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The numerical simulation of unsteady cavitation flows is sensitive to the selected models and associated parameters. Consequently, three Reynolds Average Navier-Stokes (RANS) turbulence models and the Zwart cavitation model were selected to assess their performance for the simulation of cloud cavitation on 2D hydrofoils. The experimental cavitation tests from a NACA65012 hydrofoil at different hydrodynamic conditions were used as a reference to tune the modeling parameters and the experimental tests from a NACA0015 were finally used to validate them. The effects of near wall grid refinement, time step, iterations and mesh elements were also investigated. The results indicate that the Shear Stress Transport (SST) model is sensitive to near wall grid resolution which should be fine enough. Moreover, the cavitation morphology and dynamic behavior are sensitive to the selection of the Zwart empirical vaporization, F v , and condensation, F c , coefficients. Therefore, a multiple linear regression approach with the single objective of predicting the shedding frequency was carried out that permitted to find the range of coefficient values giving the most accurate results. In addition, it was observed that they provided a better prediction of the vapor volume fraction and of the instantaneous pressure pulse generated by the main cloud cavity collapse.

The present paper assesses the performance and characteristics of fiber Bragg grating sensors, with a special interest in their applications in hydraulic machinery and systems. The hydropower industry is turning to this technology with high expectations of obtaining high quality data to validate and calibrate numerical models that could be used as digital twins of key assets, further strengthening the sector’s relevant position within industry 4.0. Prior to any validation, fiber Bragg grating sensors’ ability to perform well underwater for long periods of time with minimal degradation, and their ease of scalability, drew the authors´ attention. A simplified modal analysis of a partially submerged beam is proposed here as a first step to validate the potential of this type of technology for hydropower applications. Fiber Bragg grating sensors are used to obtain the beam’s natural frequencies and to damp vibrations under different conditions. The results are compared with more established waterproof electric strain gauges and a laser vibrometer with good agreement. The presence of several sensors in a single fiber ensures high spatial resolution, fundamental to precisely determine vibration patterns, which is a main concern in this industry. In this work, the beam’s vibration patterns have been successfully captured under different excitations and conditions.

Cavitation erosion is a major concern in hydraulic systems exposed to strong pressure fluctuations. Well-developed experimental techniques exist for detecting cavitation based on measuring induced noise or vibrations, but additional tools are needed to assess its aggressiveness under operating conditions. This study investigates the capability of acoustic emission (AE) to characterise cavitation aggressiveness during long-duration cloud cavitation. A 50 h erosion test was performed in a closed-loop cavitation tunnel using a Venturi equipped with an aluminium 7075-T6 specimen. Hydraulic conditions were controlled to maintain a constant cavity length, and AE signals were recorded every 10 min during two representative 4 h intervals at 34–38 h and 46–50 h. A new AE-derived power parameter was defined using the amplitude distribution of AE envelope peaks. Both the number of impacts and the power parameter increased markedly from the intermediate to the final interval, consistent with the growth of erosion and increasing surface roughness. Conversely, both quantities decreased systematically within each 2 h test as water temperature increased. Image analysis of selected areas confirmed the progression of pitting between 34 and 50 h. Overall, the findings demonstrate that AE can capture the combined influence of temperature and surface degradation on cavitation aggressiveness, highlighting its potential as a monitoring technique for hydraulic components.

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.

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.

A sample of healthy wind turbines from the same wind farm with identical sizes and designs was investigated to determine the average vibrational signatures of the drive train components during normal operation. The units were variable-speed machines with three blades. The rotor was supported by two bearings, and the drive train connected to an intermediate three-stage planetary/helical gearbox. The nominal 2 MW output power was regulated using blade pitch adjustment. Vibrations were measured in exactly the same positions using the same type of sensors over a six-month period covering the entire range of operating conditions. The data set was preliminary validated to remove outliers based on the theoretical power curves. The most relevant frequency peaks in the rotor, gearbox, and generator vibrations were detected and identified based on averaged power spectra. The amplitudes of the peaks induced by a common source of excitation were compared in different measurement positions. A wind speed dependency of broadband vibration amplitudes was also observed. Finally, a fault detection case is presented showing the change of vibration signature induced by a damage in the gearbox.

To have a safe structural design, an analysis of the dynamic behavior of a Francis turbine runner with consideration of the added mass effects of surrounding water is necessary during design phase. Both in design and at off-design operations, large-scale forms of attached cavitation may appear on runner blades and can change the added mass effects of the surrounding fluid in relation to a single water domain. Consequently, a numerical investigation of the modal response of a Francis runner has been carried out by reproducing the presence of various sizes of leading edge cavitation (LEC) and trailing edge cavitation (TEC). The fluid–structure interaction problem has been solved by means of an acoustic-structural coupling method. The calculated added mass effects with cavitation have been compared with those corresponding to the pure water condition without cavitation. Firstly, a single blade has been investigated to evaluate the level of significance for the proposed cavity shapes and dimensions. Afterwards, based on the results obtained, the complete runner structure has been considered, factoring in similar cavity shapes and locations. The results prove that significant added mass effects are induced on the entire runner by the attached cavitation that increase the natural frequencies of the first modes. Moreover, the added mass effects increase with cavity size and amplitude of blade deformation below the cavity.

This paper presents a vibration analysis method for detecting typical faults in gears of the drive train of a 2 MW wind turbine. The data were collected over a one-year period from an operating wind turbine with a gearbox composed of one planetary stage and two helical gear stages. Failures in two pairs of helical gears were identified: one involving pitting and wear in the gears connecting the intermediate-speed shaft to the low-speed shaft, and another one involving significant material detachment in the gears connecting the intermediate-speed shaft to the high-speed shaft. The continuous evaluation of time signals, frequency spectra, and amplitude modulations allowed the most sensitive sensors and frequencies for predicting surface damage on gear teeth in this type of turbine to be determined. A steady-state frequency analysis was performed, enabling the detection of the aforementioned surface faults. This approach is simpler compared with more complex transient-state techniques. By tracking vibration signals over time, the importance of analyzing gear mesh frequencies and their harmonics was highlighted. Additionally, it was found that the progression of gear damage was dependent on the power output of the wind turbine. As a result, the most appropriate ranges of power were identified, within which the evolution of the vibration measurement was associated with the damage evolution. Since many turbines currently in operation have similar designs and power output levels, the present findings can serve as a guideline for monitoring an extensive number of units.

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.

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.