14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
Nowadays, the tendency in the design of small-scale wind energy recovery systems is towards Ducted Wind Turbines (DWTs) exhibiting an appreciable power density increase compared to their open rotor competitor. The aerodynamic shaped of the duct enclosing the rotor allows, in fact, an increase of the amount of extracted power with the same rotor radius dimension. The strong interaction between the duct and the rotor drastically changes the entire flow field past the machine, so that accurate analysis tools are needed, if a realistic estimate of the DWT performance is sought. Better previsions generally lead to a deeper knowledge of the system characteristics which can be profited of during the design phase. The aim of this work is to build up a solid, accurate and automated methodology for the performance analysis of DWTs in view of their CFD-aided optimal design. Due to the complex flow features characterizing the device, the lowest acceptable fidelity level is represented by the blade resolved 3-D Reynolds-Averaged Navier-Stokes simulations (RANS). One blade passage is meshed via a multi-block hexahedral structured grid generator. An in-house built python routine has been designed to automatically drive the structured mesh generation process. Multigrid strategy and local time-stepping are applied to speed up the convergence, while the turbulence is modelled both with the Spalart-Allmaras and the k-ω SST closures. Low Mach number preconditioning is also adopted. The size of the computational domain has been investigated in order to minimize any interaction between the flow field around the DWT and the boundary conditions applied on the external surfaces. The analysis of the three-dimensional flow field has been focussed on the flow structures generated by the interaction between the duct and the rotor, particularly in the tip-gap and near hub regions. The distortion of the near wake at the trailing edge, as induced by the boundary layer features at the duct throat, is presented and discussed in some details.