PUBLICATIONS AND CONFERENCES

Papers we published...

Large-Eddy Simulation of Cavitating Tip Leakage Vortex Structures and Dynamics around a NACA0009 Hydrofoil, Marine Science and Engineering 2021

Abstract: The tip leakage vortex (TLV) has aroused great concern for turbomachine performance, stability and noise generation as well as cavitation erosion. To better understand structures and dynamics of the TLV, large-eddy simulation (LES) is coupled with a homogeneous cavitation model to simulate the cavitation flow around a NACA0009 hydrofoil with a given clearance. The numerical results are validated by comparisons with experimental measurements. The results demonstrate that the present LES can well predict the mean behavior of the TLV. By visualizing the mean streamlines and mean streamwise vorticity, it shows that the TLV dominates the end-wall vortex structures, and that the generation and evolution of the other vortices are found to be closely related to the development of the TLV. In addition, as the TLV moves downstream, it undergoes an interesting progression, i.e., the vortex core radius keeps increasing and the axial velocity of vortex center experiences a conversion from jet-like profile to wake-like profile

Effect of the Current-Wave Angle on the Local Scour Around Circular Piles, J. of Waterway, Port, Coastal, and Ocean Engineering 2021

Abstract: This paper studies the effect of the wave front-current angle on the scour around a circular pile. An experimental study was carried out in the Coastal, Ocean and Sediment Transport (COAST) laboratory at the University of Plymouth (UK) using a single monopile of 0.125 m in diameter and an 8 m long by 1.5 m wide by 0.2 m high sand pit. The results obtained during the test campaign show the influence of the angle between waves and currents on both the maximum scour depth and the time scale of the process. Wave fronts partially aligned with current (65°) produce deeper scour holes than perpendicular forcing conditions (90°). Wave fronts partially against the current (115°) produce less scour than any of the two previous scenarios. The addition of waves reduced the maximum scour depth, compared with the current-only case. The development of the scour hole was found to be more rapid when waves are added to the current, with 50% of the final scour achieved in half the time. The results show that wave direction relative to the current is an important component in scour prediction.

Understanding the Influence of Wake Cavitation on the Dynamic Response of Hydraulic Profiles under Lock-In Conditions, Energies 2021

Abstract: 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.

Fiber Bragg Grating Sensors for Underwater Vibration Measurement: Potential Hydropower Applications, Sensors 2021

Abstract: 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.

Numerical Investigation of the Cavitation Effects on the Vortex Shedding from a Hydrofoil with Blunt Trailing Edge, Fluids 2020

Abstract: Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 with a blunt trailing edge at free cavitation conditions and at two degrees of cavitation developments has been carried out by means of the Zwart cavitation model and the LES WALE turbulence model which permits predicting the laminar to turbulent transition of the boundary layers. To analyse the dynamic behaviour of the vortex shedding process and the coherent structures, two identification methods based on the Eulerian and Lagrangian reference frames have been applied to the simulated unsteady flow field. It is found that the cavitation occurrence in the wake significantly changes the main vortex shedding characteristics including the morphology of the vortices, the vortex formation length, the effective height of the near wake flow and the shedding frequency. The numerical results predict that the circular shape of the vortices changes to an elliptical one and that the vortex shedding frequency is significantly increased under cavitation conditions. The main reason for the frequency increase seems to be the reduction in the transverse separation between the upper and lower rows of vortices induced by the increase in the vortex formation length.

Numerical Simulation of Cavitation Erosion Aggressiveness Induced by Unsteady Cloud Cavitation, Applied Sciences 2020

Abstract: A numerical investigation of the erosion aggressiveness of leading edge unsteady cloud cavitation based on the energy balance approach has been carried out to ascertain the main damaging mechanisms and the influence of the free stream flow velocity. A systematic approach has permitted the determination of the influence of several parameters on the spatial and temporal distribution of the erosion results comprising the selection of the cavitation model and the collapse driving pressure. In particular, the Zwart, Sauer and Kunz cavitation models have been compared as well as the use of instantaneous versus average pressure values. The numerical results have been compared against a series of experimental results obtained from pitting tests on copper and stainless steel specimens. Several cavitation erosion indicators have been defined and their accuracy to predict the experimental observations has been assessed and confirmed when using a material-dependent damaging threshold level. In summary, the use of the average pressure levels during a sufficient number of simulated shedding cycles combined with the Sauer cavitation model are the recommended parameters to achieve reliable results that reproduce the main erosion mechanisms found in cloud cavitation. Moreover, the proposed erosion indicators follow a power law as a function of the free stream flow velocity with exponents ranging from 3 to 5 depending on their definition.

Improvement of cavitation mass transfer modeling by including Rayleigh–Plesset equation second order term, European Journal of Mechanics - B/Fluids 2020

Abstract: The current study evaluates the effect of taking into account the second order term of the Rayleigh–Plesset equation for the numerical simulation of cavitation with homogeneous mixture models. For that, the corrected expression for the condensation mass transfer rate has been mathematically derived and implemented in the original Zwart and Singhal cavitation models. Two tests cases of steady sheet cavitation around a NACA 0009 hydrofoil and a hemi-spherical body at different cavitation coefficients have been simulated with both the original and the corrected models. The results demonstrate that the pressure distribution at the closure region of the attached cavity is better predicted and a stronger pressure gradient is obtained. Consequently, the cavity length is slightly shortened and it gets closer to the experimental observations. Another test case of cloud cavitation around a NACA 0009 hydrofoil has confirmed that the prediction of the shedding frequency is also improved with the corrected Zwart model because the maximum cavity length is significantly reduced. In conclusion, the proposed modification of the Zwart and Singhal cavitation models helps to ameliorate the numerical simulation of both steady and unsteady cavitation flows.

Assessment of RANS turbulence models and Zwart cavitation model empirical coefficients for the simulation of unsteady cloud cavitation, Eng. Applications of Computational Fluid Mechanics 2019

Abstract: 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, Fv, and condensation, Fc, 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.

On the Rotating Vortex Rope and its induced structural response in a Kaplan Turbine Model, Energies 2022

Abstract: 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.

Prediction of cavitation erosion with different erosion risk indicators, Ocean engineering 2022

Abstract: This present work devoted to simulate the unsteady cavitating flow around a hydrofoil and assess its erosion power predicted by different erosion risk indicators. For that, the behavior of unsteady cloud cavity is numerically reproduced using density corrected Shear Stress Transport (SST) k-w turbulence model and the mass transfer between vapor and water phases is modelled with the Schnerr-Sauer cavitation model. The definitions of different erosion risk indicators are mathematically derived and their performance on predicting erosion power is compared systematically. The results demonstrate that indicator, defined only with the rapid temporal variations of the pressure, is unable to distinguish the erosion area caused by cavity collapse. And the erosion risk indicator defined by time derivative of flow properties is unable to capture the main erosion occurred on the cavity closure region because the high erosion power on such area is mainly contributed by the advection term. In addition, it is founded that the full form of erosion power definition, defined by material derivative, is the best erosion indicator which can well predict the most eroded area and thus is recommended to be applied in the industrial and practical applications

Assessment of novel modal testing methods for structures rotating in water, Applied Sciences 2023

Abstract: 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.

Assessment of Fiber Bragg Grating Sensors for Monitoring Shaft Vibrations of Hydraulic Turbines, Sensors 2023

Abstract: 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.

Fatigue damage analysis of a Kaplan turbine model operating at off-design and transient conditions, Structural Health Monitoring 2023

Abstract: 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.

On the Added Modal Coefficients of a Rotating Submerged Cylinder Induced by a Whirling Motion—Part 1: Experimental Investigation, Journal of Marine Science and Engineering 2023

Abstract: The operation of submerged rotating machines, such as marine current or tidal turbines, can present deleterious fluid phenomena that may provoke extreme structural vibrations. To predict their dynamic responses, it is necessary to know the added modal coefficients of their runners under a whirling motion. For that purpose, a bespoke test rig was designed to investigate the added modal coefficients of a submerged cylinder, which could rotate at different speeds both in air and completely submerged in water inside a cylindrical tank. First, the modes of vibration were experimentally measured by exciting the cylinder with a push-release method during steady tests or with ramps in rotating speed during transient tests. The calculated natural frequencies and damping ratios were then used in a mathematical model of the dynamic system to calculate the added modal coefficients. During steady tests, the natural frequencies and damping ratios of the whirling modes changed significantly as a function of the rotating speed. Additionally, a whirling mode was observed to change its direction at a given rotating speed. During transient tests, rotating speed ramps with high accelerations were found to present lower lock-in amplitude and frequencies. Moreover, fast downward ramps presented lock-in amplitudes four times higher than fast upward ramps. Consequently, the added modal coefficients changed accordingly as a function of the rotating speed, ramp acceleration, and ramp direction. For these reasons, it was confirmed that the modal responses of submerged rotating bodies must be calculated for each operational rotating speed, even at low velocities, and for each transient event in order to precisely predict their vibration behaviors.

On the Added Modal Coefficients of a Rotating Submerged Cylinder Induced by a Whirling Motion—Part 2: Numerical Investigation, Journal of Marine Science and Engineering 2023

Abstract: Part 2 of this work presents a numerical methodology, validated using the experimental results presented in Part 1, to calculate the added modal coefficients of a submerged cylinder in water both when it oscillates and when it rotates with a whirling motion. The numerical methodology is based on computational fluid dynamic simulations that obtain the added modal forces on the cylinder when it is forced to vibrate with mode shapes calculated using acoustic-structural modal analysis. Then, these forces are processed with a curve-fitting algorithm to extract all the coefficients. Most numerical coefficients presented a close agreement with the corresponding experimental ones, although the added modal damping was overestimated. In general, the added modal mass was found to be independent of both the rotating speed and the whirling frequency except for low whirling frequencies when it increased. The added modal damping was found to depend on both parameters, and the rest of the coefficients were independent of the whirling frequency and only depended on the rotating speed. As a conclusion, this numerical approach has permitted the study of particular conditions that could not be experimentally tested and thus broadened the knowledge of the behavior of the added modal coefficients of rotating submerged cylinders.