Nowadays, hydropower plants are being used to compensate for the variable power produced by the new fluctuating renewable energy sources, such as wind and solar power, and to stabilise the grid. Consequently, hydraulic turbines are forced to work more often in off-design conditions, far from their best efficiency point. This new operation strategy increases the probability of erosive cavitation and of hydraulic instabilities and pressure fluctuations that increase the risk of fatigue damage and reduce the life expectancy of the units. To monitor erosive cavitation and fatigue damage, acoustic emissions induced by very-high-frequency elastic waves within the solid have been traditionally used. Therefore, acoustic emissions are becoming an important tool for hydraulic turbine failure detection and troubleshooting. In particular, artificial intelligence is a promising signal analysis research hotspot, and it has a great potential in the condition monitoring of hydraulic turbines using acoustic emissions as a key factor in the digitalisation process. In this paper, a brief introduction of acoustic emissions and a description of their main applications are presented. Then, the research works carried out for cavitation and fracture detection using acoustic emissions are summarised, and the different levels of development are compared and discussed. Finally, the role of artificial intelligence is reviewed, and expected directions for future works are suggested.

This paper focuses on a resonant system used to induce a low-amplitude movement and ultrasonic frequency to complement a ball burnishing process on a lathe. The system was characterized through the combination of different techniques. A full vibratory characterization of this process was undertaken with the purpose of demonstrating that the mechanical system—composed of the tool and the machine—does not present resonance phenomena during the execution of the operation that could lead to eventual failure. This dynamic analysis validates the adequateness of the tool when attached to an NC lathe, which is important to guarantee its future implementation in actual manufacturing contexts. A further aim was to confirm that the system succeeds in transmitting an oscillating signal throughout the material lattice. To this end, different static and dynamic techniques that measure different vibration ranges—including impact tests, acoustic emission measurement, and vibration measurement—were combined. An operational deflection shape model was also constructed. Results demonstrate that the only high frequency appearing in the process originated in the tool. The process was not affected by the presence of vibration assistance, nor by the burnishing preload or feed levels. Furthermore, the frequency of the assisting ultrasonic vibration was characterized and no signal due to possible damage in the material of the specimens was detected. These results demonstrate the suitability of the new tool in the vibration-assisted ball burnishing process.

In this paper, a resonant system that produces a movement of low amplitude and ultrasonic frequency is used to achieve the vibration assistance in a ball-burnishing process. A full vibration characterization of this process performed in a lathe was done. It is carried out by a new tool designed in the research group of the authors. Its purpose is to demonstrate that the machine and the tool do not have any resonance problem during the process and to prevent possible failures. The analysis of this dynamic behaviour permits to validate the suitability of the tool when it is anchored to a numerical control lathe. This is very important for its future industrial implementation. It is also intended to confirm that the system adequately transmits vibrations through the material. To do this, a methodology to validate the dynamic tool behaviour was developed. Several techniques that combine the usual and ultrasonic vibration ranges through static and dynamic measurements were merged: vibration and acoustic emission measurements. An operational deflection shape (ODS) exercise has been also performed. Results show the suitability of the tool used to transmit the assistance vibrations, and that no damage is produced in the material in any case.

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.

Cavitation erosion is a well-known problem for hydraulic turbines working at off-design conditions which is currently worsening because their operating range needs to be widened to cope with the variability of the new renewable energy sources, such as wind and solar. The use of acoustic emission (AE) measuring ultrasonic vibrations induced by the erosion process is believed to be the most promising technique to monitor cavitation erosion. In this paper, a 50-hour cavitation test performed in a cavitation tunnel is presented. The test was divided in periods of two hours in order to limit the heating of the water generated by the pump operation. During two specific intervals between 34 to 38 h and between 46 to 50 h, respectively, acoustic emission measurements were taken every 10 minutes. At 38 and 50 h, photographs of the erosion were taken which demonstrated a significant increase of erosion. Based on the AE measurements, cavitation impacts were estimated, sorted into classes by amplitude and also using a new “power parameter”. It was seen that the amount of impacts and the mean “power parameter” corresponding to a 2-h’ time interval increased significantly between the first and the second interval. But during the 2 h period within each interval, the number of impacts and the power parameter decreased as the temperature increased.