Numerical ascertainment of improving performance of modified Savonius rotor as decreasing the minimum wind speed required for initiating the rotation is investigated using 3D flow predictions executed in ANSYS-FLUENT. For making the comparison feasible, the design represented in a similar paper is simulated and performance is compared with empirical data. Different types of nozzles are added after observing acceptable agreement between empirical and simulation results. Nozzle acts like an air intake on the advancing blade and fortifies the acting flow; it also eliminates the reverse moment caused by contacting the flow with returning blade. Therefore, increasing in power coefficient is anticipated. Systematic analysis of obtained torque and power coefficients with and without nozzle are presented. In the next step, this work is dedicated to bucket shape optimization. Buckets with multi-curvature are introduced and have been investigated in three stages. By installing a simple nozzle with tail in front of a Savonius wind turbine with double-curved two stage buckets, the Savonius turbine maximum power coefficient is improved from 0.13 to 0.39. (C) 2018 Elsevier Ltd. All rights reserved.