Blade Element Momentum Theory Model Research Articles

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines 
 Mohammad Fereidoonnezhad1, Seán Leen1, Stephen Nash1, Tomas Flanagan2, Patrick McGarry1
 1 School of Engineering, University of Galway, Galway, Ireland
 2 ÉireComposites Teo., Údarás Industrial Estate An Choill Rua, Inverin, Galway, Ireland
 E-mail: patrick.mcgarry@universityofgalway.ie 
 
 Keywords: Blade element momentum theory, Helical vertical axis tidal turbine, FEA Analysis
 Classical blade element momentum theory (BEMT) formulations are not capable of accurately simulating complex blade geometries, such as spiral or helical blade geometries. In this paper, we develop a modified BEMT model to calculate hydrodynamic forces acting on the helical vertical axis tidal turbines. Our framework accounts for curved blade geometries where the leading edge is not orthogonal to the freestream velocity, and chord vectors are not in the azimuthal-radial plane. We validate the model using experimental data for a prototype VAWT. We then perform a parametric analysis of helical bladed VATT designs. Both the helix angle of the blade and the relative orientation of the chord strongly influence the power output of a turbine. Our modified BEMT model also predicts that an increase in blade helix angle results in reduced power fluctuations. Additionally, computed fluctuations in tangential and normal forces acting on blades are shown to reduce significantly with increasing blade helix angle, suggesting a reduction in risk of fatigue failure. We also uncover a complex distribution of normal and tangential forces along the length of a helical blade. Fluctuating hydrodynamic forces computed by our modified BEMT model are input into a finite element (FE) framework to compute the stress state in a fibre reinforced composite blade material for a range of blade azimuthal positions. Analyses reveal that blade deflections are three orders of magnitude lower than the turbine radius, suggesting that sufficient structural stiffness is achieved by the blade design. Results also uncover stress concentrations in the region of strut-blade connection, revealing a higher risk of fracture and fatigue failure at this location.

Experimental investigation of propeller noise in ground effect

The aerodynamic propeller performance and aeroacoustic characteristics of a single two-bladed propeller subjected to Ground Effect are investigated using experiments in an anechoic chamber. A strong increase in thrust, torque and power coefficients when a propeller operates in Ground Effect is demonstrated, consistent with previous studies. A low-fidelity Blade Element Momentum Theory model adapted for Ground Effect is implemented to cross-validate the aerodynamic results. The near-field and far-field spectra and their coherence are studied for various propeller distances from the ground plane and compared with the isolated case. The far-field noise measurements indicate that additional tonal peaks appear with the introduction of the ground plane when compared to isolated configuration. While in-Ground Effect, additional high-frequency broadband humps are observed at some polar angles not seen in isolated configuration. Two discrete regions, a shielded and reflection zone, with respective reduction and enhancement of the Overall Sound Pressure levels are observed. The primary reason for noise increase within the reflection zone is a result of pure acoustic reflection from the ground surface. The near-field measurements from an array of embedded high-response microphones in the ground plane indicate that the main source of the propeller noise is in the tip region. Near-field measurements reconfirm flow results from literature with near-field and far-field acoustic results. The present study may find important practical applications within Urban Air Mobility (UAM), especially in the design of vertiport landing pads and propellers for effective noise mitigation, aerodynamic performance, and prediction during the vertical take-off and landing phases of flight.

Design of a novel energy harvesting mechanism for underwater gliders using thermal buoyancy engines

Underwater gliders (UGs) are becoming popular in ocean exploration. However, the main limitation for their development is their supply of energy. This paper proposes a novel energy harvesting mechanism for an underwater glider equipped with a thermal buoyancy engine. The thermal buoyancy engine changes the buoyancy of the glider, due to the difference of temperature of the ocean, and it drives the glider up and down in the water. These manoeuvres drive a turbine behind to harvest energy. Based on this harvesting mechanism, first, a new type of thermal buoyancy engine with high ballast capacity is presented. Secondly, a dedicated turbine, mounted behind the glider, is optimally designed based on the Blade Element Momentum Theory (BEMT). In order to consider the un-uniform inflow generated by the wake of the glider, an upgraded version of the BEMT model has been developed. With the results obtained in this paper, enhanced energy efficiency for a self-sustainable underwater glider can be achievable.

Extended Blade Element Momentum Theory for the Design of Small-Scale Wind Turbines

Blade element momentum theory (BEMT) has been widely used in the design of the small scale wind turbine (SSWT). However, the original BEMT has weaknesses in providing the final design values of the wind turbine blade partly due to inaccurate assumptions of infinite number of blades made in deriving the theory. As such, the theory has to be amended in certain areas to form the so called the extended BEMT. In this study, a SSWT blade is designed using the extended BEMT method considering 3 factors: tip loss, low thrust at high axial induction factor, and high angle of attack in post-stall region where 5 correction models applying also the tip loss correction factor have been compared to the original BEMT model. The SSWT rotor has a diameter of 3m. An airfoil, SG6043 known for its suitability for SSWTs is used in this study. Furthermore, the blade geometry prior to the conduct of the shape optimization process are calculated using polynomial obtained from experimental procedures. The effect of infinite number of blades can be seen here to change the axial induction factor, especially at the tip of the blade and as a result increases the lift coefficient, and overpredicts the overall power coefficient and power of the wind turbine. With the Prandtl’s tip loss factor along with the correction model, the corrected final values of aerodynamic performances have been determined.

Comparison studies on aerodynamic performances of a rotating propeller for small-size UAVs

A better prediction of key aerodynamic parameters of a propeller is necessary to improve the performances of unmanned aerial vehicles (UAVs) in different operating environments. In this work, we conduct a systematic investigation on an APC 1045 multi-rotor propeller to analyze its aerodynamic performances with wind tunnel tests, blade element momentum theory (BEMT) and 3D computational fluid dynamics (CFD). The theoretical BEMT model provides accurate and reliable estimation at the advance ratios ≤0.567, whereas the method begins to stall at advance ratios ≥0.661. Conversely, the results obtained from CFD simulations agree well with the experiments at all advance ratios, with the differences in thrust- and power coefficients around 5% and 4%, respectively. The present work compares the aerodynamic parameters while varying the freestream velocity to investigate the limitations and advantages of the use of the aforementioned three different methods. Given the limitation of the BEMT, it is found to be reliable enough to estimate the aerodynamic parameters of a small-scaled fixed-pitch propeller. In addition, a more comprehensive investigation of the BEMT shows that increasing advance ratio decreases the lifting portion of the propeller blade, i.e. incurring more losses associated with the hub and tip.

Blade element momentum theory for a skewed coaxial turbine

A coaxial turbine under skew with significant rotor spacing has the potential for increased power output compared to a flow-aligned turbine due to a portion of the downstream rotor experiencing freestream velocity, referred to as a fresh flow region. A lab-scale prototype was designed and built to investigate the skew-to-power relationship of a coaxial turbine system as it compared to a blade element momentum theory model with multiple, sheared streamtubes representing the downstream rotor fresh flow region. The inclusion of the downstream rotor fresh flow region in the theoretical analysis is compared to the experimental data. The results support that the torque and power performance of the downstream rotor and overall skewed coaxial turbine system are predicted more accurately.

XFOIL Performance Validation for Medium-Scale Variable Pitch UAV Rotor Systems

This study focuses on experimentally validating the performance of XFOIL, a sophisticated software airfoil analysis tool used for approximating lift and drag coefficients. XFOIL output data was incorporated into a theoretical model simulating a variable pitch rotor system operating in a hovering state. The output of the Blade Element Momentum Theory (BEMT) rotor model is compared to thrust and power output performance data collected from a constructed rotor test bench and analysed in MATLAB. Using XFOIL as input, the BEMT rotor model was observed to yield good robust results when compared to experimental data, but demonstrated sensitivity to airfoil performance characteristics, laying the groundwork for future empirical validation. In comparing BEMT model performance, it was interesting to find that thrust performance remained within tolerance in contrast to an overprediction of rotor power output resulting from XFOIL drag at high blade pitch angles. Upon further interrogation by means of variable isolation, XFOIL demonstrated instability resulting from sensitivity to variability of model constraints. Modification of rotor geometry definitions or environmental constants beyond the test environment framework showed simulated systems may not necessarily behave reliably nor enhance output performance. This highlights the critical importance and utility of experimentation for understanding theoretical model behaviour or validating simulation output performance.

Modeling of Hovering Intermeshing Rotor Using Blade Element Momentum Theory

This paper intends to establish a blade element momentum theory (BEMT) model for analyzing the rotor-and-rotor interference in the hovering intermeshing rotor system. The model uses the blade tip/hub losses to portray the intrinsic three-dimensional rotor aerodynamic effects and introduces the skew downwash flow to determine the rotor-and-rotor interference. Then, the model employs the interference locus considering the intersecting angle and the rotor hub spacing to distinguish the interference operating region on the rotor disk. Next, the BEMT model results are validated against experimental data of the German Autonomous Rotorcraft for Extreme Altitudes (AREA) helicopter and the computational fluid dynamics simulation results of the JZ series helicopters. Moreover, the effects of the rotor performance on the intersecting angle are also considered. It is found that the interference operating region resembles a semi-ellipse shape on the intermeshing rotor disk, which is not sensitive to the collective pitch angles under given structural parameters. Second, the emerging thrust recovery region created by raising the intersecting angle from 20 to 30 deg has a limited enhancement to the rotor thrust and the rotor performance. However, the incremental intersecting angle will produce a significant variation of the rotor thrust in the horizontal component in the body coordinate system, providing a large design numerical margin for the lateral stability in the hovering intermeshing rotor system.

An improved BEMT model based on agent actuating disk with application to ship self-propulsion simulation

Propeller body-force model is a low-cost tool for numerical simulation of the ship self-propulsion test compared with discretized propeller model. As one of the most traditional theories of propeller model, Blade Element Momentum Theory (BEMT) has been proved to have considerable application potential with stability and accuracy. However, it's still hard for BEMT to keep accuracy in a relatively large range of working conditions due to unproved prior assumptions, which limits the application of BEMT in complex inflow conditions like ship self-propulsion. In the application of BEMT, the calculation method of Angle of Attack (AOA) has a great influence on the accuracy. In this paper, an "Agent Actuating Disk (AAD)" method is proposed to determine the AOA. AAD-BEMT method uses AAD to obtain the relationship between local velocity and inflow velocity so that the AOA can be defined by geometric Angle of Attack. This consideration avoids the inaccuracy of determining AOA by local velocity from discretized propeller flow field. The KP505 propeller open-water test is chosen to verify the accuracy of the proposed AAD-BEMT method. Then the numerical simulation of a KRISO Container Ship (KCS) self-propulsion test (Fr = 0.26) is carried out to investigate the performance of the AAD-BEMT body-force model as well as other different propeller models when dealing with non-uniform ship's wake. The open-water curve, propeller load distribution, and self-propulsion factor are presented and compared with the available experimental data. The results show that the predicted load distribution is accurate. In addition, not only the thrust and torque of the AAD-BEMT model can have a stable accuracy in a wide operating condition, but also the wake agrees well with the experimental result, which means that the proposed AAD-BEMT model can economically and accurately simulate the momentum transport of propeller under complex hull-propeller interaction condition.

Performance prediction of cavitating marine current turbine by BEMT based on CFD

In the present study, the hydrodynamic performance of cavitating and non-cavitating marine current turbines (MCTs) has been investigated by blade element momentum theory (BEMT) coupled with Reynolds-averaged Navier–Stokes (RANS) technique. Before employing the BEMT, two-dimensional (2-D) lift and drag forces of cavitating and non-cavitating sections of MCT have been computed by RANS. The lift and drag forces, including cavity shapes, depend on Reynolds number (Re), cavitation number, and angle of attack. The hydrodynamic characteristics of cavitating 2-D hydrofoils (sections) were calculated via a multiphase solver “interPhaseChangeFoam” from the open-source code OpenFOAM. Cavitating flow simulations around a hydrofoil were carried out with Shear Stress Transport (SST) k−ω turbulence and Schnerr–Sauer cavitation model. The grid convergence index (GCI) was applied to confirm the numerical accuracy of the simulations. Validation of the proposed BEMT model has been done with two different model MCTs under non-cavitating conditions. Then, cavitating flow over a hemispherical head-form axisymmetric body, the NACA 66, and the Clark-Y were selected to evaluate the accuracy of the numerical methods and turbulence models available in the solver. Then, cavitating National Renewable Energy Laboratory (NREL) S814 2-D hydrofoil was later simulated for various Reynolds numbers, cavitation numbers, and angles of attacks. By using cavitating NREL S814 2-D hydrofoil data, the cavitation phenomenon was included in the method of BEMT. The distribution of cavitation on the blade has been modeled with BEMT coupled with RANS. Power coefficients at a wide range of tip speed ratios, including cavitation distribution on turbine blades obtained from BEMT, have been compared with each other and experiments. A reliable agreement has been found for all cases studied here. It is shown that cavitation has a negative effect on the power output of MCT for all tip speed ratios and cases discussed in this study. In addition, under cavitating conditions, the tip speed ratio of the operation point that corresponds to the highest power coefficient shifts to the right side of the power curve.

Tidal current turbine blade optimisation with improved blade element momentum theory and a non-dominated sorting genetic algorithm

Tidal current energy has the advantage of predictability over most of the other renewable energy resources. However, due to the harsh operating environment and complicated site conditions, developments in this domain have been gradual. Paramount to these points is device design and optimisation of hydrodynamic performance. Recent developments in the correction models of BEM theory have further improved the accuracy of the prediction model. Using an improved blade element momentum theory model that is capable of accurately capturing the downwash angle and combining it with a well-developed and reliable non-dominated sorting genetic algorithm model, an effective and efficient tidal current turbine blade optimisation tool has been developed and is presented in this paper. This novel work incorporated a NACA generator that is capable of reproducing any NACA profile, such a tool allows the solver to analyse each and every profile used in each spanwise blade element. As a result, the model is very effective at producing tidal current turbine blades that have been optimised not only for local twist angle and chord length, but also for the suitable NACA profiles to be used at a particular spanwise blade element. The use of the non-dominated sorting genetic algorithm in this work allows the model to efficiently explore a wide range of solutions, outputting a number of tidal current turbine blades suitable for a specified operating condition. The accuracy of the performance prediction of the improved BEM model is validated against an experimentally validated tidal current turbine blade. The coefficient of determination (R2) values for power and thrust coefficient are 0.99828 and 0.99488 respectively when comparing this work with experimental measurements found in the literature. Furthermore this proves that the improved BEM model is capable of efficiently predicting hydrodynamic performance of a tidal current turbine blade to a high degree of accuracy. Further work includes implementing computational fluid dynamics for further validation and evaluation.

Propeller aerodynamic optimisation to minimise energy consumption for electric fixed-wing aircraft

AbstractAn electric propulsion model for propeller-driven aircraft is developed with the aim of minimising the power consumption for a given airspeed and thrust. Blade Element Momentum Theory (BEMT) is employed for propeller performance predictions fed with aerodynamic aerofoil data obtained from a proposed combined Computational Fluid Dynamics (CFD)–Montgomerie method, which is also validated. The Two-Dimensional (2D) aerofoil data are corrected to consider compressibility, three-dimensional, viscous and Reynolds-number effects. The BEMT model showed adequate fitting with experimental data from the University of Illinois Urbana Champaign (UIUC) database. Additionally, Goldstein optimisation via vortex theory is employed to design pitch and chord distributions minimising the induced losses of the propeller. Particle swarm optimisation is employed to find the optimal value for a wide range of geometrical and operational parameters considering some constraints. The optimisation algorithm is validated through a study case where an existing optimisation problem is approached, leading to very similar results. Some trends and insights are obtained from the study case and discussed regarding the design of an optimal propulsion system. Finally, CFD simulations of the study case are carried out, showing a slight relative error of BEMT.

Experimental and numerical failure analysis of horizontal axis water turbine carbon fiber-reinforced composite blade

High-performance composites are used in many applications due to their design flexibility, corrosion resistance, high strength-to-weight ratio, and many other excellent mechanical properties. In this study, the location and magnitude of failure initiation in horizontal axis water turbine carbon fiber reinforced polymer (CFRP) blades with different lay-up orientations were investigated. Unidirectional [0°]4 and cross-ply [0°/90°]S layups were selected to study the effect of the buildup direction on the failure of the composite water turbine blades. A finite element analysis (FEA) model was generated to examine the stresses along blade span under both flexural and hydrodynamic loads. Flexural destructive tests were conducted to validate the results obtained from the numerical simulations. In addition, a blade element momentum theory model was created to calculate the hydrodynamic forces acting along the span to determine the maximum loading radial location, which was used for the fixture design and FEA simulation input. Both unidirectional and cross-ply composite blades were tested for failure. There was a general agreement between the experiments and simulations, which validated the results. Moreover, FEA simulations were performed to apply the load to the samples with different pitch angles (−10°, −5°, 0°, 5°, and 10°). Even though the unidirectional CFRP composite blades showed higher strength at 0° pitch angle than the cross-ply blades, the strength of the unidirectional blade dropped significantly when the load was applied with different pitch angles other than 0°, while the strength of the cross-ply blades was less responsive to this pitch angle variation.

Evaluation of low fidelity and CFD methods for the aerodynamic performance of a small propeller

The increasing use of unmanned aerial vehicles and micro air vehicles creates a strong demand for the accurate aerodynamic performance of small-diameters fixed-pitch propellers. The techniques range from low-fidelity to high-fidelity methods. Among the low-fidelity techniques, there is the blade element momentum theory (BEMT). Computational fluid dynamics (CFD) is categorized as a high-fidelity method. This study aims to employ these methods for the analysis of an APC propeller 14 × 7e. Geometric modeling was performed in a 3D scanner and simulations via BEMT and CFD. Analyses using the BEMT model were conducted with two formulations. The first one considered a linear lift coefficient up to the stall limits, without specifying the post-stall lift coefficients. The second one incorporates three-dimensional flow equilibrium effects, and best represents the post-stall conditions. In the CFD models, two approaches for the compatibility of the rotating and stationary domains were evaluated, the stationary frozen rotor (FR) and the transient arbitrary mesh interface (AMI). The kω SST turbulence model was employed. To investigate the influence of the transition laminar-turbulent boundary layer, the Gamma Theta transitional model was also considered. In the comparison of the results with the experimental tests, it was observed that the appropriate choice of analysis method had a strong relationship with the Reynolds number. For low advanced ratio velocities, the best results were obtained with the BEMT models. BEMT linear model overpredicted the power and efficiency for high advance ratio, while BEMT with tridimensional flow equilibrium presented consistent results with a low computational cost. On the other hand, the CFD-AMI model shows better results for higher advanced ratio velocities but is computationally more expensive. The CFD-FR model maintains a constant pattern of error that, in the analysis of efficiency, did not compromise the results.

An Inter-Comparison of Dynamic, Fully Coupled, Electro-Mechanical, Models of Tidal Turbines

Production of electricity using hydrokinetic tidal turbines has many challenges that must be overcome to ensure reliable, economic and practical solutions. Kinetic energy from flowing water is converted to electricity by a system comprising diverse mechanical and electrical components from the rotor blades up to the electricity grid. To date these have often been modelled using simulations of independent systems, lacking bi-directional, real-time, coupling. This approach leads to critical effects being missed. Turbulence in the flow, results in large velocity fluctuations around the blades, causing rapid variation in the shaft torque and generator speed, and consequently in the voltage seen by the power electronics and so compromising the export power quality. Conversely, grid frequency and voltage changes can also cause the generator speed to change, resulting in changes to the shaft speed and torque and consequently changes to the hydrodynamics acting on the blades. Clearly, fully integrated, bi-directional, models are needed. Here we present two fully coupled models which use different approaches to model the hydrodynamics of rotor blades. The first model uses the Blade Element Momentum Theory (BEMT), resulting in an efficient tool for turbine designers. The second model also uses BEMT, combines this with an actuator line model of the blades coupled to an unsteady computational fluid dynamics simulation by OpenFOAM (CFD/BEMT). Each model is coupled to an OpenModelica model of the electro-mechanical system by an energy balance to compute the shaft speed. Each coupled system simulates the performance of a 1.2 m diameter, three-bladed horizontal axis tidal turbine tested in the University of Edinburgh FloWave Ocean Energy Research Facility. The turbulent flow around the blades and the mechanical-electrical variables during the stable period of operation are analysed. Time series and tabulated average values of thrust, torque, power, and rotational speed, as well as, electrical variables of generator power, electromagnetic torque, voltage and current are presented for the coupled system simulation. The relationship between the mechanical and electrical variables and the results from both tidal turbine approaches are discussed. Our comparison shows that while the BEMT model provides an effective design tool (leading to slightly more conservative designs), the CFD/BEMT simulations show the turbulence influence in the mechanical and electrical variables which can be especially important in assessing an additional source of stresses in the whole electro-mechanical system (though at an increased computational cost).

Unsteady thrust on an oscillating wind turbine: Comparison of blade-element momentum theory with actuator-line CFD

Floating platforms are being deployed and developed further for offshore wind power generation in deep water. Platforms undergo surge and pitch in waves, causing their supported wind turbine to oscillate back and forth. An important input to modelling the platform – and hence hub – motion are the forces, moments and energy dissipation provided by the turbine rotor. Blade-element momentum theory (BEMT) provides a numerically-efficient method of calculating these quantities in steady flow, but its application to an oscillating turbine is largely untested. To test how appropriate a quasi-steady BEMT methodology is for this application, BEMT (with a high-axial-induction-factor correction) is compared with CFD using an actuator-line representation of the NREL 5 MW design, typical of the type of turbine that might be deployed. For simplicity we assume constant rotor speed and pitch angle. The paper demonstrates that, for typical wave periods and onset wind, a quasi-steady BEMT model provides a good description of loads on an oscillating turbine rotor and provides near-identical results to CFD for forces and power dissipation. However, a common time-dependent BEMT variant does not improve predictions in this application, but proves detrimental to power prediction. The paper also derives a simple formula, based on steady-flow load characteristics, to estimate dissipative damping by the rotor.

Investigation of parameters affecting horizontal axis tidal current turbines modeling by blade element momentum theory

The Blade Element Momentum Theory (BEMT) is one of the most common and widely used methods in horizontal axis wind and tidal turbine performance prediction. It mainly relies on two-dimensional (2-D) hydrodynamic, lift and drag, coefficients which are used to calculate the hydrodynamic forces within the blade element section of the model. These coefficients are greatly affected by both Reynolds numbers and turbulence transition parameter (Ncrit). The performance sensitivity of scale models tidal current turbines (TCTs) to the change of Reynolds numbers and Ncrit parameter was assessed. Initially, the BEMT was validated with experimental measurements of two scale models of horizontal axis tidal current turbines. A very good agreement was seen, thus, confidence in both the implementation of the theory and the applicability of the method was built up. Lift and drag coefficients were calculated from XFoil through the use of different sets of Reynolds numbers at different Ncrit values. The best BEMT validations with experimental data were seen when the hydrodynamic coefficients were calculated at two sets of Reynolds numbers considered at optimal performance. The first set was calculated at 75% of the blade span while the second set was calculated at every local elemental position of the blade. For facilitation, reduction of time and effort, Reynolds numbers at 75% of the blade span is considered the best choice. Furthermore, the BEMT model results showed better agreements with experimental data when lift and drag coefficients were calculated at low values of Ncrit parameter. The predicted tidal turbine performances of previous experimental data by the present BEM model were compared with those derived from the academic SERG-Tidal BEM code. The present BEM model, based on the proper selection of both Reynolds number and Ncrit parameter, showed an obvious superiority over SERG-Tidal model.

Development and assessment of a blade element momentum theory model for high solidity vertical axis tidal turbines

Tidal energy is an attractive renewable resource given its predictability. The production of reliable, efficient and cost-effective tidal energy turbines requires the development of computer models for assessment and design optimisation of turbine performance. Blade element momentum theory (BEMT) models are attractive to turbine designers as they have a very low computational cost compared to CFD models. In this paper, we develop a BEMT model for high solidity and highly loaded vertical axis tidal turbine rotors which traditional BEMT models are incapable of modelling. The double multiple stream-tube model employs a graphical approach for determination of axial induction factors rather than the iterative approach used in traditional BEMT models. Corrective methods to account for dynamic stall, flow expansion, and finite aspect ratios are also implemented in the model. Model performance is assessed against experimentally measured power performance data of both low and high solidity rotors. The model reproduced peak efficiency values to within 2.5% for a low solidity wind turbine, 8% for a high solidity wind turbine and to within 10% for a high solidity tidal turbine in confined flow.

Comparison of synthetic turbulence approaches for blade element momentum theory prediction of tidal turbine performance and loads

Turbulence is a crucial flow phenomenon for tidal energy converters (TECs), as it influences both the peak loads they experience and their fatigue life. To best mitigate its effects we must understand both turbulence itself and how it induces loads on TECs. To that end, this paper presents the results of blade element momentum theory (BEMT) simulations of flume-scale TEC models subjected to synthetic turbulent flows. Synthetic turbulence methods produce three-dimensional flowfields from limited data, without solving the equations governing fluid motion. These flowfields are non-physical, but match key statistical properties of real turbulence and are much quicker and computationally cheaper to produce. This study employs two synthetic turbulence generation methods: the synthetic eddy method and the spectral Sandia method. The response of the TECs to the synthetic turbulence is predicted using a robust BEMT model, modified from the classical formulation of BEMT. We show that, for the cases investigated, TEC load variability is lower in stall operation than at higher tip speed ratios. The variability of turbine loads has a straightforward relationship to the turbulence intensity of the inflow. Spectral properties of the velocity field are not fully reflected in the spectra of TEC loads.

Adapting conventional tools to analyse ducted and open centre tidal stream turbines

This paper details a hydrodynamic model based on Blade Element Momentum Theory (BEMT) developed to assess ’conventional’ 3-bladed tidal stream turbines (TSTs), adapted here to analyse an ’unconventional’ case of a ducted and open centre device. Validations against a more detailed coupled Reynolds averaged computational fluid dynamics (RANS-BEM) model shows excellent agreement, of within 2% up to the peak power condition, with associated computational times in the order of a few minutes on a single core. The paper demonstrates the application of hydrodynamic forces into a structural analysis tool, in order to assess blade stress distributions of a generic hubless turbine. Incorporation of parameters such as non-uniform inflows and blade weight forces are investigated, with their effects on stress profiles presented. Key findings include: i) the adapted BEMT model replicates the majority of turbine performance characteristics estimated through previous CFD assessments; ii) the proposed model reduces the computational effort by several orders of magnitude compared to the reference coupled CFD, making it suitable for engineering assessments iii) blade stress distribution profiles are quantified, detailing concentration zones and cyclic values for use in fatigue analyses. This work forms part of a greater project aimed to develop a suite of analytical tools to perform engineering assessments of bi-directional ducted TSTs.