651 research outputs found
Numerical Simulation of an Oscillating Cylinder at High Reynolds Number
No full textIn this work, we propose a numerical investigation of the main flow field characteristics around a free oscillating rigid circular cylinder that is immersed in a turbulent flow (Re≈5∙〖10〗^4). The cylinder is characterized by high value of mass ratio and mass damping (m^*=145; ξ=0.6÷1.14∙〖10〗^(-3);m^* ξ=0.1÷0.25). Then the numerical results are compared with previous experimental data obtained in the wind tunnel under very similar fluid dynamic conditions. There are few works in literature that consider both numerical and experimental results under these conditions. This is probably due to the experimental facilities limitations and the computational difficulties correlated to modeling the flow with high Reynolds number. With the aim of matching the experimental analysis, developed in the last years in the Politecnico di Milano Wind Tunnel on a suspended oscillating cylinder, with numerical results, we set-up a numerical URANS model through a CFD commercial code using a k-ω SST turbulence model in a 3D domain.
The numerical setup is characterized by the use of the DFBI-Morphing (Dynamic Fluid Body Interaction) model that allows the modeling of the body motion in response to fluid forces treating the cylinder as a mass-damping-spring system by introducing spring and damping forces acting on it.
A preliminary check of this numerical setup was provided by a benchmark involving a simple case of fixed cylinder at the same Reynolds number, where the movements of the cylinder were disabled. The numerical results of this case have been compared with experimental and numerical results reported in literature by means of Drag and Lift coefficients and Strouhal number at high Reynolds (Re≈5∙〖10〗^4). After that benchmark, the full setup has been checked by considering specific fluid dynamic conditions where the cylinder was out of the lock in region in which the oscillations of the cylinder are negligible.
Finally we have investigated two points of the steady state oscillating response curve of the cylinder in the lock in region. The numerical model gave good results in terms of amplitude response of the cylinder and aerodynamic forces if compared with experimental results.
The analysis of the numerical reconstruction of the flow field evolution are therefore considered to have more information on the vortex shedding mode especially in the transition region between 2S and 2P mode
Pressure field and wake modes analysis of an oscillating cylinder
No full textThis work proposes a numerical investigation of the main flow field characteristics and pressure field analysis around a freely oscillating rigid circular cylinder immersed in a high Reynolds number flow in the subcritical range. The cylinder is characterized by high value of mass ratio and mass damping, while the damping itself is quite low. Here the numerical results are compared with experimental data obtained in the Politecnico di Milano wind tunnel under the same fluid dynamic and mechanical conditions. The preliminary check of this numerical setup was provided by considering the case of a fixed cylinder in the subcritical region. Based on this benchmark, the full setup was checked by considering fluid dynamic conditions outside the lock-in region.
Finally, a number of points were investigated on the cylinder's steady state response curve in the lock-in region. The numerical model yielded good results in terms of cylinder amplitude response, aerodynamic forces and pressure field analysis, in agreement with the results of the experiment. Analysis of the numerical reconstruction of the flow field evolution is therefore considered to yield further information on the vortex shedding mode, especially in the transition region between 2S and 2P mode
Hydro-abrasive erosion in Pelton turbine injectors: A numerical study
Get PDFNumerical simulations were performed to investigate how the design and the operation conditions of a Pelton turbine injector affect its vulnerability to hydro-abrasive erosion, alongside with its flow control capacity. Use was made of a Volume Of Fluid (VOF) model for simulating the free nozzle jet, a Lagrangian particle tracking model for reproducing the trajectories of the solid particles, and two erosion models for estimating the mass removal. The comparison against earlier studies and the experimental evidence, integrated with a careful sensitivity analysis, gave strength to the reliability of the numerical model. Nozzle seat and needle were the injector components most vulnerable to erosion. As the valve was closing, the erosion of the needle strongly increased, whilst that of the nozzle seat remained broadly constant. The influence of the injector design was also explored, suggesting that a reduction of the needle vertex angle is likely to enhance the risk of erosive wear. Finally, it was found that the possibility to condense the effects of the needle stroke and the needle vertex angle in a single parameter (i.e. the effective opening area) is no more allowed when hydro-abrasive erosion is considered, thereby assessing the need for case-specific wear prediction analyses
VIBRAZIONI INDOTTE DA DISTACCO DI VORTICI SU UN CILINDRO CIRCOLARE
No full textL’interazione tra un cilindro oscillante ed una corrente stazionaria è un tema che continua a destare molto interesse nonostante sia ampiamente studiato. Infatti set up caratterizzati da alti valori di mass-ratio, mass-damping e numero di Reynolds sono ancora poco studiati nonostante siano di significativo interesse pratico. Sfruttando le caratteristiche della Galleria del Vento del Politecnico di Milano da qualche anno si sono avviate ricerche in questi range sperimentali. In particolare, in questo lavoro, si è approfondita l’analisi cinematica del campo di moto a valle di un cilindro oscillate al fine di verificare la transizione della modalità di distacco di vortici da 2S a 2P, in corrispondenza del salto tra l’initial branch e l’upper branch. Il significativo miglioramento dei risultati rispetto ai precedenti lavori è stato ottenuto sviluppando sia il sistema di acquisizione sia le tecniche di elaborazione delle immagini e di post-processing dei dati cinematici
Fluid-Structure Interaction and Co-Simulation: Analysis of a Beam-Supported Sphere for VIV Application
No full textThe recent developments in numerical tools and computing resources seem to provide a suitable environment to perform numerical analyses of Fluid-Structure Interaction problems. The Co-Simulation technique, in particular, develops the idea of coupling a CFD software with a structural one in order to simulate complex FSI phenomena with a partitioned approach, stressing the concept of software modularity. In this way, it is possible to adopt software tools at the cutting edge of technology. Nonetheless, several difficulties may arise in the choice of the partitioning scheme and of the algorithmic details for the step-by-step time integration.
This paper deals with the application of the Co-Simulation technique to a benchmark case experimentally investigated in previous works: the vortex-induced vibrations (VIV) of a beam supported sphere (that is, a sphere fixed to the end of a slender cantilever beam) in a free surface flow. This problem is challenging although apparently simple and it seems quite absent from literature so far. In this paper, the computational issues are thoroughly investigated and the model is validated by comparison with the experimental data. In this way, a robust framework is created in order to deal with VIV problems
Nanostructured MFI-type zeolites as catalysts in glycerol etherification with tert-butyl alcohol
No full textHierarchical zeolite possessing MFI framework type was hydrothermally prepared using C22H45 - N+(CH3)(2) - C6H12 - N+(CH3)(2) - C6H13 as a structure-directing agent in a seed-assisted synthesis method. The nanosponge-like morphology was composed of a three-dimensional disordered network of MFI layers with 2.5 nm thickness supporting each other. Catalytic performance of the MFI nanosponge was investigated in glycerol etherification with tert-butyl alcohol in liquid phase and compared to conventional microporous MFI zeolite and MFI unilamelar nanosheet. The hierarchical zeolites were much more active, which can be attributed to the acid sites located on the external surfaces accessible for the reaction of bulky reactants. (C) 2016 Elsevier B.V. All rights reserve
On discrete-time modeling of the filtered and symbol-rate sampled continuous-time signal affected by Wiener phase noise
Get PDF« ... e non vi vien voglia di venir a veder tante gran cose delle quali son certo che ne resterete pago? ». Fortunato Mandelli (1728-1797) : collections et correspondances numismatiques
No full textAt present, Fortunato Mandelli (1728-1797) is almost unknown or just mentioned in some sporadic notes about the collecting of ancient coins in Venice during the second half of the Eighteenth century. The research in several archives, on the contrary, has revealed a quite reserved person with a special interest in numismatics. On the one side, he had his own collection and, on the other side, he followed the formation of those of some of his contemporaries, such as Giacomo Gradenigo, Antonio Savorgnan, Tommaso degli Obizzi, and in particular that of Giacomo Nani of St. Trovaso. Mandelli exchanged many letters and had contacts also with other distinguished savants as Annibale degli Abbati Olivieri, Simone Assemani, Girolamo Ascanio Molin, Mariangelo Fiacchi and Giovanni Cristofano Amaduzzi. In order to construct his solid knowledge of ancient coins he elaborated a scheme of classification based especially on the observation of coin reverses, through it he managed to identify different regions and mints. This methodology was probably applied for the catalogue of Giacomo Nani’s collection: an enormous work, very detailed, a real iconographic lexicon with full indices and bibliographical references that may consent to reconstruct, at least ideally, a great part of this famous collection. Above all emerges his strong attitude for the scientific classification which was developed in two big manuscripts, a concrete proof of the aims and methods that he shared with other numismatics of his time
Phase noise impact on directly detected optical OFDM transmission in uncompensated links
No full textThe impact of large phase noise, typical of cost-effective energy-efficient sources such as VCSELs, on the transmission of a single side-band optical OFDM signal in case of direct detection over uncompensated fiber tracks is investigated thanks to Monte Carlo simulations. Comparison with the results achieved by a semi-analytical approach is reported, in particular when the laser linewidth is comparable to the OFDM subcarrier spacing
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