This video is a result of the CFD simulation of flow in a Francis turbine, operating at part load. As a result of the circumferential velocity component at the outlet of the runner, a vortex rope forms in the cone of the draft tube. Depending upon the pressure level the vortex rope may cavitate.
In this movie, the vortex rope is presented as iso-surface of vapour pressure. The simulation was performed as non-cavitating one.
Information about similar CFD simulations (cavitating or non-cavitating), with comparison with the experimental results, can be found in:
Jošt, D., Škerlavaj, A., Morgut, M., and Nobile, E., 2017. Numerical Prediction of Cavitating Vortex Rope in a Draft Tube of a Francis Turbine with Standard and Calibrated Cavitation Model. Journal of Physics: Conference Series 813(1), 012045. doi:10.1088/1742-6596/813/1/012045