This study is concerned with the numerical simulations of the flow and performance of a rotor of a hydraulic Savonius turbine. The flow was simulated using the vortex in cell (VIC) method in combination with the volume penalization (VP) method. The torque acting on the rotor was computed using a custom method, which can accurately calculate the fluid forces on solid bodies in flows simulated by the VIC-VP method, to analyze the rotational speed and power output of the rotor. The simulated flow around the rotating blades agreed qualitatively with the experimentally visualized flow, and the analyzed relation between the rotor's rotational speed and power output agreed with the experimentally measured relation. These demonstrate that the VIC-VP method in combined with the authors' method for calculating fluid forces is applicable for the simulations of flow and performance of hydraulic Savonius turbines. (C) 2020 Elsevier Ltd. All rights reserved.