This short video is a result of the CFD simulation of flow in a Francis turbine, operating at the best-efficiency point (BEP). A coherent structure in the center of the draft tube cone is visible. At BEP there is no circumferential velocity component at the inlet of the draft tube. Therefore, contrary to the part load, vortex structures move in the direction of the draft tube.
More information about the CFD simulation with the zonal LES turbulence model can be obtained in the paper:
Jošt, D., Škerlavaj, A., Morgut, M., Mežnar, P., and Nobile, E., 2015. Numerical simulation of flow in a high head Francis turbine with prediction of efficiency, rotor stator interaction and vortex structures in the draft tube. Journal of Physics: Conference Series 579(1), 012006. doi:10.1088/1742-6596/579/1/012006