This paper presents the simulation of the dynamic behaviour of variable speed pump-turbine. A power reduction scenario at constant wicket gate opening was numerically analysed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second.\udThe flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenome¬na were characterized by frequency and time-frequency analyses of several numerical signals (pressure, blade torque, flow rate in blade passages). The un¬steady pattern in return channel strengthened em-phasizing its char¬ac¬teristic frequency with the rotational velocity decreasing reaching a maximum and then disappearing. At lower rotational speed, the flow field into the wickets gates channel start to manifest a full three-dimensional flow structure. This disturbance was related to the bound¬ary layer separation and stall and it was noticed by a specific fre¬quency.
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