Computational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of vertical-axis wind turbines, helping this technology to rise towards a more mature industrial diffusion. The unsteady flow past rotating blades is, however, one of the most challenging applications for a numerical simulation and some critical issues have not been settled yet.In this work, an extended analysis is presented which has been carried out with the final aim of identifying the most effective simulation settings to ensure a reliable fully-unsteady, two-dimensional simulation of an H-type Darrieus turbine.Moving from an extended literature survey, the main analysis parameters have been selected and their influence has been analyzed together with the mutual influences between them; the benefits and drawbacks of the proposed approach are also discussed.The selected settings were applied to simulate the geometry of a real rotor which was tested in the wind tunnel, obtaining notable agreement between numerical estimations and experimental data. Moreover, the proposed approach was further validated by means of two other sets of simulations, based on literature study-cases

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Thanks to the continuous improvement of calculation resources, computational fluid dynamics (CFD) is expected to provide in the next few years a cost-effective and accurate tool to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines. This rotor type is in fact increasingly welcome by the wind energy community, especially in case of small size applications and/or non-conventional installation sites. In the present study, unique tow tank experimental data on the performance curve and the near-wake structure of a Darrieus rotor were used as a benchmark to validate the effectiveness of different CFD approaches. In particular, a dedicated analysis is provided to assess the suitability, the effectiveness and the future prospects of simplified two-dimensional (2D) simulations. The correct definition of the computational domain, the selection of the turbulence models and the correction of simulated data for the parasitic torque components are discussed in this study. Results clearly show that, (only) if properly set, two-dimensional CFD simulations are able to provide - with a reasonable computational cost - an accurate estimation of the turbine performance and also quite reliably describe the attended flow-field around the rotor and its wake

 Analysis of the 3D unsteady aerodynamics of a Darrieus wind turbine blade in motion  Highly spatially and temporally refined time-dependent simulations carried out with the COSA code  One month calculation time on more than 16.000 processors on a IBM BG/Q cluster  Detailed description of: tip losses, dynamic stall, vortex propagation and blade/wake interaction ACCEPTED MANUSCRIPT Energized by the recent rapid progress in high-performance computing and the growing The reported CFD results provide a valuable and reliable benchmark for the calibration 36 of lower-fidelity models, which are still key to industrial design due to their very high execution 37 speed.

Computational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of Vertical-Axis Wind Turbines. The unsteady flow past rotating blades is, however, a challenging application for a numerical simulation and some critical issues have not been settled yet. In particular, if some studies in the literature report detailed analyses on the assessment of the computational model, there is still no adequate convergence on the requirements in terms of spatial and temporal discretizations. In the present study, a multivariate sensitivity analysis was first carried out on a specific case study at different tip-speed ratios in order to define the optimal mesh and timestep sizes needed for an accurate simulation. Once full insensitivity had been reached, the spatial and temporal requirements needed to properly describe the flow phenomena were related to two dimensionless numbers, one for each domain, which can be used to assess the suitability of the selected settings for each specific simulation. The simulations revealed that the spatial requirements must be selected in order to ensure an accurate description of velocity gradients in the near-blade region. To this purpose, a Grid-Reduced form of vorticity is proposed as the best indicator for the quality of the mesh refinement. It is also shown that the temporal requirements are made stricter at low tip-speed ratios by the need of correctly describing the vortices detaching from the blades in the upwind region. To do so, proper thresholds for the Courant Number are highlighted in the study