The vertical axis wind turbines are simple in construction, self-starting, inexpensive and can accept wind from any direction without orientation. A combined Savonius-Darrieus type vertical axis wind rotor has got many advantages over individual Savonius or individual Darrieus wind rotor, such as better efficiency than Savonius rotor and high starting torque than Darrieus rotor. But works on the combined Savonius-Darrieus wind rotor are very scare. In view of the above, two types of models, one simple Savonius and the other combined Savonius-Darrieus wind rotors were designed and fabricated. The Savonius rotor was a three-bucket system having provisions for overlap variations. The Savonius-Darrieus rotor was a combination of three-bucket Savonius and three-bladed Darrieus rotors with the Savonius placed on top of the Darrieus rotor. The overlap variation was made in the upper part, i.e. the Savonius rotor only. These were tested in a subsonic wind tunnel available in the department. The various parameters namely, power coefficients and torque coefficients were calculated for both overlap and without overlap conditions. From the present investigation, it is seen that with the increase of overlap, the power coefficients start decreasing. The maximum power coefficient of 51% is obtained at no overlap condition. However, while comparing the power coefficients (C p ) for simple Savonius-rotor with that of the combined configuration of Savonius-Darrieus rotor, it is observed that there is a definite improvement in the power coefficient for the combined Savonius-Darrieus rotor without overlap condition. Combined rotor without overlap condition provided an efficiency of 0.51, which is higher than the efficiency of the Savonius rotor at any overlap positions under the same test conditions. r

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A steady-state two-dimensional computational fluid dynamics analysis was performed using FLUENT 6.2 software to analyze the performance of a twisted three-bladed H-Darrieus rotor. The flow over the rotor was simulated by using unstructured-mesh finite volume method coupled with moving mesh technique to solve mass and momentum conservation equations. The standard k-ε turbulence model was chosen. Second-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The aerodynamic coefficients, such as lift coefficient, drag coefficient, and lift-to-drag coefficient, were evaluated with respect to angle of attack for two chord Reynolds numbers. The power coefficient of the rotor was also evaluated. The results were validated by using experimental values for the twisted three-bladed H-Darrieus rotor. The experiments were earlier conducted in a subsonic wind tunnel available in the department. The results showed good matching between the two approaches. The effect of twist angle at the chord ends on the performance of the rotor was also evaluated.

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