## 5 3D BEM-DMST Code Experimental Validation
### Microgeneration H-Shaped 200 W  
The performance of the H-shaped VAWT (rated power 200 W) is first considered. The contribution of the 3D BEM sub-models are discussed with the support of Figure 5a,b to achieve a detailed comprehension of the sub-model effects in terms of power coefficient and power. A standard BEM curve ('BEM STD') gives, as expected, an overestimation of the performance; combined effects of dynamic stall and flow curvature ('+DS+FC' curve) shift \(C_{P}\) peak at lower TSRs, which is a peculiar effect induced by flow curvature, and reduce power beyond 10 m/s. In details, flow curvature severely decreases turbine performance however dynamic stall mitigates its detrimental effect resulting in \(C_{P}\) curve mostly affected at high TSRs, even though correspondent power depletion is limited. The effect of passive torque is remarkable ('+struts+pole') lowering the entire curve both in terms of dimensional and non-dimensional power, especially at low wind speeds; this is reasonable since the drag forces at low wind speeds 'weight' more than at high ones due to non-linear character of passive torque contributions. Same conclusion was found by CFD analysis carried out in [28] with the same H-shaped VAWT: \(C_{P}\) difference at medium and high TSRs exceeds 0.1 percentage points between 2D and 3D simulation results. The activation of all sub-models \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**VAWT Model** & **\(\Omega\) [rpm]** & **Layout** & **Airfoil** & **Re\({}^{-}-\)Re\({}^{+}\)\({}^{*}\)** \\ \hline Microgeneration H-shaped 200 W & 400 & upright & NACA0021 & 0.79–1.71 \(\times\)10\({}^{5}\) \\ \hline Microgeneration Tropsokein 450 W & 400 & upright & NACA0021 & 1.22–2.24 \(\times\)10\({}^{5}\) \\ \hline DeepWind Tropsokein Demonstrator & 200 & upright & & 0.96–1.75 \(\times\)10\({}^{5}\) \\  & 300 & tilted & DU06W200 & 0.95–1.76 \(\times\)10\({}^{5}\) \\  & 300 & upright & & 1.45–2.60 \(\times\)10\({}^{5}\) \\  & & tilted & & 1.47–2.58 \(\times\)10\({}^{5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of cases for experimental validation. (*) Maximum and minimum Reynolds numbers are referred to optimal TSRs, at equatorial plane. Figure 4: Airfoil coordinates. gives 'BEM full 3D' curve which encompasses 3D tip losses: the poor \(C_{P}\) reduction at low TSRs actually becomes as a non-negligible decrease in power from 10 m/s, resulting crucial for enhancing the accuracy of the prediction at high wind speed. Figure 5 summarizes the comparison between predictions and experiments, showing very good agreement in terms of power coefficient and power curves, both laying within the uncertainty bands. The highest deviation of power coefficient between experimental and numerical results is found at high TSRs (\(\lambda=3.3-3.5\)), Figure 4(a). However, in dimensional terms this difference appears practically negligible, being even within the experimental uncertainty as shown by Figure 4(b); the good match of experimental data at high TSRs/low wind speed is somehow expected, due to the limited angles of attack and the inherent accuracy of the polars in the fully attached region. At high wind speeds, where detached flow, post-stall and deep-stall take place, the agreement is very good, thus showing that the treatment of the blade aerodynamics beyond the static stall is appropriate for this kind of machine. This might be due to the operation of the VAWT well beyond static stall angles of attack, for most part of the rotation, somehow limiting the hysteresis loops of lift and drag forces, or alternatively due to the reliability of the dynamic stall model. Coherently with the performance prediction, in the same wind speed range the forces on the rotor tend to increase as depicted by Figure 4(b), where the 3D BEM code shows a very good agreement with the experimental data, both in terms of dimensional and non dimensional values. A slight deviation of thrust force from experiments is observed at high  
Figure 5: Comparison of experimental and 3D BEM results for the H-shaped 200 W case.