanced initial wake expansion prediction with the wake width ($\varepsilon$) behind the rotor equal to $(\beta/4 \pi)^{1/2}$ with $\beta$ being the ratio of initial wake area to the VAWT's frontal area, which addresses the limitations of previous models that under-predicted the wake onset area. Velocity deficit predictions are calculated in a series of numerical benchmarks consisting of a single and an array of four in-line vertical axis wind turbines. In comparisons with field data and large-eddy simulations, our models provide a good accuracy to represent the mean wake distribution, maximum velocity deficit, and momentum thickness, with the Gaussian model attaining the best predictions.These models will aid to drive the design of VAT arrays and accelerate this technology.Comment: 22 pages, 8 figure

An immersed boundary-based large-eddy simulation approach to predict the performance of vertical axis tidal turbines

Vertical axis tidal turbines (VATTs) are perceived to be an attractive alternative to their horizontal axis counterparts in tidal streams due to their omni-directionality. The accurate prediction of VATTs demands a turbulence simulation approach that is able to predict accurately flow separation and vortex shed- ding and a numerical method that can cope with moving boundaries. Thus, in this study an immersed boundary-based large-eddy simulation (LES-IB) method is refined to allow accurate simulation of the blade vortex interaction of VATTs. The method is first introduced and validated for a VATT subjected to laminar flow. Comparisons with highly-accurate body-fitted numerical models results demonstrate the method’s ability of reproducing accurately the performance and fluid mechanics of the chosen VATT. Then, the simulation of a VATT under turbulent flow is performed and comparisons with data from exper- iments and results from RANS-based models demonstrate the accuracy of the method. The vortex-blade interaction is visualised for various tip speed ratios and together with velocity spectra detailed insights into the fluid mechanics of VATTs are provided

Effect of blade cambering on dynamic stall in view of designing vertical axis turbines

This paper presents large-eddy simulations of symmetric and asymmetric (cambered) airfoils forced to undergo deep dynamic stall due to a prescribed pitching motion. Experimental data in terms of lift, drag, and moment coefficients are available for the symmetric NACA 0012 airfoil and these are used to validate the large-eddy simulations. Good agreement between computed and experimentally observed coefficients is found confirming the accuracy of the method. The influence of foil asymmetry on the aerodynamic coefficients is analysed by subjecting a NACA 4412 airfoil to the same flow and pitching motion conditions. Flow visualisations and analysis of aerodynamic forces allow an understanding and quantification of dynamic stall on both straight and cambered foils. The results confirm that cambered airfoils provide an increased lift-to-drag ratio and a decreased force hysteresis cycle in comparison to their symmetric counterpart. This may translate into increased performance and lower fatigue loads when using cambered airfoils in the design of vertical axis turbines operating at low tip-speed ratios

Large eddy simulation of tidal turbines

Understanding of hydrodynamics involved in the flow around tidalturbines is essential to enhance their performance and resilience, asthey are designed to operate in harsh marine environments. Dur-ing their lifespan, they are subjected to high velocities with largelevels of turbulence that demand their design to be greatly opti-mised. Experimental tests have provided valuable information aboutthe performance of tidal stream devices but these are often conductedin constricted flumes featuring turbulent flow conditions different tothose found at deployment sites. Additionally, measuring velocitiesat prospective sites is costly and often difficult. Numerical methods arise as a tool to be used complementary to theexperiments in investigations of tidal stream turbines. In this the-sis, a high-fidelity large-eddy simulation computational approach isadopted and includes the immersed boundary method for body repre-sentation, due to its ability to deal with complex moving geometries.The combination of these numerical methods offers a great balance between computational resources and accuracy. The approach is ap-plied and validated with simulations of vertical and horizontal axistidal turbines, among other challenging cases such as a pitching air-foil. An extensive validation of predicted hydrodynamics, wake de-veloped downstream of the devices or structural loadings, outlinesthe accuracy of the proposed computational approach. In the simu-lations of vertical axis tidal turbines, the blade-vortex interaction is highlighted as the main phenomenon dominating the physics of these devices. The horizontal axis tidal turbine is simulated under differ-ent flow and turbulence intensity conditions, in both flat and irregular channel bathymetries. his thesis seeks to assess and enhance the per-formance, resilience and survivability of marine hydrokinetic devicesin their future deployment at sea

Hydrodynamic loadings on a horizontal axis tidal turbine prototype

Until recently tidal stream turbine design has been carried out mainly by experimental prototype testing aiming at maximum turbine efficiency. The harsh and highly turbulent environments in which tidal stream turbines operate in poses a design challenge mainly with regards to survivability of the turbine owing to the fact that tidal turbines are exposed to significant intermittent hydrodynamic loads. Credible numerical models can be used as a complement to experiments during the design process of tidal stream turbines. They can provide insights into the hydrodynamics, predict tidal turbine performance and clarify their fluid–structure interaction as well as quantify the hydrodynamic loadings on the rotor. The latter can lead to design enhancements aiming at increased robustness and survivability of the turbine. Physical experiments and complementary large-eddy simulations of flow around a horizontal axis tidal turbine rotor are presented. The goal is to provide details of the hydrodynamics around the rotor, the performance of the turbine and acting hydrodynamic forces on the rotor blades. The simulation results are first compared with the experimental and good agreement between measured and simulated coefficients of power are obtained. Acting bending and torsional moment coefficients on the blade-hub junction are computed for idealised flow conditions. Finally, realistic environmental turbulence is added to the inflow and its impact on the turbine’s performance, hydrodynamics and rotor loadings is quantifie

Rotor loading characteristics of a full-scale tidal turbine

Tidal turbines are subject to highly dynamic mechanical loading through operation in some of the most energetic waters. If these loads cannot be accurately quantified at the design stage, turbine developers run the risk of a major failure, or must choose to conservatively over-engineer the device at additional cost. Both of these scenarios have consequences on the expected return from the project. Despite an extensive amount of research on the mechanical loading of model scale tidal turbines, very little is known from full-scale devices operating in real sea conditions. This paper addresses this by reporting on the rotor loads measured on a 400 kW tidal turbine. The results obtained during ebb tidal conditions were found to agree well with theoretical predictions of rotor loading, but the measurements during flood were lower than expected. This is believed to be due to a disturbance in the approaching flood flow created by the turbine frame geometry, and, to a lesser extent, the non-typical vertical flow profile during this tidal phase. These findings outline the necessity to quantify the characteristics of the turbulent flows at sea sites during the entire tidal cycle to ensure the long-term integrity of the deployed tidal turbines

Wake generated downstream of a vertical axis tidal turbine

Characterising the physics involved in the wake developed downstream of vertical axis tidal stream turbines (VATTs) is a cornerstone towards understanding turbine-to- turbine interactions and thus optimise their deployment in ar- rays. The flow field developed around these devices is not exempt of complexity accentuated by complex blade-vortex interactions occurring within the blades swept perimeter. This publication analyses the flow characteristics of the near-wake behind a three- bladed VATT rotating at different rotational speeds using a highly accurate computational approach. Results of mean velocities and turbulent kinetic energy are validated with experiments and compared to other RANS-based numerical results. A great match with the experimental data is achieved outlining the suitability of the proposed numerical approach to represent the complex flow around VATTs. Results evidence that at low tip speed ratios the flow passes through the turbine rotor without major obstruction of the blades. Meanwhile, at higher speeds there is a minimum incident flow entraining within the rotor swept perimeter which induces the generation of the Magnus effect. The outcomes encourage to extend the applicability of the presented method to the future design of VATT arrays enhancing inter-turbine interaction and thus maximising power generation