bluuebunny/crossref_metadata_2025_split huggingface dataset:
URANS numerical simulations of pulsating flows considering streamwise pressure gradient on asymmetric diffuser
Authors: Grecu I S , Dunca G , Bucur D M , Cervantes M J 
Published: October 2022

Abstract
The paper focuses on implementing the wall model developed by Manhart, in Reynolds Averaged Navier - Stokes (RANS) turbulence models used in the field of Computational Fluid Dynamics (CFD). This wall model considers the influence of the streamwise pressure gradient in addition to the existing wall models used in the usual CFD codes. In the present work, two RANS numerical simulations are carried out using the k-ω Shear Stress Transport (SST) turbulence model on an asymmetric diffuser geometry. One numerical simulation is carried out using the implementation of the Manhart wall model in the k-ω SST turbulence model, and the other numerical simulation is performed using the standard formulation of the k-ω SST turbulence model. The numerical simulations carried out using the Manhart wall model and the standard formulation of the k-ω SST are compared with experimental measurements made on the asymmetric diffuser experimental installation. The numerical simulations are carried out using a free, open-source CFD tool, Code_Saturne. The comparisons between numerical simulations and the experimental data are in good agreement in the boundary layer of the flow inside the diffuser. The Manhart wall model had a faster convergence resulting in a shorter simulation time.
Towards numerical simulation of high part load pulsations in Francis turbines
Authors: Chirkov D , Shcherbakov P , Skorospelov V , Turuk P 
Published: October 2022

Abstract
One of the most interesting operating points of Francis turbines is the high part load point, which is characterized by synchronous pressure pulsations of large amplitude. Physical mechanisms of these pulsations are not well understood. Experimental observations show that the frequency of the pulsations is 1.5 to 4 times higher than the runner rotation frequency and there exists a 180 degree phase shift of pressure signal measured in the spiral casing and in the draft tube. Numerical modeling this operating point is challenging as it has to account several complex phenomena: cavitation, rotating vortex rope and interaction of the turbine flow with the whole water system of the test rig. In the present paper we carried out a series of 1D-3D calculations of a model turbine in an attempt to simulate the high part load pulsations. 1D hydro-acoustic equations were used for the upstream waterways, while 3D Reynolds averaged Navier-Stokes model of two-phase mixture was used for the turbine. Although the main factors were taken into account, these computations were not able to capture the high part load pulsations. Namely, the frequency and amplitude of the pressure pulsations were significantly lower than those in the experiment, and there was no phase shift in turbine domain. In order to investigate the shortcomings of the present model, we considered a problem of propagation of disturbances in two-phase “liquid-gas” flows. Several 2D test cases were numerically investigated using both incompressible and compressible gaseous phase model. It was shown that in the absence of phase transfer the compressible model correctly represents the speed of sound in both homogeneous and stratified flow regimes. From the other hand, in case of phase transfer the model damps out propagation of pressure disturbances in upstream direction. This behavior explains the failure of the present 1D-3D model to correctly describe the acoustic properties of the two-phase flow in the draft tube, playing a crucial role in the development of high part load pulsations.
Characterization of the Fluid Damping in Simplified Models of Pump-Turbines and High Head Francis Runners
Authors: Moraga G , Valero C , Valentín D , Egusquiza M , Xia X , Zhou L , Presas A 
Published: October 2022

Abstract
In order to satisfy the power demand in the electrical grid, hydraulic turbine units frequently work under off-design operation conditions and pass through transient events. These operation conditions can lead to high vibration amplitudes in the turbine runners, decreasing their useful life, and in some cases to premature failures. To determine and to understand the behaviour of the fluid damping is a relevant topic, because this parameter limits the maximum amplitude in resonance conditions. The runner of some types of turbines, such as reversible pump-turbine and high head Francis turbine, can be modelled as a disk-like structure, due to their similar mode shapes. Because of this, in this work, the fluid damping of a vibrating disk was studied. The disk was submerged in water and was put in a resonant state at different vibration amplitudes. Moreover, this structure was excited at different distances to a rigid surface, in order to analyse the effects of the distance between the runner and the casing. The main effects on the fluid damping were determined and characterized, showing a dependency of the fluid damping ratio on the different parameters.
On-site measurements of the dynamic behaviour of Pelton turbines in the context of predictive maintenance
Authors: Chiarelli M , Hasmatuchi V C , Amini A , Decaix J , Vetsch D F , Boes R M , Münch-Alligné C 
Published: October 2022

Abstract
The operating modes of pumped storage power plants, such as that of the Forces Motrices Hongrin-Léman (FMHL) in Switzerland, have drastically changed over the past decades due to the emergence of new renewable energies. The number of starts and stops experienced by machines increased significantly leading to broader fatigue cycles on the different parts of the machine. Therefore, the development of predictive maintenance tools is paramount to increase the availability and reinforce operational safety of the power plant. In this context, vibration measurements were performed on a 60 MW Pelton turbine of one of the ternary units of the FMHL power plant during dynamic and steady state operations. Non intrusive instrumentation has been deployed including accelerometers on the bearing. Comparing the ramp-up and ramp-down of the Pelton turbine, a hysteresis of the vibration level for a similar power has been observed. A reduction of the vibration levels when an additional injector is engaged during the ramp-up is observed. This reduction correlates strongly with the load reduction on the buckets when the total flow rate is distributed over more injectors, leading to a smaller flow rate per injector.
Validation of deep part load dynamic stresses for axial runners
Authors: Monette C , Chamberland-Lauzon J , Nennemann B 
Published: October 2022

Abstract
To accommodate renewable energy production and load demand variability, hydropower plant owners need to increase their operating range regardless of their units’ original design load envelop. When this increased operating range is an issue for the fatigue life of the old runners, solutions need to be found with the design of a new runner to sustain those new challenging loads of the increased operating range. In recent years, many papers have been published to show the challenging loads on Francis runners at speed-no-load and deep part load conditions. Andritz demonstrated a good numerical prediction capability for stress levels at deep part load conditions for Francis runners. However, for axial units, very little has been published. Very recently, some papers showed good predictability by CFD of the flow behavior at deep part load including the vortices present at those conditions. This paper demonstrates the prediction capability of the numerical tools by comparing strain gage measurements on an axial runner to CFD-FEA stress predictions. The measurement campaign was conducted conjointly by the unit owner and the manufacturer for research purposes. In the deep part load operating zone under the effect of columnar vortices, frequency analysis of the measured vibrations and strain gage signals confirmed the flow behavior predicted by CFD, and the measured dynamic strain amplitudes were well predicted. Numerical prediction of dynamic stress in the complete range from 0-100% power of the measured unit as well as detection of high vibration zones was successful.
Anti-interference characteristics of variable speed pumped storage units for load cases of hydraulic disturbances
Authors: Huang Yifan , Yang Weijia , Li Yulan , Zhao Zhigao , Yang Jiandong 
Published: October 2022

Abstract
Variable speed pumped storage unit (VSPSU) is an innovative technology, and investigations regarding operating characteristics of VSPSU are significant. However, there are relatively few studies towards hydraulic disturbances of VSPSUs. The hydraulic disturbance means that pumped-storage units, which were originally in steady operation, are inevitably affected by suddenly operating changes of other units in the same waterway system. Normally, it will cause the power output of fixed-speed units to oscillate, and adversely affect the safety of generators and stability of power systems. Hence, investigations of VSPSUs’ characteristics under hydraulic disturbances are necessary. In this paper, VSPSU using doubly fed induction machine (DFIM) is adopted as a study case, and fast power control strategy is used in the controlling of VSPSU. The simulation result shows that VSPSU has good anti-interference performance, and normally operating VSPSU can still maintain stable power output and stator current even under disturbance from other units. However, despite the good anti-interference performance of VSPSU, the mechanical power of VSPSU lags far behind the electromagnetic power, and the hydraulic-mechanical transient processes of VSPSU are similar to that of a fixed-speed unit. The findings in this paper could provide understanding and guidance for practical operation of VSPSUs.
Contribution for the roadmap of hydraulic short circuit implementation: Case of Grand-Maison pumped storage power plant
Authors: Landry C , Nicolet C , Badina C , Pichon H , Drommi J-L 
Published: October 2022

Abstract
The objectives of the 2050 energy policy based on the decarbonization of the electric power networks generate drastic changes for grid balancing with a massive integration of non-dispatchable Renewable Energy Sources. Hydroelectric power plants already significantly support electricity power system flexibility with innovative solutions such as variable speed units, fast frequency control, fast generating to pumping modes transition, high ramping rate, inertia emulation, etc.
For pumped storage power plants (PSP), a quick solution to increase the flexibility without large investment is to operate the power plant in hydraulic short circuit (HSC) mode. This technological solution is simple to implement, but requires an in-depth study of various technical aspects among which the hydraulic transients of the new operating modes is of high importance from the installation’s safety perspective. In the framework of XFLEX HYDRO H2020 European research project, the exploitation of this solution is under implementation at Grand-Maison PSP. Located in the French Alps, Grand-Maison PSP is equipped with 8 reversible multi-stage Francis pump-turbines and 4 Pelton turbines, for a total installed capacity of 1800MW, thus being the largest PSP in Europe and one of the major PSP in the world. The waterway includes a headrace tunnel, a headrace surge tank, 3 parallel penstocks feeding the 12 units operated under a maximum gross head of 955mWC.
In this paper, after a description of the general HSC considerations, the 1D model of the Grand Maison PSP and the related validation are presented. Finally, the most critical load cases in HSC operation are described to identify the potential hydraulic transient issues, such as extreme water levels in the upstream surge tank, maximum static pressure along the pressure shaft and minimum static pressure along the tunnels. The analysis performed for Grand Maison PSP is a contribution to the roadmap for the implementation of HSC operation in pumped storage power plant and will be made available as a public deliverable of the XFLEX HYDRO H2020 European research project.
Optimization of turbine start-up sequence of a full size frequency converter variable speed pump-turbine
Authors: Schmid J , Alligné S , Biner D , Münch-Alligné C , Hugo N , Nicolet C 
Published: October 2022

AbstractThe stability of the electricity grid will be disrupted by the massive integration of new renewable energies. Hydropower plants have a major role to play in this transformation of the electricity market by increasing their operational flexibility and their ability to provide ancillary services. However, this flexibility may lead to an accelerated degradation of mechanical components. By changing the turbine operating point far from the best efficiency point or by increasing the number of transient manoeuvres such as start and stop sequences, unsteady flow phenomena, cavitation development and additional wear and tear stress the unit’s components and impact its lifetime. The present work aims to provide preliminary insight on the optimization of the start-up sequence of a 5 MW reversible Francis pump-turbine equipped with a Full Size Frequency Converter (FSFC). The goal of the optimization approaches are to determine a start-up sequence which minimizes the runner damage, the penstock fatigue and the water losses. The objective functions are evaluated by 1D hydraulic transient simulations with the SIMSEN software and which it allow to compare the relative mitigation between the conventional fixed-speed start-up and the linear variable-speed start-up equipped with a Full Size Frequency Converter.
Heat treatment optimization of 13Cr4Ni stainless alloy used as hydro turbine materials
Authors: Dahal D R , Khatri K.C R H , Tandukar S , Kapali A , Ghimire A , Panthee A , Chitrakar S , Thapa B 
Published: October 2022

Abstract
In Nepal and the regions along with the Andes, sediment erosion is one of the major causes of the failure of hydro turbines. Every year lots of the components of turbines exposed to sediment-laden water got eroded resulting unexpected shutdown of the power plants. The cheapest method of repairing these eroded runners and other components of turbines are welding which may induce residual stress, embrittlement, and increase carbide content. Therefore, post welding heat treatment especially stress relieving annealing is always carried out to reduce the induced residual stresses, enhance strength and other mechanical properties as well as reduce the risk of cracking. As heating time is relative of low importance, great care should be taken during heat treatment to maintain the soaking time and cooling rate. Holding time for heat treatment will vary with the thickness of casting sections involved, but should be sufficiently long enough to heat all sections to a uniform temperature throughout. The cooling rate determines the material properties at room temperature and rapid cooling avoids the formation of chromium carbide. In this paper, the post-weld heat treatment used for repair and maintenance of turbines in Nepal is studied along with the investigation of heat treatment conditions on mechanical properties and microstructures of 13Cr4Ni stainless steel used as turbine materials to optimize the heat treatment itself. First of all relative analysis of five different heat treatment conditions with respect to ‘no heat treatment’ regarding mechanical properties i.e. toughness, hardness, and fatigue life was carried out. Heat treatments of the specimen were done in a muffle furnace. Hardness, toughness, and fatigue strength of the specimen using the Rockwell Hardness Test, Charpy Impact Test, and Fatigue Life Test were conducted respectively. Subsequently, microstructures of all the heat treatment conditions are closely examined using the metallurgical microscope. It was observed that holding time plays a great impact on improving mechanical properties.
Energy scale of dielectric coupling in antiferromagnetic insulators
Authors: Zhang Jiarui , Chen Cai , Xiang Shuqing , Zhang Junchao , Qi Qi , Huang Rui , Yu Yang , Chen Kai , Han Zhida , Yuan Guoliang , Liu Junming , Zhu Jinsong 
Published: October 2022

Abstract
To probe the energy of dielectric coupling between the dipole chains and the quasipolarons based on extension of the polaron concept, dielectric and magnetic properties of antiferromagnetic insulators, namely Ca4-xCuxTi4O12 (x = 3, 2 and 1), are investigated at different temperatures. The energy is estimated larger than 56 meV of the dipole interaction energy, but smaller than 88 meV of the dipole-chain thermal activation energy which is for the Dissado-Hill type dielectric relaxation. In particular, the quasipolaron ingredient is at least 28 meV of the total superexchange interaction which bond one singe titanium ion of the dipole chain with four quasipolarons. Then, the energy of the coupling is in the range from 84 meV to 88 meV. The experimental observation, suggests the dielectric coupling may provide an approach to obtain large permittivity in strongly-correlating systems of antiferromagnetic insulators.
Changes in compound extremes of rainfall and temperature over West Africa using CMIP5 simulations
Authors: Diba Ibrahima , Diedhiou Arona , Famien Adjoua M , Camara Moctar , Fotso-Nguemo Thierry C 
Published: October 2022

Abstract
This study aims to characterize changes in compound extremes of rainfall and temperature over West Africa. For this purpose, data from CHIRPS observations, the ERA5 reanalysis, and twenty-four (24) climate models involved in the CMIP5 Project were analyzed. First, climate models were evaluated in terms of their capacity to simulate summer mean climatology and compound extremes during the historical period (1981–2005), and secondly, changes in compound extremes were examined under RCP8.5 emission scenario between the near future (2031–2055) and the far future (2071–2095) relative to the historical period. Despite the presence of some biases, the ensemble mean of the models well reproduces the compound extremes patterns over West Africa at the seasonal and intraseasonal timescales. The analysis over the historical period with CHIRPS/ERA5 dataset shows a strong occurrence of the dry/warm mode over the northern Sahel during the June-July-August-September period (JJAS; main rainy season) and over the Guinean region during the February-March-April-May season (FMAM; first and main rainy season). These strong occurrences are due to a weak and highly frequent precipitation recorded in these zones. The compound wet/warm mode is frequent in JJAS over the Sahel and the Sudanian zone (transition area between Sahel and Guinean regions), while in FMAM, its occurrence is maximum over the Guinean region. The study also shows that the dry/warm mode will increase in the whole Sahel (western and central) and in the Guinean zone in the near and far futures while the compound wet/warm mode will decrease in the whole region. This study suggests that the West Africa region will be prone to drought intensified by warmer temperatures and calls for climate action and adaptation strategies to mitigate the risks on rain-fed agriculture, energy, and on animals and human health.
Roadmap on chalcogenide photonics
Authors: Gholipour Behrad , Elliott Stephen R , Müller Maximilian J , Wuttig Matthias , Hewak Daniel W , Hayden Brian E , Li Yifei , Jo Seong Soon , Jaramillo Rafael , Simpson Robert E , Tominaga Junji , Cui Yihao , Mandal Avik , Eggleton Benjamin J , Rochette Martin , Rezaei Mohsen , Alamgir Imtiaz , Shamim Hosne Mobarok , Kormokar Robi , Anjum Arslan , Zeweldi Gebrehiwot Tesfay , Karnik Tushar Sanjay , Hu Juejun , Kasap Safa O , Belev George , Reznik Alla 
Published: October 2022

Abstract
Alloys of sulfur, selenium and tellurium, often referred to as chalcogenide semiconductors, offer a highly versatile, compositionally-controllable material platform for a variety of passive and active photonic applications. They are optically nonlinear, photoconductive materials with wide transmission windows that present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet, visible and infrared frequencies, in addition to an, non-volatile, electrically/optically induced switching capability between phase states with markedly different electromagnetic properties. This roadmap collection presents an in-depth account of the critical role that chalcogenide semiconductors play within various traditional and emerging photonic technology platforms. The potential of this field going forward is demonstrated by presenting context and outlook on selected socio-economically important research streams utilizing chalcogenide semiconductors. To this end, this roadmap encompasses selected topics that range from systematic design of material properties and switching kinetics to device-level nanostructuring and integration within various photonic system architectures.
A comparative study of different machine learning methods for dissipative quantum dynamics
Authors: Rodríguez Luis E Herrera , Ullah Arif , Espinosa Kennet J Rueda , Dral Pavlo O , Kananenka Alexei A 
Published: October 2022

Abstract
It has been recently shown that supervised machine learning (ML) algorithms can accurately and efficiently predict long-time population dynamics of dissipative quantum systems given only short-time population dynamics. In the present article we benchmarked 22 ML models on their ability to predict long-time dynamics of a two-level quantum system linearly coupled to harmonic bath. The models include uni- and bidirectional recurrent, convolutional, and fully-connected feedforward artificial neural networks (ANNs) and kernel ridge regression (KRR) with linear and most commonly used nonlinear kernels. Our results suggest that KRR with nonlinear kernels can serve as inexpensive yet accurate way to simulate long-time dynamics in cases where the constant length of input trajectories is appropriate. Convolutional gated recurrent unit model is found to be the most efficient ANN model.