The rotating vortex rope, which can be decomposed in the rotating and the plunging modes, is the origin of pressure fluctuations in the draft tube cone when hydraulic turbines operate at part load, compromising the structural integrity and limiting the output load. A measurement campaign was carried out in a Kaplan turbine model which is a replica of the experimental 10 MW Porjus U9 prototype machine along a propeller curve to study the rotating vortex rope’s excitation levels and the induced structural responses. A complete set of sensors mounted on-board and off-board was used to measure pressures, forces, torques, accelerations, displacements, and strains. The characteristic frequencies and amplitudes of the pressure fluctuations and of the corresponding induced loads and vibrations associated with the two modes were quantified in a wide range of operating conditions at part load. The two modes are detected at different frequencies depending on the sensor position. Moreover, their frequencies change depending on the discharge and present different amplitudes depending on the mode. Particularly, the rotating mode shows higher amplitudes than the plunging mode in the majority of positions and directions measured.

The current renewable energy market forces hydraulic turbines to operate for longer periods of time at off-design and transient conditions. Their life expectancy is then decreased due to the wear provoked by flow instabilities and stochastic flow excitations. This study presents an experimental investigation into the fatigue damage induced on the runner blades of a Kaplan turbine model when working at speed-no-load (SNL), part load (PL) and during ramps of load. The unit was equipped with on-board sensors on the blades and the shaft as well as with off-board sensors installed on the supporting structure and the draft tube cone. The results reveal that operation at SNL induces more fatigue damage on the runner blades than at PL. The damage is then mainly induced by stochastic flow excitations at SNL and by the rotating mode of the rotating vortex rope (RVR) at PL. The ramps of load, when crossing each operating condition, cause levels of damage similar to those found in stationary operation. Finally, it was proved that the blade fatigue damage can be estimated from on-board shaft measurements at any condition. However, the blade fatigue damage can only be estimated using off-board measurements when the RVR is fully developed at PL.

The structural dynamic response of hydraulic turbines needs to be continuously monitored to predict incipient failures and avoid catastrophic breakdowns. Current methods based on traditional off-board vibration sensors mounted on fixed components do not permit inferring loads induced on rotating parts with enough accuracy. Therefore, the present paper assesses the performance of fiber Bragg grating sensors to measure the vibrations induced on a rotating shaft–disc assembly partially submerged in water resembling a hydraulic turbine rotor. An innovative mounting procedure for installing the sensors is developed and tested, which consists of machining a thin groove along a shaft line to embed a fiber-optic array that can pass through the bearings. At the top of the shaft, a rotary joint is used to extract, in real time, the signals to the interrogator. The shaft strain distribution is measured with high spatial resolution at different rotating speeds in air and water. From this, the natural frequencies, damping ratios, and their associated mode shapes are quantified at different operating conditions. Additionally, the change induced in the modes of vibration by the rotation effects is well captured. All in all, these results validate the suitability of this new fiber-optic technology for such applications and its overall better performance in terms of sensitivity and spatial resolution relative to traditional equipment. The next steps will consist of testing this new sensing technology in actual full-scale hydraulic turbines.

To accelerate the integration of fluctuating renewable energy technologies in the power systems, it is necessary to increase the flexibility of hydropower by operating turbines at off-design conditions. Unfortunately, this strategy causes deleterious flow phenomena such as von Kármán’s vortices at the wake of the vanes and/or blades. When their shedding frequency lies in the vicinity of a structure’s natural frequency, lock-in occurs and vibration amplitudes increase significantly. Moreover, if cavitation occurs at the centers of these vortices, the structure’s dynamic response will be modified. In order to understand this interaction and to avoid its negative consequences, the vibration behavior of a NACA 0009 hydrofoil under a torsional lock-in condition was numerically simulated for cavitation-free and cavitating-flow conditions. The results showed that the presence of vortex cavitation modified the formation and growth process of shed von Kármán vortices in the near-wake region which, in turn, caused an increase of the work performed by the hydrofoil deformation on the surrounding flow and a sharp decrease of the maximum vibration amplitude under resonance conditions.

The growing installation of intermittent renewable energy sources is forcing hydraulic turbines to work more frequently at speed no load when dangerous vaneless space vortex structures and stochastic flow phenomena can occur. An experimental campaign has been performed in a reduced-scale Kaplan turbine model at speed no load for different combinations of guide vane and runner blade angles under non-cavitation and cavitation conditions. Several simultaneous vaneless space vortex structures, all of them inducing torsional rotor vibrations, have been observed. Nonetheless, only one of them has been found to dominate over the rest depending on the blade and guide vane angles. Off-board pressures, torques and vibrations as well as on-board strains have been measured to characterize their nature, intensity, dynamic behavior and induced structural response. Their precession frequencies have been found to depend on the discharge factor, the number of vortices and their location inside the vaneless space. Under cavitation conditions, their dynamic behavior has not been significantly altered but the induced structural response has increased at the low-pressure side of the turbine.

The current paper presents an investigation into novel modal testing methods applied to a disk–shaft structure at different rotating speeds in air and water. The structure was excited using three different methods: an instrumented hammer, a piezoelectric PZT patch glued on the disk and a transient ramp-up. The structural response was measured using an accelerometer and strain gauges mounted on board as well as accelerometers and displacement lasers mounted off board. The potential to excite the natural frequencies using each excitation method and to detect natural frequencies with each sensor was analyzed and compared. Numerical structural and acoustic–structural modal and harmonic analyses of the non-rotating disk in air and water were also performed, taking into consideration the PZT patch. The numerical results showed a close agreement with the experimental ones in both air and water. It was found that the rotating speed of the disk modified the detected natural frequencies, depending on the frame of reference of the sensor. Finally, the PZT patch and the transient ramp-up were proven to be reliable methods to excite the natural frequencies of the current test rig and to be potentially applicable in full-scale hydraulic turbines under operating conditions.