Numerical detection and characterization of blade vortex interactions using a free wake analysis

Like their horizontal-axis counterparts, as the blades of vertical-axis wind turbines increase in size they typically become relatively more flexible so a better understanding of their aeroelastic behaviour is required. This research addresses the challenges of large, flexible, vertical-axis wind turbines using methods previously unavailable, or impractical due to limited computational resources. A weakly-coupled aeroelastic simulation was developed to investigate the dynamic behaviour of vertical-axis wind turbines. The aeroelastic simulation comprised a free vortex wake model to represent the aerodynamics, and a multibody systems model to represent the structural dynamics, with the models coupled together via an interface. A modified version of the Beddoes- Leishman dynamic stall model was used to account for unsteady aerodynamic effects on the blades. The modal characteristics of the structural model were extracted using the Eigensystem Realisation Algorithm. The aerodynamic aspects of the simulation were validated against a wide range of experimental data. The structural aspects of the simulation were verified against an industry-standard multi-body systems package, and validated against a combination of analytical, numerical and experimental data. An extensive set of tests were conducted on the aerodynamic and structural models demonstrating that quite computationally undemanding methods for the computational parameters, including the time-step sizes and levels of blade discretisation, were capable of producing results very similar to those predicted using much more computationally demanding values. The aeroelastic simulation was used to conduct a number of numerical experiments designed to aid an investigation into how the aeroelastic behaviour of a large-scale baseline turbine configuration changed in response to various parameters. It was shown that the asymmetric loading along the blades, caused by a combination of the gravitational loads and the vertical variation in height of the wind speed, results in a periodic vertical rocking motion of the blades. This rocking motion occurred at a frequency of twice per revolution. It enabled new insight into the dynamic motion of VAWT blades. It was also found that the periodic rocking motion could be reduced by increasing the stiffness of the blades and reducing the linear density of the blades.

Development of an Aeroelastic Simulation for the Analysis of Vertical-Axis Wind Turbines

The present work presents a study of unsteady aerodynamic modelling of horizontal axis wind turbine performance. The unsteady aspects addressed in this work include effects of variations in turbine inflow velocity due to operation in yawed flow, in the atmospheric boundary layer, in a wind tunnel, and due to the tower wake. In each case, the basis for the analysis is a prescribed wake vortex model, the development and enhancement of which has been the main focus of the work.

A high resolution model has been developed to meet the requirement for adequate representation of the tower shadow effects. A near wake dynamic model has been enhanced with appropriate modifications and integrated into the prescribed wake scheme to produce a hybrid method capable of predicting the detailed high resolution unsteady response in the tower shadow region. The azimuthal interval used within the shadow region can be reduced to 0.5° whilst the computational cost introduced by the high resolution near wake model is almost negligible.

A low order source panel method and the prescribed wake model have been combined into a coupled scheme capable of assessing the basic effect of wind tunnel walls on wind turbine flow and performance. The wind tunnel walls are discretised into a series of panels on which source singularities are placed. The source strengths are related to the turbine bound and wake vorticities via their induced velocities. The geometry of the turbine wake is obtained by superposition of the contribution of the disturbance velocities due to the source panels upon the prescribed wake. This new wake structure modifies the wind turbine aerodynamic performance in turn.

Unsteady aerodynamic modelling of horizontal axis wind turbine performance

Ce travail a pour objectif de caracteriser par le biais de la modelisation experimentale et numerique le comportement hydrodynamique d’une hydrolienne et les caracteristiques du sillage genere pour des conditions realistes de fonctionnement. Les maquettes d’hydroliennes considerees sont des turbines tri-pales a axe horizontal de 0,7 m de diametre. Une attention particuliere est portee sur les effets des caracteristiques de l’ecoulement sur les performances de la turbine, mais egalement sur le sillage genere. Les essais sont menes au bassin d’essais d’Ifremer a Boulognesur-Mer et au canal de traction de Brest, ou differents profils de courant, taux de turbulence de l’ecoulement amont et conditions houle/courant ont ete considerees. Les developpements d’un code tridimensionnel base sur les methodes vortex, prenant en compte l’evolution instationnaire du sillage genere par une turbine sont presentes. Les resultats numeriques obtenus sont compares aux resultats experimentaux, permettant de valider les outils en termes de rendement et d’evolution du sillage.

Caractérisation numérique et expérimentale des effets d'interaction entre une hydrolienne et le milieu marin

Unmanned aerial vehicles, especially agricultural unmanned helicopters (AUH), are nowadays extensively used in precision agriculture. AUHs have recently become responsible for spraying fertilizers and pesticides for crop yields. The strong downward rotating flow produced by the main rotor helps very uniform crop spraying which determines that how important is the aerodynamics of rotor blade in AUH. In this work, the aerodynamic performance of AUH rotor blades is evaluated and an efficient blade is obtained by numerically investigating the influence of design variables on the aerodynamics of rotor blades. The design variables consist of airfoil shape, pitch settings, and twist angle. The limited power available in hover and aerodynamic requirements (lift and drag) are the aerodynamic constraints. This analysis only considers the hovering flight condition at a constant rotational speed. The aerodynamically efficient rotor blade which is based on gradually varying and linearly twisted airfoil shapes, show a significant improvement in hover performance with relatively uniform blade loading. After testing, the optimum blade can be used as the main rotor in the AUH to perform precision farming.

https://jafmonline.net/JournalArchive/download?file_ID=43344&issue_ID=242

R.H. Miller

Papers

Methods for rotor aerodynamic and dynamic analysis