###### Abstract  
Low order models based on the Blade Element Momentum (BEM) theory exhibit modeling issues in the performance prediction of Vertical Axis Wind Turbines (VAWT) compared to Computational Fluid Dynamics, despite the widespread engineering practice of such methods. The present study shows that the capability of BEM codes applied to VAVTs can be greatly improved by implementing a novel three-dimensional set of high-order corrections and demonstrates this by comparing the BEM predictions against wind-tunnel experiments conducted on three small-scale VAVT models featuring different rotor design (H-shaped and Troposkein), blade profile (NAAC0021 and DU-06-W200), and Reynolds number (from \(0.8\times 10^{5}\) to \(2.5\times 10^{5}\)). Though based on the conventional Double Multiple Stream Tube (DMST) model, the here-presented in-house BEM code incorporates several two-dimensional and three-dimensional corrections including: accurate extended polar data, flow curvature, dynamic stall, a spanwise-distributed formulation of the tip losses, a fully 3D approach in the modeling of rotors featuring general shape (such as but not only, the Troposkein one), and accounting for the passive effects of supporting struts and pole. The detailed comparison with experimental data of the same models, tested in the large-scale wind tunnel of the Politecnico di Milano, suggests the very good predictive capability of the code in terms of power exchange, torque coefficient, and loads, on both time-mean and time-resolved basis. The peculiar formulation of the code allows including in a straightforward way the usual spanwise non-uniformity of the incoming wind and the effects of skew, thus allowing predicting the turbine operation in a realistic open-field in presence of the environmental boundary layer. A systematic study on the operation of VAVTs in multiple environments, such as in coastal regions or off-shore, and highlighting the sensitivity of VAVT performance to blade profile selection, rotor shape and size, wind shear, and rotor tilt concludes the paper. vertical axis wind turbine; BEM; DMST; experimental validation; dynamic stall; performance; design; wind shear; Reynolds number; polars +
Footnote †: journal: Applied Sciences  
cult development of VAWT technology, that lie in the flow complexity and aerodynamic forcing of a machine which features unsteady operation by design. However, the text also highlights several aspects that might make it advantageous with respect to HAWTs: insensitivity to yaw angle, low sensitivity to gust and skew, lower noise (due to lower optimal tip-speed ratios), the possibility to mount gearbox, electric generator and other systems on the ground. In the last decade, the development of VAWT technology has seen a significant acceleration, and recent works have enriched the experimental database on prototypes at laboratory scale [5; 6; 7; 8; 9; 10; 11; 12; 13], have improved the modeling techniques [14; 15; 16; 17; 18; 19; 20; 21; 22; 23], and have investigated issues related to the machine operation [21; 24; 25; 26; 27; 28]. This impulse has improved the VAWT concept, making it progressively competitive against the conventional HAWT. These considerations have led to reconsider the VAWT as a promising technology for the efficient energy harvesting in non-conventional environments, especially for small-scale urban/distributed installations. The same arguments, however, apply also in another, extremely relevant application branch, e.g., Floating Offshore Wind Turbines (FOWT), which experience a significant inflow complexity due to the movement of the platform. FOWTs operate under time-periodic skew and yaw conditions and, in the case of HAWTs, have to cope with large overturning moments produced by the heaviest components of the turbine, such as the drivetrain and the generator, which have to be placed high above the sea level.