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Il lavoro riguarda l’implementazione di una catena modellistica per l’analisi delle colate detritiche in grado di simulare sia l’evento critico di innesco di precipitazione, attraverso modelli idrologici di piena, che la successiva propagazione a valle del materiale già distaccato posto in alveo, attraverso la modellazione idrodinamica. Gli aspetti di tale modellazione affrontati nella tesi riguardano: (i) la calibrazione e validazione di un modello idrologico in bacini montani, spesso caratterizzati da mancanza di misure o dati poco attendibili; (ii) la stima della portata solida a partire dalla portata liquida; (iii) la scelta di un modello reologico per la corretta caratterizzazione della miscela solido-liquida; (iv) l’implementazione di un modello di propagazione che tenga conto delle pendenze ripide tipiche degli alvei torrentizi e di un criterio di arresto. La framework modellistica è costituita dal modello idrologico semi-distribuito GEOframe-NewAge, qui adottato per la prima volta ad una scala temporale di 5 minuti, adeguata a catturare gli eventi meteorici intensi che causano l’innesco; dal modello idrodinamico in-house DAM_DEBRIS, che integra le classiche equazioni alle basse, qui appositamente modificate per tener conto delle alte pendenze di cui sono caratterizzati i bacini montani. Inoltre, è stato implementato un opportuno criterio di arresto e sono state confrontate più leggi reologiche per la caratterizzazione della miscela. L’area di studio analizzata è la parte montana del bacino del fiume Fella (Alpi Giulie), situato nella parte nord-est della regione Friuli Venezia Giulia. Quest’area, infatti, è altamente suscettibile a fenomeni di colate detritiche in virtù sia delle caratteristiche geomorfologiche che di quelle pluviometriche, con precipitazioni particolarmente abbondanti ed intense. I risultati della modellazione idrologica hanno dimostrato l’attendibilità degli idrogrammi simulati anche in sezioni non strumentate. Relativamente alla parte idrodinamica, primi test effettuati su prove di laboratorio hanno mostrato risultati incoraggianti nell’ottica di una sua applicazione al caso reale. La disponibilità di un simile strumento modellistico, potrà permettere lo studio di possibili scenari futuri indotti, per esempio, dall’intensificarsi di eventi meteorici particolarmente estremi, fornendo un ulteriore strumento per una efficiente pianificazione territoriale. The work of this thesis aims at implementing a hydro-morphodynamic modelling framework for debris flow analysis capable of simulating both the precipitation triggering event, through the hydrological modelling, and the subsequent downstream propagation of debris flow, through the hydrodynamic modelling. The aspects addressed in the thesis concern: (i) the calibration and validation of an hydrological model in mountain basins, often characterized by scarcity of measurements or unreliable data; (ii) the estimation of solid hydrograph from the liquid one; (iii) the choice of a rheological model to characterize the solid-liquid mixture; and (iv) the implementation of a propagation model, which takes into account the steep slopes typical of river beds, and a runout criterion. The framework consists of the semi-distributed hydrologic model GEOframe-NewAge, here adopted for the first time at a 5-minute time-step, which is adequate to capture the intense precipitation events that trigger the debris flowing; the in-house hydrodynamic model DAM_DEBRIS, which integrates the classical shallow water equations, here modified to account for the steep slopes of mountains basins. In addition, an appropriate runout criterion was implemented, and multiple rheological laws were compared for the mixture characterization. The study area is the mountainous side of the Fella River basin (across the Julian Alps), located in the northeastern part of the Friuli Venezia Giulia region. This area is highly susceptible to debris flow phenomena due to both geomorphologic and rainfall characteristics, characterized by abundant and intense precipitations. The results of the hydrological modelling demonstrated the reliability of the simulated hydrographs in ungauged areas. With regard to the hydrodynamic part, first numerical tests carried out on laboratory tests showed encouraging results with a view to its application to the real case. The availability of such a modelling tool, may allow the study of possible future scenarios induced, for example, by the intensification of extreme weather events, providing an additional tool for an efficient territorial planning.

The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface. The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface.

We present results of numerical simulations of a stratified reservoir with a three-layer stratification, subject to an oscillating surface shear stress. The simulations are carried out at a laboratory scale, using Large Eddy Simulation. We solve the three-dimensional Navier-Stokes equations under the Boussinesq approximation using a second-order accurate finite volume solver. The model was validated by reproducing experimental results for the reservoir response to surface shear stress and resonant frequencies of internal waves. In the first part of our research, we investigate the effect of sloped end-walls on mixing and internal wave adjustment to forcing within the basin, for three different periods of forcing. We find interesting combinations of wave modes and mixing under variation of the forcing frequencies and of the inclination of the end-walls. When the frequency of the forcing is close to the fundamental mode wave frequency, a resonant internal seiche occurs and the response is characterized by the first vertical mode. For forcing periods twice and three times the fundamental period the dominant response is in terms of the second vertical mode. Adjustment to forcing via second vertical mode is accompanied by the cancellation of the fundamental wave and energy transfer towards the high-frequency waves. The study shows that the slope of the end-walls dramatically affects the location of mixing, which has feedback on the wave field promoting higher vertical modes. The second part of our research is devoted to the investigation of the influence of Earth's rotation on the wave field. In this case, the ratio of forcing to inertial period plays the important role in discerning whether Kelvin or Poincare type waves will be excited which has direct consequences on the wave modal structure and the turbulent quantities. Superinertial forcing frequency excites Poincare waves that are observed to increase the amount of turbulent dissipation rate in the system. In the two cases, where the forcing period was half and twice the fundamental one, we observed that superinertial forcing frequency pushed the dominant internal wave response from first to second vertical mode. The study shows that the increase of the importance of the rotation rate over forcing increases the turbulent dissipation rate in the interior while decreasing it in the near-surface and near-boundary regions.

Il 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.

This thesis concerns the investigation of the meandering phenomena, focusing on the hydrodynamic (Part I) and on the morphodynamic (Part II) of meandering rivers. In the first part of this work, sharp curved single-bend open channel flow with a flat bed, representative of the early phase of bed erosion, is investigated by the use of Large Eddy Simulation (LES). The three-dimensional (3D) numerical simulation can provide flow field information that are difficult to obtain in the laboratory or in a real river. The focus is to provide insight into the physics of sharp meandering bends, highlighting the main flow and secondary flow characteristics and the role of turbulence. The latter plays an important role in many processes that are key in natural rivers, such as the phenomena of spreading and mixing of suspended matter, of sediment transport and scour processes. Turbulence affects the strength of the curvature-induced secondary flow in the core of the flow domain, a typical feature of curved open-channel flow. It rules the intensity of the bed shear stresses and the friction losses along the bend. It is especially important in the flow regions near the banks, affecting the stability of the channel banks. At the inner bank, the model predicts, rather accurately, the boundary layer detachment and the formation of an internal shear layer. Furthermore, the model adequately reproduces the outer-bank cell of secondary flow and the local increase of turbulent kinetic energy. In this work, two curved channels are investigated with the intent to underline the influence of the water depth on the flow features. Finally, the effects of the super-elevation of the free surface on the meandering hydrodynamics are analyzed using a Detached Eddy Simulation (DES) model available in the free software OpenFOAM. In the second part of this work, a mathematical model for meandering rivers with spatial width variations is developed. The mathematical modeling of the long-term evolution of meandering rivers needs an efficient computation of the flow field. Therefore, the development of a mathematical model based on the complete response of a meandering river to spatially varying channel axis curvature and width is necessary. For this purpose, we elaborate a morphodynamic model able to predict the spatial distribution of the flow field and the equilibrium bed configuration of an alluvial river characterized by arbitrary distributions of both the channel axis curvature and the channel width. Owing to analytical character of the model, it provides a computationally efficient tool that can be easily incorporated in long-term river planform evolution models. Furthermore, it can be used to rapidly evaluate the morphological tendencies of an alluvial river in response to variations in planform geometry or hydrodynamic forcing. The model is tested by comparison with the bed topography observed in a typical reach of the Po River, showing that in presence of wide, mildly curved and long bend and weak width variations, the river topography is described with a good accuracy.

Single-phase Euler-Euler based wall-resolving LES with the dynamic Smagorinsky model is used to investigate suspended sediment transport in a turbulent open channel flow. Aspect ratio of the open channel is high. Bottom bed is smooth. For the clear water flow, streamwise and vertical turbulence intensities in experiments of Muste et al. [2005] are much bigger than those in the wall-resolving LES and DNS by Hoyas and Jimenez [2008]. Bulk velocity in the sediment-laden flow is lesser than that in the clear water flow. Friction velocity is same for both flows. While Muste et al. [2005] recorded that in the inner region, the streamwise velocity of the sediment-laden flow was higher than that of the clear water flow, wall-resolving LES shows no alteration. In the outer region, the depth-resolved streamwise velocity of the sediment-laden flow is lower than that of the clear water flow. Introduction of suspended sand particles into the turbulent open channel flow of the clear water results in the decrease of the drag force while shear stress on the channel bed is constant. To get reduction of the bulk velocity, the fast Eulerian method in the two-way coupling should be employed. Single-phase Euler-Euler based unresolved wall-function LES with the Smagorinsky model under the equilibrium stress assumption is implemented to understand interactions between turbulence and suspended sand particles in a turbulent open channel flow. Channel bed is rough. Aspect ratio of the open channel is low. To treat erosion from the bed, the reference concentration method together with the Shields diagram is used. Results are compared against experiments of Cellino [1998]. Suspended particles engender reduction of the friction velocity, bulk velocity and roughness in contrast to the clear water flow. Streamwise velocity is decreased in the outer region while it is expedited in the super-saturated region near the channel bed. It is due to high inter-particle collisions between sediment particles which are not bounded by viscosity. In upper levels, remarkable weakening of the vertical turbulence intensity is seen. When the buoyancy term is deactivated, the sediment concentration gets high and unsatisfactory turbulence statistics are obtained. Wall shear stress on the sidewalls of narrow open channels must be considered. Effects of lateral and bottom macro-rough boundaries on the propagation of a suspended sediment wave in a turbulent open channel flow are shown experimentally. Least decay of the normalized concentration is for the reference case in absence of trapping zone. Least sedimentation among lateral configurations is for case L4 with the highest roughness aspect ratio, cavity density and medium flow discharge. Length of the inlet reach is lowest and it boosts mixing. Highest deposition of the Polyurethane particles is for case L5 with the lowest roughness aspect ratio and cavity density. Discharge is also low. Turbulence is most attenuated for case L2 with the medium flow discharge due to adequate lengths of the inlet and outlet reach. Deposition of the Polyurethane particles and turbulence in the lateral macro-rough flows depend on the cavity aspect ratio, flow discharge, roughness aspect ratio, cavity density and location of the lateral cavities. In the bottom macro-rough flows, effects of spacing between bottom macro-rough elements on the deposition and turbulence characteristics are seen. Turbulence characteristics of upstream C1 and downstream C2 signals are identical for reference, B1 and B1.5 cases. When spacing is 2, highest trapping and deposition take place. Most weakening of the turbulence is seen for spacing 2. When spacing is augmented from 2 to 5, the turbulence is enhanced and trapping becomes lesser. Bottom macro-rough elements change significantly flow pattern and turbulence characteristics in a specific spacing. As spacing gets larger, effects of bottom macro-roughness elements on each other reduces more.

L'obiettivo di questo lavoro è quello di sviluppare un metodo efficiente di decomposizione dei domini per la simulazione di vari flussi di fluido, di interesse per la meccanica dei fluidi, geofisica e ambientale, utilizzando griglie non sovrapposte in un solutore LES. Viene mostrato un metodo di decomposizione del dominio di sub-strutturazione esplicita che applica le condizioni di compatibilità presenti tra flussi di fluido non miscibili. L'attuale metodologia è utilizzata per lo studio dei flussi archètipo accoppiati in direzione co-corrente e controcorrente, per numeri di Reynolds bassi. Inoltre, viene mostrato lo studio sui ostacoli immersi nei flussi di Poiseuille-Couette accoppiati. The objective of this work is to develop an efficient domain decomposition method for the simulation of multiple fluid flows, of interest in geophysical and environmental fluid mechanics, using non-overlapping grids. An explicit sub-structuring domain decomposition method that enforces the compatibility conditions present between non-miscible fluid flows is shown. The current methodology is used for the study of coupled canonical co-current, and counter-current, flows for low Reynolds Numbers. Additionally, the study around immersed obstacles in coupled Poiseuille-Couette flows is made.

Le correnti di gravità sono flussi prevalentemente orizzontali causati da variazioni di densità rispetto al fluido circostante. Tali flussi, possono scatenare il trasporto di notevoli volumi di sedimenti provocando così rapide deformazioni del letto dei corsi d’acqua, dei bacini causando importanti problemi di natura ambientale. Nonostante la rilevanza di tale fenomeno, non è ancora ben compreso il meccanismo che causa la ri-sospensione dei sedimenti. Un nuovo modello idro-morfodinamico viene presentato nel presente lavoro di Tesi al fine di simulare i processi di trasporto dei sedimenti indotti dalla propagazione delle correnti di gravità su fondo mobile. L’idrodinamica del flusso è risolta mediante il modello LES-COAST, che utilizza il metodo Large-eddy mentre i cambiamenti morfologici causati dall’erosione e dalla deposizione sono simulati mediante il modello level-set. L’accoppiamento dei due strumenti modellistici è realizzato attraverso la strategia dei confini immersi. Il modello numerico accoppiato è stato sviluppato in ambiente parallelo, condizione che consente di simulare problemi a scala reale. Il modello è stato applicato alla simulazione della migrazione delle forme di fondo al fine di valutarne le capacità. I risultati ottenuti sono stati confrontati con i dati esito di un analogo studio numerico ed è emersa la capacità del modello di catturare correttamente l’idrodinamica dei flussi acqua-sedimento e i connessi mutamenti morfologici del fondo. La sospensione dei sedimenti indotta dalle correnti di gravità è inizialmente simulata assumendo condizioni al contorno stazionarie e i risultati numerici sono validati con dati sperimentali. Le impostazioni numeriche adottate in questo caso permettono di indentificare la regione del flusso con maggiore capacità erosiva e determinare le condizioni del flusso che dominano la dinamica della sospensione dei sedimenti. Inoltre, è stato approfondito l’effetto dei sedimenti sospesi sulla condizione di stabilità della corrente di gravità nonché l’influenza di tale condizione sulla distribuzione dei sedimenti sospesi. Infine, il modello accoppiato idro-morfodinamico è utilizzato per simulare le forme di fondo generate da tale tipologia di correnti. È importante notare che la propagazione di correnti di gravità con elevato potere erosivo su fondi mobili può causare notevoli deformazioni del letto che possono avere un effetto non trascurabile sulla dinamica del flusso. Per la prima volta sono state realizzate delle simulazioni numeriche 3D delle correnti di gravità che interessano letti deformabili. L’evoluzione istantanea dei pattern di erosione e deposizione sono quindi presentati, contribuendo così a una conoscenza più dettagliata dell’interazione flusso- sedimenti e flusso-fondo in correnti di gravità. Gravity currents are essentially horizontal flows driven by a density difference with respect to the ambient fluid. Such flows, can trigger the transport of large amounts of sediment resulting in rapid deformations on the bed of rivers and reservoirs, and causing important environmental issues. Despite the importance of such phenomena, the main mechanisms that lead to sediment entrainment in such flows are still poorly understood. In this thesis a coupled hydro-morphodynamic model is presented in order to simulate sediment transport processes induced by the propagation of gravity currents over erodible-beds. The hydrodynamics of the flow is resolved using the LES-COAST model, that employs the large eddy simulation method, and the topological changes due to erosion and deposition are tracked by the level-set method. The coupling between the two models is achieved through the immersed boundary methodology. The resulting hydro-moprhodynamic tool works in parallel environment that makes the simulation of real systems possible. This model is applied to the simulation of the ripple migration problem in order to test its capabilities. The obtained results are compared with data provided by a similar numerical study and indicate that the model is able to capture correctly the hydrodynamics of the sediment-laden flow and the related topological changes. The sediment pick-up induced by gravity currents is initially simulated assuming stationary flow boundaries and the numerical results are validated by experimental data. The numerical setting considered in this case allows to identify the region of the flow with higher erosive capacity and determine the flow features which play a dominant role on the sediment entrainment. Moreover, the effect of the suspended sediment on the stability condition of the gravity current is examined, along with the influence of the latest on the suspended sediment distribution. Finally, the developed hydro-morphodynamic model is used to simulate the bed forms generated by such flows. It is worth-noting that the propagation of highly erosive gravity currents over loose beds can lead to considerable bed deformation that significantly influence the flow dynamics. For the first time, 3D numerical simulations of gravity currents flowing over a deformable beds are performed. The instantaneous evolution of the generated scour and deposition patterns is presented providing a better insight on the flow-sediment and flow-bed interaction in buoyancy driven flows.

L’oceano copre circa il 70% della superficie terrestre e riveste un ruolo fondamentale per la regolazione climatica. In oceano le misure sono sparse nel tempo e nello spazio, cosi i modelli numerici oceanici sono un importantissimo strumento per lo studio della circolazione oceanica e per predire gli scenari futuri. Le aree costiere rappresentano solo una piccola frazione dell’oceano globale, ma hanno un grande impatto su molti aspetti dell’ambiente marino e sulle attività umane. Migliorare la rappresentazione delle aree costiere nei modelli globali è cosi una delle grandi sfide della comunità scientifica. Sulla base di tale considerazione, in questo lavoro tesi, si è cercato di realizzare uno strumento che possa risolvere adeguatamente anche le scale di interesse costiero. A tale scopo si è utilizzato il codice “LES-COAST”, sviluppato presso l’Università di Trieste. In un primo momento la capacità di due differenti codici (MITgcm e LES-COAST) di simulare la circolazione in un dominio di calcolo semplice è stata testata. Una serie di esperimenti sono stati condotti e l’obiettivo è stato confrontare i risultati ottenuti. Successivamente, è stato realizzato un downscaling dinamico partendo dal modello di circolazione generale (MITgcm) fino ad arrivare ad una adeguata risoluzione numerica, grazie all’utilizzo del suddetto codice LES, di determinate aree costiere di interesse specifico. Il modello di circolazione generale, MITcgm (modello numerico tridimensionale alle equazioni primitive ad alta risoluzione spaziale), si è annidato (nesting) con il modello LES-COAST, che risolve quei processi fisici altrimenti sottoposti a parametrizzazione. Il codice LES è un modello numerico tridimensionale, alle differenze finite e ad altissima risoluzione spaziale (capace di risolvere scale fino ad una risoluzione di 10 metri in orizzontale ed 1 metro in verticale) che fa uso dell’approccio “large-eddy simulation” per parametrizzare la turbolenza. In particolare, per adeguarsi al meglio alla conformazione dei bacini costieri che prevedono tipicamente una lunghezza scala orizzontale molto maggiore di quella verticale, fa uso di un modello Smagorinsky, anisotropico, a doppia viscosità per i flussi di sotto-griglia (SGS) orizzontali e verticali. Si è sviluppato un terzo codice che gestisce l’accoppiamento e lo scambio d’informazioni tra i due modelli descritti. The ocean covers 70% of the earth’s surface and it plays a major role for the climate of the earth. In the ocean measurements are sparse both in time and space. Numerical ocean models are a considerable tool for studying the ocean circulation and to predict future scenarios. Coastal and shelf seas represent a small fraction of the area of the global ocean but have a large impact on many aspects of the marine environment and human activities, so improving the representation of coastal and shelf seas in global models is one of the grand challenges in ocean modelling and Earth system science. Investigating the large-scale impacts on smaller-scale processes in the coastal ocean can often be successfully treated by nested regional studies. In my project the ability of two different models (global model and coastal model) to simulate a flow in a simple closed domain has been tested. In this way a series of numerical simulations have been conducted and our goal has been to compare the results derived from two modeling approaches: Ocean General Circulation Model (OGCM) and Large Eddy Simulation Model (LES). Later, considering that nesting remains an important approach for investigating of regional systems and providing fine-scale information, a novel approach is proposed. A nesting between these two codes has been designed and developed. Comparing the results of simulations “OGCM stand-alone” and “LES after nesting” it has been shown that the resolution of the velocity field has been clearly improved using the new methodology.

Il presente studio è incentrato sulla valutazione delle performance e sviluppo della scia di una turbina marina soggetta a condizioni di stratificazione stabile. Lo studio numerico è stato condotto per mezzo della tecnica di ‘Large Eddy Simulation’ (LES) mediante l’utilizzo del modello LES-COAST, sviluppato dal gruppo di ricerca. Il modello numerico è stato validato attraverso la comparazione tra i risultati numerici e dati di laboratorio(reperiti in letteratura) riguardanti la potenza estratta e la forza di thrust generata dalla turbina. Data l’importanza della stratificazione nelle dinamiche marine, l’ obbiettivo è studiare l’interazione tra la stratificazione e il campo della turbina. Inoltre la produzione globale di un parco di turbine marine e la produzione delle singole turbine possono essere influenzati dagli effetti della stratificazione sul campo di moto. Per quanto riguarda la letteratura, non sono state individuate ricerche con lo scopo di studiare gli effetti della stratificazione sullo sviluppo della scia e della produzione di energia della turbina. I siti di marea nei quali le turbine vengono istallate possono essere soggetti alla presenza di stratificazione stabile di densità. La stratificazione di densità è causata da variazioni di temperatura e/o salinità tra le acque superficiali e le acque del fondo marino. Allo scopo di valutare gli effetti della stratificazione sulla produzione di energia e sul recupero del deficit di velocità, sono stati simulati due tipi di stratificazione stabile: una condizione intensa e una condizione debole. La stratificazione di tipo debole è stata simulata imponendo un salto di temperatura in modo da ottenere un profilo di densità a forma di gradino. Per quanto riguarda la stratificazione intensa, è stato imposto un salto di salinità, anche in questo caso a forma di gradino, il quale produce una maggiore variazione di densità rispetto al caso debole. Successivamente la turbina è stata introdotta all’interno dei due campi stratificati e i conseguenti risultati sono stati comparati. L’analisi dei risultati dimostra che la stratificazione ha un impatto sulla produzione di energia della turbina e sull’evoluzione e recupero della scia. Inoltre l’effetto di mescolamento causato dalla turbina è stato analizzato The present study concerns the evaluation of the effects of stable stratified condition on turbine performance and wake development. The numerical analysis has been carried out by means of Large Eddy simulation (LES) through the in-house LES-COAST model coupled with a turbine module that computes the turbine induced forces. The capability of the model in reproducing power and thrust characteristics of a turbine has been proved from the comparison between the numerical results and the experimental data supplied by literature. Since the stratification is an important factor in the marine dynamics, the aim is to evaluate the interaction between the stratification and the turbine. Moreover the overall power production of a tidal farm and the power output of the single turbines can be influenced by the effects that stratification entails on the flow field. To the best of our knowledge, in the marine field there is no research aimed at studying the influence of marine stratification on wake recover and turbine performance. Tidal sites where marine turbines are installed can be subject to the presence of density stable stratified conditions. The density stratification is due to temperature and/or salinity variations between the superficial and the bottom water. In order to evaluate the influence of stratification on power output and wake development and recover, we simulated to types of stable stratified condition: a weak and a strong stratification. The weak stratification has been simulated imposing a temperature jump in order to obtain a vertical density profile with a step shape. For the strong stratification, we imposed a salinity jump with a step shape which gives rise to an higher density variation compared to the weak case. Then the turbine has been introduced into the stratified fields and the results have been compared. The analysis of the results highlights that the stratification has an impact on turbine production and on wake development and recover. Moreover the turbine mixing effect is analyzed.

L'attività di ricerca riportata in questa tesi riguarda lo studio numerico della generazione e propagazione del rumore idroacustico. Le onde sonore possono essere emesse ogni volta che esiste un movimento relativo tra due fluidi o tra un fluido e una superficie. Nei decenni passati è stata prestata molta attenzione ai problemi di rumore aeroacustico. Nel corso degli anni, modelli teorici e numerici adatti per flussi transonici o super-sonici sono stati sviluppati e la loro efficacia è stata testata. La principale fonte di rumore è stata identificata nei termini detti di thickness e loading. Per rilevare questo tipo di fonte di rumore è sufficiente considerare i termini lineari dell'equazione di Ffowcs-Williams e Hawkings. Nell'ambiente subacqueo le onde acustiche, quindi i disturbi della pressione, viaggiano a velocità molto più alta di quella del moto del flusso, così che la maggior parte dei fenomeni idrodinamici si trovano in un regime incomprimibile. La lunghezza d'onda è comunemente molto più grande della scala di lunghezza del problema considerato. Inoltre, il vortice che si sviluppa nella parte posteriore di un corpo immerso persiste sulla scia dando contributo considerevole al campo acustico. In queste condizioni, i meccanismi di produzione e propagazione del rumore necessitano di modelli diverso. In questo lavoro, vengono analizzate e discusse diverse metodologie di soluzione dell'equazione FW-H per tenere conto dei termini non lineari. In particolare, la forma avvettiva dei termini di volume viene derivata. Il campo fluidodinamico, considerato come una raccolta di impulsi di rumore, deve essere riprodotto accuratamente. Una simulazione Large-Eddy (LES) è qui considerata come lo strumento più vantaggioso per riprodurre flussi turbolenti e, allo stesso tempo, affrontare casi di interesse pratico. La prima parte dello studio è dedicata alla valutazione del modello, in seguito viene eseguita una LES di un flusso turbolento attorno a un cilindro quadrato di lunghezza finita. Si confronta il contributo dei diversi termini dell'equazione FW-H con la pressione dinamica del fluido. Attraverso l'analisi dimensionale si osserva che per problemi idrodinamici, dove la velocità di un corpo è piccola rispetto alla velocità del suono, l'integrazione diretta del termine del volume è lecita e pratica. Il calcolo diretto dei termini non lineari, sulla regione del volume che circonda il corpo immerso, viene quindi impiegato, nella seconda parte della tesi, per lo studio del rumore generato da un flusso attorno a tre diverse geometrie: sfera, cubo ed ellissoide. L'ultima parte della tesi è dedicata ad uno studio preliminare del campo acustico emesso da un flusso cavitante. La cavitazione può essere interpretata come la formazione di bolle di vapore a causa di una forte variazione di pressione, scendendo questa al di sotto della pressione di saturazione del liquido. L'importanza di studiare i flussi cavitanti è correlato alla loro presenza in varie applicazioni tecniche, come pompe, turbine, eliche di navi e sistemi di iniezione di carburante, nonché in scienze mediche. Esistono diversi tipi di cavitazione, tra le più importanti ci sono: sheet cavitation, bubble cavitation and vortex cavitation. La sheet cavitation può verificarsi sui profili alari, su pale di pompe ed eliche, in particolare quando il carico è elevato. Questo tipo di cavitazione difficilmente può essere evitato, a causa dei requisiti di alta efficienza. La dinamica della sheet cavitation spesso genera forti fluttuazioni di pressione dovute a il collasso delle strutture del vapore del capannone, che potrebbe portare all'erosione del materiale superficiale e ad una emissione acustica intensa e complessa. In questa tesi viene proposto uno studio preliminare sul rumore di cavitazione, considerando prima una bolla isolata poi una nuvola di bolle e poi un hydrofoil. The research activity reported in this thesis concerns the numerical study of hydroacoustic noise generation and propagation. Sound waves may be emitted whenever a relative motion exists between two fluids or between a fluid and a surface. In the past decades much attention has been paid to aeroacoustic noise problems. Over the years, theoretical and numerical models suitable for transonic or super-sonic flows have been developed, and their effectiveness has been tested. The main source of noise has been identified with the thickness and loading noise terms. To detect this type of noise source is enough to consider the linear terms of the Ffowcs-Williams and Hawkings equation. In underwater environment the acoustic waves, thus the pressure disturbances, travel at speed much higher than that of the flow motion, such that most of hydrodynamic phenomena are in an incompressible regime. Wave length is commonly much larger than the length scale of the considered problem. Moreover, vortex developing at the rear of an immersed body, persists on the wake until braking downstream thus giving a considerable contribution to the noise signature. Under these conditions, the mechanisms of noise production and propagation need a different modeling. Thus, in this work, different solution methodologies of the FW-H equation are analyzed and discussed in order to account for the non-linear terms. In particular, the advective form of the non-linear terms, suitable for wind-tunnel type of problems, is derived. The flow field, regarded as a collection of noise impulses, needs to be reproduced accurately. A Large-Eddy Simulation is here considered as the most advantageous tool to reproduce turbulent flows and, at the same time, deal with cases of practical interest. The first part of the study is dedicated to the assessment of the model: we perform a LES of a flow around a finite-size square cylinder. We compare the contribution from different terms of the FW-H equation with the fluid dynamic pressure. Different methods which are proposed in literature were considered. The direct integration of the volume terms was found to give the most accurate results. Moreover, through dimensional analysis it is observed that for hydrodynamic problems, where velocity of a body is small compared the speed of sound, the direct integration of the volume term is licit and practical . The direct computation of non-linear terms, by integrating on the volume region surrounding the immersed body, is then employed, in the second part of the thesis, for the study of noise signature generated by a flow around three different geometries: sphere, cube and prolate spheroid. Last part of the thesis is devoted to a preliminary study of the acoustic field emitted by a cavitating flow. Cavitation may be interpreted as the rupture of the liquid continuum due to excessive stresses. It is the evaporation of a liquid in a flow when the pressure drops below the saturation pressure of that liquid. The importance of understanding cavitating flows is related to their occurrence in various technical applications, such as pumps, turbines, ship propellers and fuel injection systems, as well as in medical sciences. There are several types of cavitation, such as: sheet cavitation, bubble cavitation and vortex cavitation. Sheet cavitation may occur on hydrofoils, on blades of pumps and propellers, specifically when the loading is high. This type of cavitation can hardly be avoided, because of high efficiency requirements. The dynamics of sheet cavitation often generates strong pressure fluctuations due to the collapse of shed vapor structures, which might lead to erosion of surface material and intense and complex noise track. In this thesis a preliminary study on the cavitation noise is proposed, first considering an isolated bubble then a bubble cloud and then an hydrofoil.

In the present thesis I investigate the wind-driven circulation and mixing in a peri-alpine lake (Lake of Ledro) located in Trentino Alto Adige (Italy). It represents an interesting case study because of two main reasons: it is surrounded by mountains, which affect wind distribution and consequently produce a non homogeneous wind stress of the lake surface; in recent years the lake has suffered for intense blooms of the harmful cyanobacterium \textit{Planktothrix Rubescens}, occurring in the winter season, characterized by the absence stratification. The study is carried out numerically, using a high-resolution, eddy resolving Large Eddy Simulation (LES) model (LES-COAST). The model resolves directly the large scales of motion and parametrizes the small ones by means of the Anisotropic Smagorinsky Model (ASM). The complexity of the coastline and bathymetry is treated by the Immersed Boundary Method which, combined with a curvilinear grid, provides a feasible tool for the study of the hydrodynamics of semi-closed or closed areas like a peri-alpine lake. To the best of author's knowledge this work represents the first application of a LES model to an actual lake as whole, considering its shape and its dimensions. Two types of boundary condition for the time changing wind stress are used: spatially homogeneous and non-homogeneous. The former is reproduced by applying to the lake's surface the wind velocity and the direction measured by a floating meteo station located in the center of the lake during the week 16-22 of January 2012, whereas the latter is obtained by the regional scale Weather Research and Forecasting model (WRF) for the same week, making a first attempt of coupling air-water models. The analysis of the wind-driven circulation, as well as of the high-order statistics, has been performed with the aforementioned wind conditions. The analysis shows the presence of downwelling/upwelling areas along the windward/leeward coastline respectively, providing an explanation of the bloom of cyanobacterium at the lake surface observed under winter conditions. Moreover, peculiar superstreaks, which span over the entire depth of the lake, have been simulated, enhancing the vertical mixing in the water column. The bottom boundary layer has been studied in both unstratified and stratified case. The role of the Coriolis force in such a small lake is highlighted, together with the effect of the simplification of the computational domain with respect to the actual geometry. The eddy viscosities values have been computed and they have shown, among other analysis, the presence of a minimum stress plane which has been confirmed by the study of the Turbulent Kinetic Energy. The evolution in time of the TKE and its dissipation rate in both wind regimes indicate the presence of equilibrium turbulence even under the unsteady conditions herein studied, moreover it provide an estimation of phase lag of the two quantities between the surface and the bottom of the lakes. Under wind inhomogeneity, turbulent mixing appears enhanced with respect to the simplified homogeneous wind case. In presence of thermal stratification the eddy viscosities, the turbulent kinetic energy and TKE dissipation rate are smaller with respect to the non stratified case, and evidences of the presence of the BBL in the stratified case are provided. Convergence and divergence areas confined in the surface mixed layer are shown, in particular in presence of strong wind squalls and inhomogeneity they appear aligned with the isotherms. The present thesis has to be considered as a first step toward the study of the hydrodynamics and of the internal waves in stratified lakes with different wind conditions.

Biogeochemical extreme events are analysed for the special case of the surface chlorophyll in the Mediterranean open sea. The extremes are defined statistically as values over the 99th percentile threshold that are connected in space and time. A multidecadal simulation with daily output (1979-2012) was run to collect the statistics needed to carry out the study. The online coupled hydrodynamic-biogeochemical (MITgcm-BFM) model has a spatial resolution of 1/12°, with 75 vertical levels. The model was corroborated with available observations and other validated models. To account for the heterogeneity of the chlorophyll features and dynamics across the Mediterranean area, the definition of the extremes as peaks over the threshold was initially applied at each grid point and then refined to include in one extreme event all the points near in space with extremes at the same time. The macro-events were then characterized by a set of indexes, related to their spatio-temporal localisation, shape and impact on the local marine ecosystem due to their supply of high and/or anomalous biomass production. Following, the macro-events occurred in the Mediterranean area in the winter-spring months of the 1994-2012 period were described and classified using the introduced set of indexes. Overall, the macro-events occurred in each year during the winter months, especially in the northern part of the Mediterranean Sea. They have a wide spectrum of possible shapes, mainly corresponding to a duration shorter than 20 days and a radius contained within the larger Mediterranean mesoscale (200 km). The most persistent macro-events (uniformity index higher than 60%) are relatively small and short within the total spectrum, with an inverse proportionality between area and duration. The most severe macro-events (mean severity higher than 1.1 kgChl/km2/day in the first meter of depth) have intermediate area and duration; generally they are not very persistent, but sometimes they are very anomalous (anomaly higher than 15%). The most anomalous macro-events have typically medium or long duration (up to 84 days) and they can be also very large (up to the whole sub-basin scale), but not very persistent. In agreement with literature, no significant trends are observed in the macro-events indexes on the basin-scale. Some subdomains were analysed in detail: the North Western Mediterranean Sea (NWM), the Ionian Sea (ION), the South Adriatic Sea (ADS) and the Levantine Sea (LEV). The decreasing eastward gradient of the mean severity across the subdomains reflects the well known gradients of the mean surface chlorophyll. More than 70% of the macro-events of chlorophyll occurred in NWM and ADS are classified as events of high severity and high anomaly with respect to the medians of the distributions of the mean severity and anomaly on the whole domain. Overall, the most severe macro-events are in NWM. ADS is the subdomain with the most persistent macro-events, possibly ascribed to the spatial constraints of the topography and of the circulation structure. In ION and LEV more than the 50% of macro-events show low values of both indexes, even if hot spots of macro-events heavily affecting the ecosystem can be observed in the North Ionian Sea, close to the southern coasts of Sicily and in the Rhodes Gyre regions. Focusing on some representative examples of macro-events further investigated, we observed that the model is able to reproduce the main features of timing and location of the chlorophyll patterns observed by remote sensing and that the physical forcing anomalies with respect to climatology appear to play the major role as trigger of the macro-events. Finally, the selected macro-events are characterized by phytoplankton subgroups (BFM parametrization of picophytoplankton, flagellates, diatoms, large phytoplankton) that evolve in space and time in different ways in the subdomains.

The understanding and prediction of the monsoon variability over South Asia region is one of the biggest challenges for climatologist and meteorologist today. The Indian Summer Monsoon (ISM) has different temporal and spatial scales of variability and it is mainly driven by strong air sea interactions. In this thesis we evaluate the performance of the new regional Earth System Model (ESM) RegCM-ES in reproducing the main characteristics of the ISM rainfall (ISMR). We performed two sets of simulations, one with the new RegCM-ES and another with the stand-alone version of the atmospheric component i.e. the regional climate model (RCM) RegCM4. RegCM-ES is composed mainly by three components, the RegCM4, the ocean model MITgcm and the hydrological model HD. Another experiment, performed using RegCM-ES with a more high-resolution hydrological model implemented ad hoc for this study, has been added to this two set of experiments. The climatological mean state of the monsoon is well represented by mostly all the experiments although not with the same skills. The most interesting results are observed in simulating the variability of the monsoon and here we highlight some of them. The intraseasonal northward(eastward) propagation of the convection has been analysed using lag/lead map of the regressed anomalies as a function of latitudes(longitudes). The two propagations are better reproduced by the set of ESM simulations thanks to the role of the air-sea coupling. For what concerns the interannual variability (IAV) of the ISMR the air-sea coupling plays an important role. The time series of simulated anomalies by RCM exhibit no correlation with the observed anomalies obtained from the dataset of the Indian Meteorological Department (IMD). On the other hand, the corresponding ESM simulations exhibits good skills in reproducing the IAV of ISMR and good correlation coefficients are observed with IMD. One new finding of this study is a new source of predictability with one-year lag for the ISMR. It is well known that El Niño Southern Oscillation (ENSO) plays a quite important role in modulating the precipitation over most of the intertropical belt and over the South Asia region. As seen in previous findings, the response of the ISMR to ENSO can be delayed of two seasons through the contribution of different air-sea coupled mechanisms as the decrease of Western Arabian Sea Upwellingthe Indo-western Pacific ocean capacitor and the Indo-Tropical northwest Pacific ocean-atmosphere interaction. Our findings extend and confirm the possibility that this response may have a longer feedback time, a year or so. The coupled simulations are used to explain the mechanism and investigate the models response. They appear quite similar to those proposed in the previous studies but further investigations are needed to understand more in deep the phenomena involved. The role of the new hydrological model (CHyM) implemented inside RegCM-ES on the ISMR is also investigated. Due to the main role that the freshwater discharge plays on the formation of a shallow mixed layer depth on the Bay of Bengal that influences the air-sea coupling and the formation of deep convection over this area, the correct estimation of the freshwater discharge is quite important. Although the new hydrological model produces a more realistic annual cycle of the discharge (verified only for a limited set of data due to the lack of observations over this region), this doesn't seem to have a relevant effect on both the mean climatological state of the monsoon and in its variability. A possible explanation for this comes from a missing representation of the barrier layer (BL) in the Bay of Bengal (BoB) due to many different reasons. Two of them are a not enough resoluted ocean model as well as a too strong wind stress forcing that doesn’t allow the formation of the BL.

In this thesis we want to model all steps of a CO2 injection in deep saline aquifers, from the injection to the monitoring by means of seismic methods. After outlining the main physical and chemical processes which allow the CO2 to be trapped for very long periods of time, we present two numerical examples. In the first one, CO2 is injected in a complex aquifer, part of an anticlinal structure, with characteristics resembling those of the Sleipner field. The injection is monitored via an active cross-hole seismic experiment; in particular, we perform a tomographic inverion of the direct arrivals. In the second example, CO2 is injected at a constant rate for one hour in a homogenous aquifer. It is then monitored with a passive seismic method. In fact, the overpressure caused by the injection, may lead to the formation of microcracks and therefore to the emission of seismic waves. We approximate the position of the CO2 plume by finding these emitting points by means of a reverse-time migration algorithm.

The challenge of reconstructing the growth and decay of palaeo-ice sheets represents a critical task to better understand mechanisms of global climate change and associated sea-level fluctuations in the present and future. The Eurasian palaeo-Ice Sheet (EIS) at its maximum volume and extent (between 21,000 and 20,000 years ago) comprised the Scandinavian (SIS), Svalbard/Barents/Kara seas (SBKSIS), and British-Irish (BIIS) palaeo-ice sheets. The SBKSIS was a so called marine based ice sheet, as it rested several hundreds of meters below the sea level in the Barents and Kara seas. From a bathymetric and topographic point of view, there is a strong similarity between the SBKSIS and the present-day West Antarctic Ice Sheet (WAIS). Consequently, to reconstruct the dynamic processes driving the last deglaciation of the SBKSIS may represent an important task to better understand the WAIS present and future evolution. In this study, the evolution of the EIS during the last deglaciation is reconstructed with two hybrid Shallow-Ice/Shallow-Shelf-Approximation numerical Ice Sheet Models (ISMs), namely PSU and GRISLI. These two ISMs differ mainly in the ice stream parametrization and in the complexity with which grounding line migration is treated. A particular focus in this study is given to the Storfjorden glacial system in the north-western Barents Sea. In fact, several palaeo-data from this area furnish insights on the Storfjorden ice stream dynamics history, providing a good testing ground for ISMs. The ISMs are forced with macro-regional indexes representative of the climate evolution over Siberia/Kara Sea, Svalbard/Barents Sea and Fennoscandia during the last deglaciation. The climate in- dexes are based on TraCE-21ka, a transient climate simulation of the last 21,000 years carried out with the Atmosphere-Ocean General Circulation Model CCSM3. Two different ocean basal melting parametrizations based on ice-ocean heat fluxes are tested. The ocean basal melting parametrizations are forced with time-varying ocean temperature and salinity from TraCE-21ka transient climate simulation. Ocean temperature and salinity representative of Arctic Ocean, Norwegian Sea, north-western and south- western Barents Sea are employed. In order to deal with ISMs poorly constrained model parameters, a statistical approach is adopted. A Latin Hypercube Sampling (LHS) of five GRISLI parameters is performed. Due to large computational costs it is not possible to perform such a statistical approach with PSU ISM. Therefore, a restricted number of simulations performed with PSU ISM employ model parameter values from GRISLI significant simulations. GRISLI-simulated ice streams dynamics has a strong control on the deglaciation of the EIS, in particular in the Southern SIS and in the Western SBKSIS. In terms of total ice volume there is a good agreement between GRISLI simulations, ICE-5G reconstruction and global Eustatic Sea Level data. In contrast, GRISLI-simulated ice sheet extent evolution presents discrepancies with geological observation in the Southern SIS and in the Eastern SBKSIS. The use of different basal melting parametrizations in GRISLI has a strong impact both on the deglaciation of the SBKSIS and on the retreat of Storfjorden ice stream. The Storfjorden ice stream dynamics history in GRISLI simulations is in good agreement with palaeo-data in terms of timing of the ice stream retreat, Grounding Zone Wedges formation and response to Meltwater Pulse 1A. The different treatment in PSU of ice streams and Grounding Line migration has a remarkable effect on the EIS deglaciation. In particular, in PSU the Grounding Line dynamics plays a primary role with respect to ocean basal melting, thus exerting a strong control on the decay of the marine-based SBKSIS. Finally, an inverse-type approach is adopted to match PSU simulations with ICE-5G reconstruction and the other palaeo-data.

Heat transfer in fluids and between a fluid and a surrounding solid body is often encountered in practical applications. The numerical investigations of such phenomena is of great interest for a number of practical applications, both on industrial and environmental side. Among the others we mention optimization of home appliances (oven, dishwasher, refrigerators), building design (ventilation and heating systems), electronic equipment design (solar collectors, device cooling systems), environmental flow analysis (atmospheric thermal stratification, evaporation from lakes and channels, solar thermal radiation). Despite the wide range of application, the correct numerical simulation of such systems poses big challenges. First, the physics governing wet-surface evaporation/condensation processes and thermal radiation is extremely complex; thus, a mathematical model is derived only under simplification hypotheses. Second, the transient nature of surface heat transfer along with the complex geometry and anisotropic turbulence flow, requires a careful numerical resolution technique of the fluid flow equations. Third, particular attention has to be payed when the different heat transfer modes are coupled, because of their mutual strong interaction. The present work focuses on numerical investigation of the heat transfer mechanisms in fluid dynamics systems, considering different physical processes. Specifically, this topic is declined in three different studies: (i) turbulent buoyant flow in a confined cavity with conjugate heat transfer; (ii) thin film evaporation and condensation process from a vertical plate inside an enclosure; (iii) radiative heat transfer in systems with participating media. All these points are tackled. A complete numerical solver is presented and employed. In all cases we use large-eddy simulation, carried out in conjunction with a dynamic Lagrangian subgrid-scale turbulent model. Also, the thermal interaction between fluid and solid media is considered by means of conjugate heat transfer; the problem with evaporation and condensation over solid surfaces is studied taking advantage of the thin film assumption; a radiative heat transfer model is integrated in the numerical solver. The heat exchange, through fluid and solid interface, is due to conduction, water change of phase and surface thermal radiation. The temperature alteration of conductive solids, can substantially change the system thermodynamic equilibrium and can eventually lead to significant variations on the overall fluid dynamics. An in-house solver, developed within the open-source software package OpenFOAM, is used for the numerical studies and, later, extended to include the effects of thermal radiation and surface radiative heat exchange. The influence of various solid boundary materials on natural convection in closed cavity is investigated, pointing out the different effects induced on the fluid motion by thermal conductors and insulators (this part of the research has been published in Physics of Fluid). The study has shown that thermal conductive boundaries strongly influence the fluid flow; the use of simplify boundary condition (such as the adiabatic condition) instead of conjugate heat transfer, leads to unrealistic results. Successively, the cooling effects of water evaporation from a plate are studied, changing the plate materials, and the different behaviour of each substance is analysed. This study has shown that water change of phase completely rules the interface heat exchange and that material heat capacity governs the cooling process of wet bodies. Finally, the physics of thermal radiation is reviewed, the mathematical derivation of the spherical harmonic approximation model is reported and its accuracy studied, the numerical implementation carefully documented and validated.

The stability of slab protection at the bottom of spillway stilling basins or plunge pools downstream of large dams came to the great practical interest when the protection under the hydraulic jump stilling basins in some hydraulic plants was seriously damaged by floods smaller than maximum design value. This thesis offers novel practical design criteria to define the concrete thickness of spillway stilling basins and plunge pools lining. In the case of spillway stilling basins, the study presents a new experimental procedure that can define the global instantaneous uplift force. Results from detailed experiments of the statistical structure of turbulence pressure fluctuations at the bottom of hydraulic jumps is reported. Here, the whole spatial correlation structure of the fluctuating pressure field is required in order to evaluate slab stability. This is computed via simultaneous acquisition of the point pressure fluctuations on a dense grid in the hydraulic jump region, requiring a severe experimental work. As an alternative, one can evaluate the pressure spatial correlation structure via auto-correlation using one point pressure acquisition and applying the Taylor hypothesis. To adopt the Taylor hypothesis, one must know the pressure propagation celerity in space that can be obtained by comparing the whole spatial pressure correlation with the pivot point pressure auto-correlation. The experiments were performed by simultaneous pressure acquisitions at the bottom of a hydraulic jump for Froude numbers of the incident flow ranging from 4.9 to 10.3. From experiments, a criterion to define the pressure celerity as a function of the incident flow velocity is presented. The results highlight a good agreement between the relevant pressure statistical parameters as measured and the ones computed using the Taylor hypothesis. The comparison between the slab thicknesses, as computed via Taylor hypothesis, with the ones retrievable in literature, as obtained by direct force measurement on instrumented slabs in laboratory conditions, highlights the accuracy of the proposed approach that presents undeniable practical advantages. While this simplified approach based on Taylor hypothesis is used to assess the pressure field acting on the slab, the pressure propagation at the lower surface of the slab is evaluated using a 3D model based on unsteady flow analysis of seepage through porous media. By this approach, it is possible to consider the effect of finite thickness foundation layers, typical in the case of earth dams, rock-fill dams and in other dam types. Slabs with unsealed joints are considered and compared to the case of sealed joints. The dynamic behavior of anchored slabs is also investigated. These results are relevant to a robust and safe design and maintenance of stilling basins downstream of large dams. In the case of plunge pools, the stability of concrete slabs or rock blocks under the impact of an impinging jet is theoretically analyzed, with reference to the mean characteristics of the flow field: pressure and velocity. In cases when the mean components are relevant respect to the fluctuating ones this analysis is exhausting. In other cases, a separate evaluation of the fluctuation effects in lining design is treated, by means of experimental evidences. The mean dynamic pressure at the bottom depends strongly on the impingement angle that assumes a relevant role in the design of floor protections. In plunge pools, that are confined upstream by the presence of the drop structure, the impingement angle is theoretically determined by mass balance and momentum conservation, resulting independent on the jet entrance angle at the plunge pool water surface. The theoretical results are compared with literature experimental evidences and numerical simulations. This highlights the capability of the proposed theoretical framework to correctly interpret the physical phenomena and to produce suitable engineering approaches.

The link between variations of the wintertime large-scale climatic patterns and thermohaline properties of the Mediterranean Sea was studied using correlation coefficients and the difference of the patterns between positive and negative modes. High NAO index is associated with the higher (lower) water temperature in the western (eastern) basin, less precipitation as well as sea level height; and increase of the net evaporation, northerlies and buoyancy flux particularly over the northern part of the basin. Some differences in the relationship between NAO index and oceanographic conditions in the Mediterranean sub-basins have also been evidenced. More specifically, the sea level in the North Ionian gyre is not influenced by NAO. The temperature in the west Alboran Sea is out of phase with the index in spite of the positive correlation in the western basin. In addition, the warmer water in the eastern coasts of the Sicily during the NAO+ is not related to the surface heat fluxes. NAO doesn’t have a weak impact on the SST of the Adriatic sea although the heat gain is more pronounced during the positive NAO. EA pattern reveals a strong influence on the Western Mediterranean and the negative phase of this index is associated by the stronger winter convection. The influence of MOI is more evidenced in the eastern basin by the lower temperature due to heat loss intensification during the positive MOI.

This study aims to shed light on the dark sides of sediment transport and meandering channel flow patterns. For this purpose, the Large Eddy Simulation methodology was used to investigate sediment transport in open-channel flow and also to predict the meandering channel flow pattern in two sine-generated channels. In the turbulent flows the transported suspended sediments is usually assumed in a passive form. In other word, the feedback effect of sediment suspension on near-wall momentum transport is neglectable. The first phase of this research was devoted to quantify the just mentioned effect in the open channel flow. A single phase approach was used to gain better description of sediment-turbulence interaction. The contribution of the small scales was parametrized by a dynamic Smagorinsky model. Results revealed that - in presence of large suspension- the buoyancy effect modifies the dynamics of flows. The second phase of the research was dedicated to the turbulent flow in meandering channels. These channels had initial deflection angles of 45 and 95 degrees. In both cases the width-to-depth ratio was equal to 15. The super-elevation of water in channel, which is impressed by secondary motion in the meandering channels, was considered in the numerical model. Analysis of numerical results was carried out on the basis of both instantaneous and time-averaged velocity fields. In addition, to provide an accurate description of the mean flow, this work explored the characterization of the cross-circulatory motion and the internal turbulence structure of flows. In particular, the sediment transport was investigated in the meandering channel to clarify the effect of bed shear stress variation induced by the channel curvature.