Hydraulic turbines are increasingly required to operate under off-design conditions, which often leads to the development of hydraulic excitations such as the rotating vortex rope in the draft tube. When these excitations coincide with the natural frequencies of the structure, resonance phenomena may occur, posing risks to turbine integrity and performance. This paper presents a comprehensive investigation into the modal and dynamic response of a prototype Kaplan turbine with six blades subjected to the rotating vortex rope excitation, combining experimental and numerical approaches. An experimental modal analysis of the turbine rotor and runner was conducted in dry conditions using the hammer rowing technique. Excitations were applied with both an instrumented impact hammer and with a conventional hammer, and the structural responses were measured using industrial accelerometers fixed on the runner and the shaft. A numerical model of the complete rotor and the surrounding water around the runner was developed using a coupled acoustic–structural approach. Comparison between experimental and numerical results showed deviations below 11%, validating the numerical model. Results of the runner reveal that blade mode shapes form mode families characterized by blades vibrating with identical shapes but different phase shifts at six closely spaced frequencies. The addition of the shaft and/or the draft tube cone casing on the runner numerical model is found to have a negligible effect on the main modes, while the presence of water significantly influences the natural frequencies, particularly those associated with the runner modes. Finally, the dynamic analysis indicates that the axial and radial components of the rotating vortex rope primarily induce the highest strains on the roots of the blades and on the shaft next to the turbine bearing, respectively. • Experimental modal response of a full-scale Kaplan turbine rotor in air. • Acoustic–structural model predicts rotor modes in air and in water. • Mesh-resolution guidelines improve vibration accuracy at lower computational cost. • Water and casing added-mass effects on runner and rotor modes are quantified. • Strain hotspots under rotating vortex rope axial and rotating modes are predicted.
Lumbreras et al. (Sun,) studied this question.