Modal Decomposition of the Precessing Vortex Core in a Hydro Turbine Model

We report on the experimental study of a precessing vortex core (PVC) in an air model Francis turbine. The focus is placed modal decomposition PVC that occurs draft tube turbine for range operation conditions. turbulent flow fluctuations are assessed using stereo particle image velocimetry (PIV) measurements. Proper orthogonal (POD) applied to antisymmetric and symmetric components velocity fields distinguish dynamics azimuthal instabilities. pressure pulsations induced by measured four sensors mounted wall hydro tube. Spatial Fourier signals identify contributions modes, m=1 m=2, total fluctuations. analysis based data shows similar results regarding identification PVC. contribution m=2 mode overall kinetic energy significant part load regimes, where rates twice as low at best efficiency point (BEP). It also shown this not higher harmonic PVC, suggesting it driven different instability. Finally, we show linear fit saturation amplitudes oscillations determine critical bifurcation points these modes. This yields swirl numbers Scr=0.47 0.61, respectively. fact turbines two individual instabilities relevant development tailored active control