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Draft tube vortex is a sign of flow instability of Francis turbine, and even leads to unit fatigue failure in serious cases. In order to accurately capture the transient turbulence characteristics of fluid flow in draft tube under different working conditions, the newly developed Liutex vortex identification method was used to capture and compare the draft tube vortex rope based on slip grid technology and SST k-ω turbulence model. The influence of upstream disturbance on formation, development, rupture of vortex rope and low-frequency pressure pulsation of wall was analyzed emphatically. The results showed that the accuracy of result was verified through a comparison with literature and experimental results; The shape of vortex rope in draft tube was different under different upstream disturbances. In the best efficiency point, a stable spindle-shaped vortex structure was formed, which had little effect on the flow field. When the flow rate was reduced to 81% of the best efficiency point, the spiral vortex rope was formed, and the eccentric rotational motion of the vortex rope had a great disturbance effect on main flow. For example, due to the squeezing effect of vortex, an obvious highspeed zone appeared between the vortex structure and wall surface, and the amplitude of pressure coefficient was increased by 1.36~4 times. The pressure fluctuation in draft tube presented typical characteristics of low frequency and high amplitude. As the opening continued to be decreased, the volume of vortex rope was increased greatly, forming a large cavity vortex zone, and occupying a wide range and hit against the draft tube wall directly. With the decrease of the guide vane opening, the chaotic vortex and unstable flow field structure generated in draft tube were increased. When the opening dropped to the lowest, the original shape of vortex rope was damaged, and the broken little vortex filled the whole draft tube.