1,354,298 research outputs found
Von Karman Vortex Induced Vibration in a Francis turbine: as reduced numerical model for estimating the shedding frequency.
No full textThe present paper presents a numerical analysis of the von Karman vortex shedding at the blades trailing edge of a recently installed small hydro Francis turbine manifesting very loud and high frequency acoustic pulsation when operating close to the rated discharge.
A reduced single passage numerical model is developed to reduce the computational effort of the simulation while ensuring high accuracy in the assessment of fluid flow.
The potentials of the numerical model are investigated by comparing the frequency spectrum of the experimentally acquired acoustic frequency and the numerical pressure signals allowing to confirm the nature of the vibrations affecting the machine. Thanks to the so demonstrated ability in the evaluation of the fluid flow, the proposed numerical model represents a valid alternative to the traditional correlation-bases approach for the evaluation of the von Karman shedding frequency with a less computational effort compared with a transient simulation of the entire machine
Von Karman Vortex Induced Vibration in a Francis Turbine: a reduced numerical model for estimating the shedding frequency
No full textThe present paper presents a numerical analysis of the von Karman vortex shedding at the blades trailing edge of a recently installed small hydro Francis turbine manifesting very loud and high frequency acoustic pulsation when operating close to the rated discharge. A reduced single passage numerical model is developed to reduce the computational effort of the simulation while ensuring high accuracy in the assessment of fluid flow. The potentials of the numerical model are investigated by comparing the frequency spectrum of the experimentally acquired acoustic frequency and the numerical pressure signals allowing to confirm the nature of the vibrations affecting the machine. Thanks to the so demonstrated ability in the evaluation of the fluid flow, the proposed numerical model represents a valid alternative to the traditional correlation-bases approach for the evaluation of the von Karman shedding frequency with a less computational effort compared with a transient simulation of the entire machine
Numerical analyses of a cavitating pelton turbine
No full textErosive wear of hydro turbine runners is a complex phenomenon, which depends upon many parameters and which leads to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. Consistently, the study of this wearing process is an important step to improve the impeller design, to avoid or minimize the rise of extraordinary maintenance. In the present paper the cavitation mechanics of a Pelton turbine was investigated using CFD analyses. A Pelton affected by pitting cavitation was taken as test case. The Pelton geometry was modelled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. The homogeneous approach was used to describe the multiphase flow composed by water, water vapour and air. Numerical results discriminated the vapour production processes during the cut in of the bucket on the water jet. The design and the part load flow rates were analyzed and the cavitation process compared. A simple procedure to identify the locations of higher damage risk was presented and verified on the test case runner
Numerical investigation of the interaction between jet and bucket in a Pelton turbine
No full textThis article presents the numerical investigation of the interaction between the jet and the bucket in a Pelton turbine. Unsteady numerical analyses were carried out on a single jet Pelton turbine installed in the north of Italy. A two-phase inhomogeneous model was used.
Two different jet configurations were analysed and compared. In the first configuration, the interaction between the runner and an axial-symmetric jet characterized by a given velocity jet profile was investigated, whereas in the second configuration the runner was coupled with the needle nozzle and the final part of the penstock and the interaction between the jet and the bucket was analysed.
A detailed analysis of the torque highlighted the influence of the shape of the water jet on the turbine losses and the influence of the stator on the efficiency of this type of hydraulic machines was shown. The numerical results were compared with the experimental data derived from the installation test of the turbine in order to validate the numerical analysis
Influence of the bucket geometry on the Pelton performance
No full textThe increasing share of hydropower in world electricity production requires the development of standardized and
optimized design procedures leading to increasingly higher efficiency values.
To date, despite a certain amount of support from computational fluid dynamics, Pelton turbines are still characterized by semiempirical design criteria that do not make it possible to optimize the jet–bucket interaction in order to maximize turbine performance. Based on an analysis of particle flow tracks, this paper presents a hybrid Eulerian–Lagrangian method to investigate the influence of bucket geometry on the Pelton efficiency at two different operating conditions.
Jet–bucket interaction was numerically analyzed by means of a traditional mesh-based numerical approach, using a transient multi-phase homogeneous model. Subsequently, the numerical results were integrated using a predictor–corrector algorithm, combining a fourth order Adams-Bashforth method as predictor and a fourth order Adams-Moulton method as corrector, in order to determine the fluid particle trajectories on the rotating buckets. The particle flow tracks were analyzed in detail to evaluate the single-particle performance in terms of discharged kinetic energy, momentum variation, and total energy variation during the jet–bucket interaction. Moreover, on the basis of the particle discharging position, the contribution of the different bucket areas to the total torque of the turbine was investigated to determine the time-depending influence of the bucket geometry on the turbine energy exchange and to suggest possible design solutions for improving bucket performance
Numerical Analyses of Cavitating Flow in a Pelton Turbine
No full textErosion and wear of hydraulic surfaces are frequent problems in hydraulic turbines, which lead to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. These circumstances have negative repercussions on the annual produced power due to the decay of the efficiency, the delivered power, and to the off line periods as result of ordinary and extraordinary hydraulic profiles maintenances. Consistently, the study of this wearing process is an important step to improve the impeller design, and to avoid or minimize the rise of extraordinary maintenance. While mechanical damages are well documented and studied, little information can be found on cavitation in Pelton turbines. In this paper, a CFD model was applied to study the cavitation mechanics on a Pelton turbine. A Pelton runner affected by pitting cavitation was taken as a test case. The bucket geometry was modeled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. Numerical results allowed us to highlight the different vapor productions during the cut-in water jet processes by the bucket. Furthermore, a simple procedure to identify the locations of higher damage risk was presented and verified in the test case runne
Procedure per la progettazione standardizzata delle turbine delle piccole centrali idroelettriche
No full textThe thesis presents the definition and development of standardized design procedures of three type of hydraulic turbines for small hydraulic power plants: Pelton, Francis and Kaplan. In order to properly define the design parameter of each type of turbine, a technical-economical criterion was first applied to the flow duration curves of the turbine installation sites. Then, the critical aspects of the design procedure of each type of turbine were analyzed. As regards the Pelton turbines, a numerical investigation of the interaction between the jet and the bucket is presented. Unsteady numerical analyses were carried out on a single jet Pelton Turbine installed in the north of Italy. A two-phase inhomogeneous model was used. Two different jet configurations were analyzed and compared. In the first configuration the interaction between the runner and an axial-symmetric jet characterized by a given velocity jet profile was investigated, whereas in the second configuration the runner was coupled with the needle nozzle and the final part of the penstock and the interaction between the jet and the bucket was analyzed. A detailed analysis of the torque highlighted the influence of the shape of the water jet on the turbine losses and the influence of the stator on the efficiency of this type of hydraulic machines was shown. The numerical results was compared with the experimental data derived from the installation test of the turbine in order to validate the numerical analysis. As regards the Francis turbines, the analysis was focused on the design procedure of the statorical part. Numerical analyses, carried out on an installed and working machine, highlighted problems on the prediction of the operating conditions due to the distributor design procedure. To overcome these problems, an alternative distributor design procedure was presented and validated by numerical analysis. Finally, for the Kaplan turbines a new design methodology of the blade profiles is presented. Moreover, in order to overcome the prediction problems highlighted for the Francis turbines, a prediction criterion of the flow deflection obtainable from the distributor is also proposed.La tesi presenta procedure di progettazione standardizzata sviluppate per tre tipologie di turbine idrauliche impiegate in centrali idroelettriche di piccola potenza: turbine Pelton, Francis e Kaplan. In primo luogo, per definire in modo appropriato e razionale i parametri progettuali delle turbine, le curve di durata delle portate dei siti di installazione sono state analizzate mediante un criterio sviluppato sulla base di parametri sia tecnici che economici. Si sono poi analizzati in dettaglio gli aspetti critici della procedura di progettazione delle varie tipologie di turbine. Per quanto riguarda le turbine Pelton, oggetto dell'indagine è stata l'interazione tra il getto e la pala a doppio cucchiaio. Analisi numeriche non-stazionarie multifase sono state condotte su di una turbina Pelton a singolo getto installata nel Nord Italia della quale si sono prese in considerazione due diverse configurazioni. Nella prima si è analizzata l'interazione tra la girante e un getto ideale assial-simmetrico a cui è stato assegnato un definito profilo di velocità, mentre nella seconda configurazione la girante è stata accoppiata con l'ugello e la parte finale della condotta forzata per analizzare l'interazione tra la girante e un getto dalle caratteristiche più vicine alla realtà. Un'analisi dettagliata della coppia all'albero ha consentito di valutare l'influenza della forma del getto e dell'elemento statorico sulle perdite e l'efficienza di questa tipologia di macchine idrauliche. I risultati numerici sono stati quindi confrontati con i dati sperimentali dei test di installazione della turbina allo scopo di valutare l'accuratezza dell'analisi numerica. Per quanto riguarda invece la turbine Francis, analisi numeriche condotte su di una macchina installata e funzionante, hanno evidenziato problemi di predizione della condizione di funzionamento dovuti alle procedure di progettazione dell'organo statorico. Per ovviare a questi problemi, si è sviluppata una procedura alternativa per il dimensionamento del distributore che consentisse di prevedere la condizione di funzionamento della macchina in modo più accurato. Tale procedura è stata convalidata tramite analisi numerica. Per quanto attiene le turbi ne Kaplan, è stata sviluppata una metodologia standardizzata di calcolo dei profili palari. Inoltre per ovviare ai problemi di predizione riscontrati anche per questa tipologia di turbine si è sviluppato un criterio di predizione della deviazione all'uscita dell'organo statorico, verificato per via numerica
Three-dimensional evolution of the flow unsteadiness in the S-shape of pump-turbines and its correlation with the runner geometry
No full textPump-turbines (RTP) are the most common mechanical equipment adopted in pumped-hydro power plants and, for grid balancing purposes, are required to sharply switch from pumping to generating mode, and to extend their operative, jeopardizing not only the machine operability but also its life. New design approaches to avoid the onset of unstable behaviours are still far from being defined, and control strategies for accelerating start-up/shut-down procedures are still not effective since these are based on semi-empirical approaches, due to the lack of identification of precursors of the unstable behavior. In this paper, a numerical analysis of the unstable behavior of an RPT during the transition from partial load up to the turbine-brake area was carried out. The fluid-dynamics in different operating points (partial load, run-away condition, turbine brake) was deeply investigated, identifying the rotor-stator mechanisms causing the 3D evolution of the flow field leading to the development of the unstable behavior. Three evolution phases (inception, growth and consolidation) were identified and clearly correlated with the runner geometry and with the S-Shape of the RPT characteristic curve. Customized signal processing strategies were adopted for spectrally characterizing each phase so as to identify potential triggers for new monitoring and control strategies. Moreover, for the first time, a clear fluid-dynamic explanation of the empirical results found in literature on the influence of the runner geometry is provided
Techno-economical method for the capacity sizing of a small hydropower plant
No full textThis paper presents a method for the capacity sizing of a small hydropower plant on the basis of techno-economical analyses of the flow duration curve. Seven technical and economical parameters were considered: the turbine type, the turbine dimensions, the annual energy production, the maximum installation height to avoid cavitation inception, the machine cost, the Net Present Value (NPV) and the Internal Rate of Return (IRR).
A proper model was proposed to study the effects of the design operating conditions on these parameters. The model, applied to the flow duration curve, allowed to analyse the feasibility, the profitability and the performance of the plant in the available flowing range of the site.
To verify the effectiveness of the proposed method, three sites having different flow duration curves were analysed
Learning Regularities from the Visual World
No full textPatterns of visual objects, streams of sounds, and spatiotemporal events are just a few examples of the structures present in a variety of sensory inputs. Amid such variety, numerous regularities can be found. In order to handle the sensory processing, individuals of each species have to be able to rapidly track these regularities. Statistical learning is one of the principal mechanisms that enable to track patterns from the flow of sensory information, by detecting coherent relations between elements (e.g., A predicts B). Once relevant structures are detected, learners are sometimes required to generalize to novel situations. This process can be challenging since it demands to abstract away from the surface information, and extract structures from previously-unseen stimuli. Over the past two decades, researchers have shown that statistical learning and generalization operate across domains, modalities and species, supporting the generality assumption. These mechanisms in fact, play a crucial role in organizing the sensory world, and developing representation of the environment.
But when and how do organisms begin to track and generalize patterns from the environment? From the overall existing literature, very little is known about the roots these mechanisms. The experiments described in this thesis were all designed to explore whether statistical learning and generalization of visual patterns are fully available at birth, using the newborn domestic chick (Gallus gallus) as animal model. This species represents an excellent developmental model for the study of the ontogeny of several cognitive traits because it can be tested soon after hatching, and allows complete manipulation of pre- and post-natal experience.
In Chapter 2, four statistical learning experiments are described. Through learning-by-exposure, visually-naive chicks were familiarized to a computer-presented stream of objects defined by a statistical structure; in particular, transitional (conditional) probabilities linked together sequence elements (e.g., the cross predicts the circle 100% of the times). After exposure, the familiar structured sequence were compared to a random presentation (Experiment 1) or a novel, structured combination (Experiment 2) of the familiar shapes. Chicks successfully differentiated test sequences in both experiments. One relevant aspect of these findings is that the learning process is unsupervised. Despite the lack of reinforcement, the mere exposure to the statistically-defined input was sufficient to obtain a significant learning effect.
Two additional experiments have been designed in order to explore the complexity of the patterns that can be learned by this species. In particular, the aim of Experiments 3 and 4 was to investigate chicks’ ability to discriminate subtle differences of distributional properties of the stimuli. New sequences have been created; the familiar one was formed by a pairs of shapes that always appear in that order whereas the unfamiliar stimulus was formed by shapes spanning the boundaries across familiar pairs (part-pairs). Unfamiliar part-pairs were indeed created by joining the last element of a familiar pair and the first element of another (subsequent) familiar pair. The key difference among pairs and part-pairs lied on the probabilistic structure of the two: being formed by the union of two familiar elements, part-pairs were experienced during familiarization but with a lower probability. In order to distinguish test sequences, chicks needed to detect a very small difference in conditional probability characterizing the two stimuli. Unfortunately, the animals were unable to differentiate test sequences when formed by 8 (Experiment 3) or 6 (Experiment 4) elements. My final goal would have been to discover whether chicks are effectively able to pick up transitional probabilities or whether they simply track frequencies of co-occurrence. In Experiments 1 and 2, since the frequency of appearance of each shape was balanced across stimuli, it was impossible to tell if chicks detected transitional probabilities (e.g., X predicts Y) or frequencies of co-occurrence (e.g., X and Y co-occur together, but any predictive relation characterize them) among elements. However, since the animals did not succeed in the first task, being unable to discriminate pairs vs. part-pairs, data are inconclusive as regards to this issue. Possible explanations and theoretical implications of these results are provided in the final chapter of this thesis.
In Chapter 3, the two studies described were aimed at testing newborn chicks’ capacities of generalization of patterns presented as stings of visual tokens. For instance, the pattern AAB can be defined as “two identical items (AA) followed by another one, different from the formers (B)”. Patterns were presented as triplets of simultaneously-visible shapes, arranged according to AAB, ABA (Experiment 5), ABB and BAA (Experiment 6). Using a training procedure, chicks were able to recognize the trained regularity when compared to another (neutral) regularity (for instance, AAB displayed as cross-cross-circle vs. ABA displayed as cross-circle-cross). Chicks were also capable of generalizing these patterns to novel exemplars composed of previously-unseen elements (AAB vs. ABA implemented by hourglass-hourglass-arrow vs. hourglass-arrow-hourglass).
A subsequent study (Experiment 6) was aimed at verifying whether the presence/absence of contiguous reduplicated elements (in AAB but not in ABA) may have facilitated learning and generalization in previous task. All regularities comprised an adjacent repetition that gave the triplets asymmetrical structures (AAB vs. ABB and AAB vs. BAA). Chicks discriminated pattern-following and pattern-violating novel test triplets instantiating all regularities employed in the study, suggesting that the presence/absence of an adjacent repetition was not a relevant cue to succeed in the task.
Overall, the present research provides new data of statistical learning and generalization of visual regularities in a newborn animal model, revealing that these mechanisms fully operate at the very beginning of life. For what concerns statistical learning, day-old chicks performed better than neonates but similar to human infants. As regards to generalization, chicks’ performance is consistent to what shown by neonates in the linguistic domain. These findings suggest that newborn chicks may be predisposed to track visual regularities in their postnatal environment. Despite the very limited previous experience, after a mere exposure to a structured input or a 3-days training session, significant learning and generalization effects have been obtained, pointing to the presence of early predispositions serving the development of these cognitive abilities
- …
