*7.1. Torque Coe*ffi*cient Comparison for Di*ff*erent (DR*/*DT)*

CFD simulations for different Darrieus to closed Savonius rotor diameter ratios (DR/DT) are performed in search of the optimum rotor diameters. The AHDT in closed configuration is simulated for five complete revolutions instantaneously recording the torque coefficient of blades for every degree of rotation. However, the solution is converged after three revolutions of the turbine. The coefficient of power (*Cp*) is calculated from (*Ct*) as shown in Equation (9).

$$\mathbf{C}\_p = \mathbf{C}\_t \times \text{TSR} \tag{9}$$

The operating TSR range (λ) considered in the present work is 1 to 4 for the two-bladed rotor at an increment of 0.2. Computing at a TSR, below the mentioned range, is of no significance for this optimization study, and above the indicated range means operating beyond stall conditions. For high solidities, power and efficiency rapidly decrease at stall conditions. The free stream wind speed for which the AHDT configurations are simulated is between 4 m/s and 9 m/s which is converted to Re for further descriptions in this study. The instantaneous torque coefficient (*Ct*) versus azimuth angle (θ) is computed for a range of Re <sup>=</sup> 1.2 <sup>×</sup> <sup>10</sup><sup>5</sup> to Re <sup>=</sup> 2.7 <sup>×</sup> <sup>10</sup>5. The results for the single blade are plotted in Figure 5a. For each Re, the various DR/DT ratios are evaluated for one complete revolution (0◦ to 360◦). It shows that as DR/DT ratio increases, the peak torque coefficient in the *Ct* cycle increases. The torque coefficient for the rotor at different Re and different DR/DT is displayed in Figure 6. It can be seen that the two peak torque coefficients constantly decrease as the diameter of the cylinder increases. Also, it is evident that as the diameter of the cylinder increases, the azimuthal angle at peak *Ct* shifts. This can be attributed to the change in the relative wind velocity due to vortices from the cylinder.

**Figure 5.** (**a**) Torque coefficient of single blade; (**b**) *Cp*max for different diametrical ratio.

**Figure 6.** Effect of Re on Torque Coefficient.
