Abstract
To clarify the cause of "S" instability, the three-dimensional simulation of the steady flow in a pump-turbine are performed using by the commercial CFD code. For lower flow rates and negative flow rates i.e., reverse pump region, the positive slope was observed in the performance curve and "S" curve is caused by the increase of the head width the decrease of flow rate. As the flow rate decreases, the backflow region near the pressure side of the leading edge of a blade is widened. For die reverse pump region, the high pressure region near the suction side of the trailing edge of a blade is observed.
