Effect of the number of blades and solidity on the performance of a vertical axis wind turbine

Two, three and four bladed phgr-shape Vertical Axis Wind Turbines are simulated using a free-wake vortex model. Two versions of the three and four bladed turbines are considered, one having the same chord length as the two-bladed turbine and the other having the same solidity as the two-bladed turbine. Results of the two-bladed turbine are validated against published experimental data of power coefficient and instantaneous torque. The effect of solidity on the power coefficient is presented and the instantaneous torque, thrust and lateral force of the two-, three- and four-bladed turbines are compared for the same solidity. It is found that increasing the number of blades from two to three significantly reduces the torque, thrust and lateral force ripples. Adding a fourth blade further reduces the ripples except for the torque at low tip speed ratio. This work aims to help choosing the number of blades during the design phase of a vertical axis wind turbine

Momentum balance in a fully developed boundary layer over a staggered array of NREL 5MW rotors

3D Reynolds-averaged Navier-Stokes (RANS) simulations of a fully developed wind farm boundary layer over a staggered array of NREL 5MW turbines are presented. The turbine is modeled as an actuator disk and as a fully resolved rotor to compare the effect of the turbine model on the wind farm aerodynamics, in particular the streamwise momentum balance across the farm. Results show that the difference in the turbine model affects the average wind speed through the farm as well as the local flow pattern around each turbine; both contributing to the difference in the prediction of farm performance. Results are also compared with a simple theoretical model of very large wind farms proposed recently. The actuator disk simulations agree very well with the theoretical model, whereas the fully resolved rotor simulations show some consistent and expected differences from the model. Paper presented at the Wake Conference 2017 30 May to 1 June 2017, Visby, Sweden

Numerical study of airfoil stall cells using a very wide computational domain

The formation of stall cells over a NACA 0012 airfoil at a Reynolds number of one million has been investigated numerically, using unsteady Reynolds-averaged Navier–Stokes (URANS) and delayed detached-eddy simulation (DDES) approaches. The simulations are performed with a very wide computational domain (10 chord length) to minimize the influence of spanwise periodic boundary conditions. For the URANS simulations, four different spanwise mesh resolutions are tested to determine the minimum resolution required to capture the formation of stall cells. Both URANS and DDES results show a sudden decrease in lift and increase in drag between 16° and 17° angle of attack, accompanied by a significant change of separated flow patterns. Stall cell structures are observed clearly in the URANS solutions between 17° and 19° with a spanwise spacing of about 1.4 to 1.8 chord length, which agrees well with a theoretical prediction based on the slope of the lift curve in this angle-of-attack range. The DDES results show much more complex flow patterns over the airfoil at these high angles of attack, although the spectral analysis of wall shear stress suggests the existence of flow structures having a similar spanwise length scale to the stall cells

Analyse de l'écoulement transitionnel sur un hydrofoil : application aux hydroliennes à axe transverse avec contrôle actif de l'angle de calage

This work studies the laminar-turbulent transition and the pitch control effects on the performances of a vertical axis tidal turbine (SHIVA) developed at the French naval academy research institute. Firstly, experimental and numerical results are compared to study the transitional flow around a hydrofoil. The experiments were carried out in the hydrodynamic tunnel of the French naval academy research institute and the laminar-turbulent transition was triggered by a laminar separation bubble mechanism. Comparisons are based on the local analysis of pressure data and velocity profiles in the vicinity of the laminar separation bubble. Lift, drag and moment coefficients measured on a fixed hydrofoil and on a hydrofoil undergoing a pitching movement are also used for comparison. 2D RANS calculations carried out with or without a transition modal (ɣ — Reo), 3D RANS calculations and 2.5D LES calculations were run so as to assess the accuracy of each type of simulation. This study shows that the ‘y Reo transition modal clearly improves the accuracy of the results compared to a fully turbulent turbulence model (k— w SST) when considering a transitional flow. The influence of the laminar-turbulent transition on the performance of the SHIVA turbine is then studied. Results of 2D calculations run with and without transition model are compared. The use of the transition modal is relevant at the tip speed ratio value ʎ = 2 for which the blades undergo dynamic stall. The transition modal leads to a better prediction of the leading edge vortex development and then allows a better prediction of the dynamic stall. The use of the transition model at higher ʎ values is less relevant since the transition modal appears to predict a drag too low. Finally, the effect of the pitch control on the SHIVA turbine performance is .studied at ʎ = 3, for which the power coefficient is the highest. Advanced pitching laws are developed to modify the blades’ angle of attack independently on the upstream and downstream halves of the turbine. The best pitching law tested in this study leads to an improvement of the power coefficient by 34% and smooths the torque distribution.Cette thèse vise à étudier les effets de la transition laminaire - turbulent et du contrôle actif de l’angle de calage des pales sur les performances de l’hydrolienne à axe transverse SHIVA (Système Hydrolien Intelligent à Variation d’Angle) développée à l’institut de Recherche de l’Ecole-Navale (IRENav). L’écoulement transitionnel autour d’un hydrofoil est, d’abord étudié en comparant des résultats expérimentaux et numériques. Les résultats expérimentaux ont été obtenus dans le tunnel hydrodynamique de l’IRENav. La transition s’effectue par un mécanisme de bulbe de séparation laminaire. Les comparaisons sont fondées sur l’analyse locale des pressions, des profils de vitesse dans la zone du bulbe de séparation laminaire et sur l’analyse des portances, traînées et moments mesurés sur un profil fixe et en mouvement de tangage forcé. Des calculs RANS 2D, avec et sans modèle de transition (ɣ— Reo), RANS 3D et LES 2.5D ont été menés afin de comparer les approches et évaluer la précision des simulations. L’étude montre que le modèle de transition ɣ — Reo améliore nettement les résultats obtenus par rapport à un modèle tout turbulent (k — w SST) dans le cas d’un écoulement transitionnel. L’influence de la transition laminaire - turbulent sur les performances de la turbine SHIVA est ensuite étudiée en comparant les résultats de calculs effectués avec et sans modèle de transition. L’approche est bidimensionnelle. L’utilisation du modèle de transition est intéressante au paramètre d’avance ʎ = 2 pour lequel les pales subissent un décrochage dynamique important. Le développement du tourbillon de bord d’attaque, favorisé par le modèle de transition, permet en effet une meilleure prédiction du décrochage. Les valeurs de ʎ supérieures sont moins concluantes du fait de la prédiction d’une tramée trop faible par le modèle de transition. Enfin, l’influence du contrôle actif du calage des pales sur les performances de la turbine est étudiée au point de fonctionnement optimal de la turbine ʎ = 3. Des lois de calage avancées sont développées, permettant d’agir indépendamment sur la moitié amont ou aval de la turbine. La meilleure loi testée permet une augmentation du coefficient de puissance de 34% tout en lissant la répartition du couple

Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines

A Double Multiple Streamtube model, a free-wake vortex model (both widely used for vertical axis wind turbine design) and RANS CFD simulations are used in this work to predict the performance of the 17 m Vertical Axis Wind Turbine, field tested by Sandia National Laboratories. The three-dimensional, full scale calculations are compared with the experiments in terms of power coefficient, power and instantaneous turbine torque to assess the validity of each model. Additionally, the two aerodynamic models and RANS CFD are compared to each other in terms of thrust and lateral force. The two models and CFD agree well with the experiments at the turbine optimal tip speed ratio. However, away from the optimal tip speed ratio, the streamtube model significantly deviates from the experimental data and from the other numerical models. RANS CFD gives a good agreement with the experiments, slightly underestimating the power coefficient at every tip speed ratio tested. The vortex model proves to be a useful tool with a better accuracy than the streamtube model and a much lower computational cost compared to RANS CFD

Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines

A Double Multiple Streamtube model, a free-wake vortex model (both widely used for vertical axis wind turbine design) and RANS CFD simulations are used in this work to predict the performance of the 17�m Vertical Axis Wind Turbine, field tested by Sandia National Laboratories. The three-dimensional, full scale calculations are compared with the experiments in terms of power coefficient, power and instantaneous turbine torque to assess the validity of each model. Additionally, the two aerodynamic models and RANS CFD are compared to each other in terms of thrust and lateral force. The two models and CFD agree well with the experiments at the turbine optimal tip speed ratio. However, away from the optimal tip speed ratio, the streamtube model significantly deviates from the experimental data and from the other numerical models. RANS CFD gives a good agreement with the experiments, slightly underestimating the power coefficient at every tip speed ratio tested. The vortex model proves to be a useful tool with a better accuracy than the streamtube model and a much lower computational cost compared to RANS CFD

Blade-resolved CFD simulations of a periodic array of NREL 5 MW rotors with and without towers

A fully resolved (FR) NREL 5 MW turbine model is employed in two unsteady Reynolds-averaged Navier–Stokes (URANS) simulations (one with and one without the turbine tower) of a periodic atmospheric boundary layer (ABL) to study the performance of an infinitely large wind farm. The results show that the power reduction due to the tower drag is about 5% under the assumption that the driving force of the ABL is unchanged. Two additional simulations using an actuator disc (AD) model are also conducted. The AD and FR results show nearly identical tower-induced reductions of the wind speed above the wind farm, supporting the argument that the AD model is sufficient to predict the wind farm blockage effect. We also investigate the feasibility of performing delayed-detached-eddy simulations (DDES) using the same FR turbine model and periodic domain setup. The results show complex turbulent flow characteristics within the farm, such as the interaction of large-scale hairpin-like vortices with smaller-scale blade-tip vortices. The computational cost of the DDES required for a given number of rotor revolutions is found to be similar to the corresponding URANS simulation, but the sampling period required to obtain meaningful time-averaged results seems much longer due to the existence of long-timescale fluctuations

Structural optimisation of vertical-axis wind turbine composite blades based on finite element analysis and genetic algorithm

A wind turbine blade generally has complex structures including several layers of composite materials with shear webs, making its structure design very challenging. In this paper, a structural optimisation model for wind turbine composite blades has been developed based on a parametric FEA (finite element analysis) model and a GA (genetic algorithm) model. The optimisation model minimises the mass of composite blades with multi-criteria constraints. The number of unidirectional plies, the locations of the spar cap and the thicknesses of shear webs are taken as design variables. The optimisation model takes account of five constraints, i.e. stress constraint, deformation constraint, vibration constraint, buckling constraint, and manufacturing manoeuvrability and continuity of laminate layups constraint. The model has been applied to the blade structural optimisation of ELECTRA 30 kW wind turbine, which is a novel VAWT (vertical-axis wind turbine) combining sails and V-shape arm. The mass of the optimised blade is 228 kg, which is 17.4% lower than the initial design, indicating the blade mass can be significantly reduced by using the present optimisation model. It is demonstrated that the structural optimisation model presented in this paper is capable of effectively and accurately determining the optimal structural layups of composite blades

Blinding and sham control methods in trials of physical, psychological, and self-management interventions for pain (article II): a meta-analysis relating methods to trial results

Sham interventions in randomised clinical trials (RCTs) of physical, psychological, and self-management (PPS) therapies for pain are highly variable in design and thought to contribute to poor internal validity. It has, however, not been formally tested whether the extent to which sham controls resemble the treatment under investigation consistently affects trial outcomes, such as effect sizes, differential attrition, participant expectancy, and blinding effectiveness. Placebo or sham-controlled RCTs of PPS interventions of clinical pain populations were searched in twelve databases. The similarity of control interventions to the experimental treatment was rated across 25 features. Meta-regression analyses assessed putative links between employed control interventions, observed effect sizes in pain-related outcomes, attrition, and blinding success. The sample included 198 unique control interventions, dominated by manual therapy and chronic musculoskeletal pain research. Meta-analyses indicated small to moderate benefit of active treatments over control interventions, across subgroups of manual therapies, exercise, and rehabilitation, and psychological intervention trials. Multiple meta-regression modelling demonstrated that similarity between sham control and tested interventions predicted variability in pain-related outcomes, attrition, and blinding effectiveness. Influential were differences relating to the extent of intervention exposure, participant experience, and treatment environments. The results support the supposed link between blinding methods and effect sizes, based on a large and systematically sourced overview of methods. Challenges to effective blinding are, however, complex, and often difficult to discern from trial reports. Nonetheless, these insights have the potential to change trial design, conduct, and reporting and will inform guideline development