Spatial-Temporal Evolution Analysis of the Three-Dimensional Coherent Structure in the Draft Tube Flow Field of a Francis Turbine

抄録

In order to analyze the flow characteristics of the internal flow field in the draft tube of the Francis turbine and reveal the coherent structure evolution, the flow in Francis-99 model turbine are simulated under three working conditions. The three-dimensional coherent structure and dynamic information of single fluctuation frequency in the draft tube under different working conditions are extracted based on numerical computation by dynamic mode decomposition (DMD) method. The averaged flow field (mode 0) occupy the most flow field energy (>96%), and the first 6th modes occupy more than 99.5% flow field energy, representing the dominant flow structure in draft tube. The spatial coherent structures of mode 1 to mode 6 show multiple spiral vortex ropes surrounding the center vortex rope of mode 0 coherent structure, and the number of the vortex ropes equal to mode number. The temporal evolutions of unsteady flow structures of modes with nonzero frequency repeat the cycle that, generating from the draft tube inlet, growing to entire flow passage downward, disappearing slowly and arising again.