Water Quality modelling of the Southern North sea using TELEMAC-3D coupled with AED2

Hydrodynamic

Large Eddy Simulations of sediment entrainment induced by a lock-exchange gravity current

Large Eddy simulations of lock-exchange gravity currents propagating over a mobile reach are presented. The numerical setting allows to investigate the sediment pick up induced by the currents and to study the underlying mechanisms leading to sediment entrainment for different Grashof numbers and grain sizes. First, the velocity field and the bed shear-stress distribution are investigated, along with turbulent structures formed in the flow, before the current reaches the mobile bed. Then, during the propagation of the current above the erodible section of the bed the contour plots of the entrained material are pre- sented as well as the time evolution of the areas covered by the current and by the sediment at this section. The numerical outcomes are compared with experimental data showing a very good agreement. Overall, the study confirms that sediment pick up is prevalent at the head of the current where the strongest turbulence occurs. Further, above the mobile reach of the bed, settling process seems to be of minor importance, with the entrained material being advected downstream by the current. Additionally, the study shows that, although shear stress is the main mechanism that sets particles in motion, turbu- lent bursts as well as vertical velocity fluctuations are also necessary to counteract the falling velocity of the particles and maintain them into suspension. Finally, the analysis of the stability conditions of the current shows that, from one side, sediment concentration gives a negligible contribution to the stability of the front of the current and from the other side, the stability conditions provided by the current do not allow sediments to move into the ambient fluid

NUMERICAL SIMULATION OF SEDIMENT ENTRAINMENT BY LOCK-EXCHANGE GRAVITY CURRENTS

Gravity currents are flows driven by buoyancy differences between two contacting fluids caused by differences in temperature, salinity, or by the presence of suspended particles. Such flows can reach high velocities near the bed, especially on the area behind the front of the current. As a result, rapid morphological changes may take place in river and estuarine beds due to the passage of these flows. Essential to determine the erosion induced by the current, are the spatial and temporal distributions of the bed shear stress. However, these are troublesome to measure in laboratory or in the field. To bridge this difficulty, the eddy-solving numerical simulations may be used. This study presents here the three-dimensional numerical simulations of lock-exchange salinity currents flowing over a mobile bed. It is aimed at the characterization of the sediment entrainment capacity of the current. The large eddy simulation technique is employed for analyzing the evolution and the structure of the current. For the sediment simulation, an Euler-Euler methodology based on a single phase approach is used. The main features of the current are compared with experimental data obtained in the laboratory. Velocity fields and bed shear stress distributions for different initial current densities are analyzed and linked to entrainment scenarios. The influence of small variations in particle size of the mobile bed is also discussed

Large eddy simulations of gravity currents over an erodible bed with suspended sediment transport

Le correnti di gravit\ue0 sono flussi prevalentemente orizzontali causati da variazioni di densit\ue0 rispetto al fluido circostante. Tali flussi, possono scatenare il trasporto di notevoli volumi di sedimenti provocando cos\uec rapide deformazioni del letto dei corsi d\u2019acqua, dei bacini causando importanti problemi di natura ambientale. Nonostante la rilevanza di tale fenomeno, non \ue8 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 gravit\ue0 su fondo mobile. L\u2019idrodinamica del flusso \ue8 risolta mediante il modello LES-COAST, che utilizza il metodo Large-eddy mentre i cambiamenti morfologici causati dall\u2019erosione e dalla deposizione sono simulati mediante il modello level-set. L\u2019accoppiamento dei due strumenti modellistici \ue8 realizzato attraverso la strategia dei confini immersi. Il modello numerico accoppiato \ue8 stato sviluppato in ambiente parallelo, condizione che consente di simulare problemi a scala reale. Il modello \ue8 stato applicato alla simulazione della migrazione delle forme di fondo al fine di valutarne le capacit\ue0. I risultati ottenuti sono stati confrontati con i dati esito di un analogo studio numerico ed \ue8 emersa la capacit\ue0 del modello di catturare correttamente l\u2019idrodinamica dei flussi acqua-sedimento e i connessi mutamenti morfologici del fondo. La sospensione dei sedimenti indotta dalle correnti di gravit\ue0 \ue8 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 capacit\ue0 erosiva e determinare le condizioni del flusso che dominano la dinamica della sospensione dei sedimenti. Inoltre, \ue8 stato approfondito l\u2019effetto dei sedimenti sospesi sulla condizione di stabilit\ue0 della corrente di gravit\ue0 nonch\ue9 l\u2019influenza di tale condizione sulla distribuzione dei sedimenti sospesi. Infine, il modello accoppiato idro-morfodinamico \ue8 utilizzato per simulare le forme di fondo generate da tale tipologia di correnti. \uc8 importante notare che la propagazione di correnti di gravit\ue0 con elevato potere erosivo su fondi mobili pu\uf2 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 gravit\ue0 che interessano letti deformabili. L\u2019evoluzione istantanea dei pattern di erosione e deposizione sono quindi presentati, contribuendo cos\uec a una conoscenza pi\uf9 dettagliata dell\u2019interazione flusso- sedimenti e flusso-fondo in correnti di gravit\ue0.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