When measuring turbine prototype flow, a number of modes with different flow rates and cavitating vortex rope settings have been measured. The turbulent flow in the draft tube of diameter 240 mm reaches the Reynolds number of 10 5 . The vortex and highly swirling flow is developed in the draft tube. The usage of kHz-order frame rates was necessary to resolve the velocity field. The investigated area in the mixing vessel was examined by the stereoscopic time-resolved PIV technique. Further mode

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... valuation allows comparison of cross-sectional velocities in the vector map. The Proper orthogonal decomposition analysis was applied on 3D vector maps, those including u, v, w velocity compounds. In the selected regimes -operating points with volume flow rate 97 l/sec and 114 l/sec, the greatest risk of transition to cavitation mode was identified, so they were studied in detail.