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The Savonius hydrokinetic turbine (SHT) is widely used for generating electricity from running water. However, most optimization work has been carried out on conventional blades with similar concave and convex profiles. This study aims to enhance SHT performance by modifying the rotor blades' outer surface radius (0.079, 0.087 and 0.095 m) to create a semi-elliptical shape, thus reducing opposing forces. The tip speed ratio (TSR) varies from 0.5 to 1.3 with an interval of 0.1. A constant channel velocity of 0.8 m/s at Re = 2.25 × 105 is considered for the analysis. The flow field has been numerically investigated using the SST k - ω model. This study comprises the angular variation in the coefficients of power (Cp) and torque (Cm), performance curves of the rotor, and pressure distribution on the blade surface at different angular positions. It is observed that the rotor with a radius of 0.095 m has a maximum Cp value of 0.142, which is 7.57% and 18.33% higher than the Cp values of rotors with radii of 0.079 m and 0.087 m, respectively. The maximum power output of the rotor with a radius of 0.095 m is 2.32 W, whereas the power outputs of the rotors with radii of 0.087 m and 0.079 m are 2.16 W and 1.96 W, respectively. An increase in the instantaneous values of Cm between rotation angles 0◦ to 115◦ is observed, during which the returning blades mainly interact with the incoming stream. The pressure decreases as the radius of the semi-elliptical outer surface increases at rotor positions ranging from 0◦ to 225◦, but it increases at rotor positions ranging from 270◦ to 315◦.