The effect of draft tube oscillations on the performance of the turbine is very important. These oscillations create unwanted pressure and velocity fluctuations in the turbine system, which result in large power swings. Especially when the turbine is operating at off design conditions. The oscillations were studied at different cross sections of the draft tube and their FFT plots were analyzed. Vortex breakdown occurs at extreme operating conditions of the turbine. Further, the breakdown is of spiral type, with centerline stagnation preceded by a large disturbance signal. The predicted frequencies agree well with the measured ones.

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