On the generation mechanisms of low-frequency synchronous pressure pulsations in a simplified draft-tube cone

•The swirling flow in a Venturi-tube is experimentally and numerically studied.•Two types of pressure fluctuations with different frequencies are identified.•Low-frequency synchronous pressure fluctuations are observed.•The vortex breakdown location fluctuates with a low frequency.•The vortex center stretches spatially near the breakdown location.