1,721,064 research outputs found
Computational hydroacoustic analysis of the BB2 submarine using the advective Ffowcs Williams and Hawkings equation with Wall-Modeled LES
Full text linkIl lavoro di ricerca riportato nella presente tesi riguarda la caratterizzazione idrodinamica e idroacustica di una geometria non classificata di sottomarino, tramite simulazioni numeriche. La geometria in esame è il modello sviluppato dal "Defence Science and Technology Organization", noto come BB2. Questo modello è rappresentativo di un sottomarino diesel, con un largo "casing", due timoni anteriori posizionati sulla torretta e quattro timoni posteriori disposti ad "X". Le simulazioni sono state effettuate in scala modello e le condizioni di flusso sono quelle di un classico test in galleria del vento. Sono state effettuate delle simulazioni preparatorie utilizzando metodologie numeriche basate sulle equazioni di Navier-Stokes mediate alla Reynolds (RANS). Sono stati utilizzati due approcci differenti: in un caso, le equazioni RANS sono state integrate lungo tutta l’altezza dello strato limite (WR-RANS); nell’altro caso, il comportamento del flusso nella zona vicino alle pareti solide è stato approssimato tramite i cosiddetti “modelli di parete” (WM-RANS). Le simulazioni sono state effettuate per due diverse velocità del flusso, alle quali corrispondono i numeri di Reynolds (basati sulla lunghezza del sottomarino, L): Re=9.57x10^6 e Re=1.2x10^6. La soluzione ottenuta utilizzando l’approccio WM-RANS è stata utilizzata per inizializzare il calcolo della simulazione LES, anch'essa effettuata utilizzando modelli di parete (WMLES); quest’ultima analisi è stata condotta a Re_L=1.2x10^6. Per quanto concerne la simulazione LES, la griglia utilizzata è composta da 40 milioni di celle. Le celle della griglia attorno al sottomarino e in scia sono sostanzialmente ortogonali con fattore di forma vicino ad uno. L'analisi idroacustica è basata sulla tecnica delle "Analogie-Acustiche" (AA) che permette di disaccoppiare la simulazione fluidodinamica dal calcolo acustico. Nell'ambito delle AA, la pressione acustica è governata dall'equazione di Ffowcs Williams e Hawkings che, nel presente lavoro, è stata considerata nella sua formulazione advettiva, adatta all'analisi di problemi con condizioni simili a quelle di un test in galleria del vento. Il termine di quadrupolo acustico, non lineare, è calcolato a partire dai dati fluidodinamici forniti dalla LES. Lo strato limite, quasi assialsimmetrico, predetto dalla LES esibisce un coefficiente di attrito (Cf) minore rispetto quello di uno strato limite piano, a parità di Reynolds; anche confrontata con la soluzione WR-RANS, la simulazione WMLES mostra una sottostima del contributo viscoso alla resistenza. Al contrario, la RANS con modelli parete mostra una sovrastima del valore di Cf. Per quanto riguarda la scia, le stime fornite dalla LES mostrano un buon accordo con i dati presenti in letteratura; l’accordo è buono anche a notevoli distanze dal sottomarino (18 D a valle del corpo, dove con D si è indicato il diametro del corpo centrale dello scafo). I profili di velocità assiale media, opportunamente scalati con le grandezze caratteristiche della scia (velocità e lunghezza) collassano su un’unica curva, come indicato dalla teoria dell'auto-similarità in equilibrio. Inoltre, è stato osservato che la scia mostra uno stato di evoluzione verso un regime di alto-Reynolds locale; cioè, non si è osservata nessuna transizione da regime ad alto a regime a basso Reynolds. E' stato infine evidenziato che i moti turbolenti (di dimensioni comparabili con D) forniscono il contributo più importante al rumore idrodinamico. Il rumore provocato dal carico di pressione sulla superficie del sottomarino risulta trascurabile se comprato agli effetti non lineari. Gli spettri della pressione acustica in corrispondenza della torretta sono caratterizzati da un picco alla frequenza di 1Hz. Il rumore alle alte frequenze aumenta a valle del corpo come risultato del processo di evoluzione verso strutture turbolente sempre più piccole, a scapito di quelle di dimensioni maggiori.The research activity reported in this thesis concerns the numerical characterization of the hydrodynamics and the hydroacoustics of a notional submarine geometry; the selected geometry is the model developed by the Defence Science and Technology Organization (DSTO) currently known as the BB2 submarine. It is representative of a Diesel submarine, with a large casing, two fore fins placed on the side of the sail and an X-form rudder arrangement.
Simulations are performed at model scale with flow conditions mimicking those of a classical wind-tunnel test. Preparatory Wall-Resolved and Wall-Modeled Reynolds Averaged Navier-Stokes Simulations (hereafter denoted as WR- and WM-RANS, respectively) have been performed at two speeds, corresponding to length based (model scale) Reynolds numbers of Re_L=9.57 x 10^6 and Re_L=1.2 x 10^6. WM-RANS solution was used to initialize Wall-Modeled Large-Eddy Simulation (WMLES) at Re_L=1.2 x 10^6. For what LES simulation concerns, the grid utilized is composed of approximately 40 million cells. The grid cells in a layer surrounding the submarine surface and in its wake are of hexahedral shape. The hydroacoustic analysis is based on the Acoustic-Analogy (AA), which allows to decouple the fluid dynamic simulation from the acoustic estimation. The acoustic pressure is governed by the Ffowcs Williams and Hawkings (FW-H) equation, that, in the present case, has been considered in its advective formulation, which is the most suitable form for wind-tunnel-like problems. The non-linear quadrupole term is estimated by the fluid-dynamic data from the WMLES.
The quasi-axisymmetric turbulent boundary layer predicted by LES exhibits a skin-friction coefficient (Cf) lower compared to a planar turbulent boundary layer under similar conditions, as well as to the WR-RANS solution. On the other hand, WM-RANS overestimates Cf.
Concerning the wake, LES predictions are in a good agreement with available data in literature, up to 18 D downstream (with D the diameter of the submarine). In accordance with the equilibrium self-similarity theory, the mean streamwise velocity profiles, when made non-dimensional with the characteristic velocity and length scales of the wake, collapse into a single curve. Moreover, it has been determined that the wake is in a state of evolution to high-Re regime (i.e. no shift from high- to low-Re solution was observed.
Finally, it has been highlighted that, turbulence furnishes the most important contribution to the hydroacoustic noise. The contribution to the noise of the pressure load on the submarine surface results negligible compared to the non-linear effects. Sound pressure level spectra at the sail side are characterized by peak at 1 Hz. High frequency noise increases downstream as result of the eddies breaking up process in the wake
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Full text linkPrecision oncology is perceived as a way forward to treat individual cancer patients. However, knowing particular cancer mutations is not enough for optimal therapeutic treatment, because cancer genotype-phenotype relationships are nonlinear and dynamic. Systems biology studies the biological processes at the systems’ level, using an array of techniques, ranging from statistical methods to network reconstruction and analysis, to mathematical modeling. Its goal is to reconstruct the complex and often counterintuitive dynamic behavior of biological systems and quantitatively predict their responses to environmental perturbations. In this paper, we review the impact of systems biology on precision oncology. We show examples of how the analysis of signal transduction networks allows to dissect resistance to targeted therapies and inform the choice of combinations of targeted drugs based on tumor molecular alterations. Patient-specific biomarkers based on dynamical models of signaling networks can have a greater prognostic value than conventional biomarkers. These examples support systems biology models as valuable tools to advance clinical and translational oncological research
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
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Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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