## 7 Conclusions  
The paper presented a new formulation of well-known DMST algorithm employing already fine-tuned sub-models implemented into an up-to-date 3D BEM code, such as:  
* Glauert correction;
* polar extension;
* flow curvature;
* Strickland dynamic stall model;
* distributed tip loss correction;
* pole and struts parasitic torque correction;
* wind shear;
* wind skew/rotor tilt. The spanwise distribution of the corrections implemented led to develop a fully 3D BEM code, especially featuring the tip losses. The treatment of the correction factor \(F\) is therefore addressed and a novel implementation into the BEM algorithm is proposed. The overall effect of the tip loss model herein implemented shows good agreement with high-fidelity CFD. The in-house code was shown to be a flexible and fast tool to investigate VAWT performance with no limitations in the rotor shape: H-shaped and Troposkein layouts have been specifically investigated in the paper. The research has benefited from a wide and comprehensive experimental assessment with data available in the literature, especially for micro-generation VAWTs at different level of detail. Excellent or very good agreement has been found in terms of averaged quantities as \(C_{p}\), power and thrust forces. Phase resolved analyses for H-shaped VAWT have shown a trend very similar to the experimental one, with nearly identical peak values. Rotor tilt implementation has also been validated against measurements performed for the DeepWind Demonstrator for different rotational speeds. Figure 15: DeepWind 5 MW power coefficient and power with varying-(1) Airfoil (**a,d**); (2) Wind shear coefficient (**b,e**); (3) Rotor tilt (**c,f**). By virtue of the extended validation, the code has been applied to perform sensitivity analyses on rotor and airfoil shape according to diversified scenarios for VAWTs installation, from micro-scale or small-scale machines for distributed generation up to large scale offshore wind turbines. Analyses have also been performed varying the wind shear profile, considering a constant velocity at the equatorial plane, showing low effects on small Troposkein rotors and slightly higher impact on small H-shaped VAWTs. Conversely, the impact of wind shear has been found quantitatively significant for large-scale VAWT rotors, even in the less disturbed off-shore environment. The analysis was also aimed at obtaining a fast-response in evaluating the sensitivity to geometric parameters in the preliminary design stages of VAWTs; for this reason, two airfoil shapes have been considered for selected test cases (symmetrical NACA0021 and DU06W200). It has been found that NACA0021 exhibits higher performance as long as the Reynolds number remains below 200'000, while for higher Reynolds number values the DU06W200 starts outperforming NACA0021. This result suggests a careful approach when designing a VAWT according to Reynolds characteristic regimes. In light of such results, the here-proposed 3D BEM tool appears to be inherently suitable for estimating the energy harvested by a turbine during long-term operation: introducing a wind velocity distribution (e.g., Weibull distribution), or alternatively measured wind data at site such as those reported in [55], the 3D BEM code may allow predicting the Annual Energy Output with very limited computational cost. As a further application area, the tool developed herein may be exploited to perform a design optimization of the machine according to the specific site conditions. Many aspects still play a paramount role in trustful performance predictions: for instance, tests at low Reynolds number are still subjected to uncertainty due to laminar-turbulent transition on which research efforts are underway. Moreover, polars are sensible to the detailed surface quality and roughness; this sheds light on the importance in having accurate preliminary databases to enhance prediction accuracy, otherwise widely precluded to low-fidelity models.