Axisymmetric three-dimensional gravity currents generated by lock exchange

Unconfined three-dimensional gravity currents generated by lock exchange using a small dividing gate in a sufficiently large tank are investigated by means of large eddy simulations under the Boussinesq approximation, with Grashof numbers varying over five orders of magnitudes. The study shows that, after an initial transient, the flow can be separated into an axisymmetric expansion and a globally translating motion. In particular, the circular frontline spreads like a constant-flow-rate, axially symmetric gravity current about a virtual source translating along the symmetry axis. The flow is characterised by the presence of lobe and cleft instabilities and hydrodynamic shocks. Depending on the Grashof number, the shocks can either be isolated or produced continuously. In the latter case a typical ring structure is visible in the density and velocity fields. The analysis of the frontal spreading of the axisymmetric part of the current indicates the presence of three regimes, namely, a slumping phase, an inertial-buoyancy equilibrium regime and a viscous-buoyancy equilibrium regime. The viscous-buoyancy phase is in good agreement with the model of Huppert (J. Fluid Mech., vol. 121, 1982, pp. 43-58), while the inertial phase is consistent with the experiments of Britter (Atmos. Environ., vol. 13, 1979, pp. 1241-1247), conducted for purely axially symmetric, constant inflow, gravity currents. The adoption of the slumping model of Huppert & Simpson (J. Fluid Mech., vol. 99 (04), 1980, pp. 785-799), which is here extended to the case of constant-flow-rate cylindrical currents, allows reconciling of the different theories about the initial radial spreading in the context of different asymptotic regimes. As expected, the slumping phase is governed by the Froude number at the lock's gate, whereas the transition to the viscous phase depends on both the Froude number at the gate and the Grashof number. The identification of the inertial-buoyancy regime in the presence of hydrodynamic shocks for this class of flows is important, due to the lack of analytical solutions for the similarity problem in the framework of shallow water theory. This fact has considerably slowed the research on variable-flow-rate axisymmetric gravity currents, as opposed to the rapid development of the knowledge about cylindrical constant-volume and planar gravity currents, despite their own environmental relevance

Density currents flowing up a slope

The dynamics of lock-release density currents propagating up a sloping bottom are investigated by Large Eddy Simulations (LES). The flow dynamics are deduced by the analysis of the density and velocity fields obtained by LES.The density current, during its propagation, is observed to develop different flowregimes: a slumping phase followed by a self-similar phase. A decrease in the velocities of the current is observed with the increase of the angle between the bottom boundary and the horizontal direction, Θ.A smoother behaviour of the current profile is also visible for high values of Θ. The presence of a backward flow close to the bottom of the domain is detected and it is found to depend on the inclination of the bottom Θ.An accumulation of dense fluid in the lock region of the tank caused by the reverse flow is also observed. Entrainment processes occurring between the ambient fluid and the dense current are observed and investigated. In particular, during the propagation of the current, light ambient fluid is entrained by the dense current, which increases in volume. It is found that the entrainment is affected by the inclination of the bottom and, in particular, a decrease of the entrainment with the increase of the steepness of the bottom is observed

Sviluppo del sistema costiero di previsione dello stato del mare

La simulazione della generazione e propagazione del moto ondoso associato al vento è un campo di applicazione che, nell’ultimo decennio, ha avuto ampio sviluppo. Nonostante importanti aspetti delle onde ancora poco noti, il forecast dello stato del mare, grazie allo sviluppo di modelli numerici in grado di simulare accuratamente i complessi processi che regolano l’evoluzione dello spettro energetico delle onde, è un prodotto ormai consolidato per tutti i centri meteorologici. Per capire e prevedere lo stato del mare è stato realizzato un sistema costiero in grado di descrivere gli effetti della propagazione ondosa in sei aree costiere italiane, partendo dalle previsioni sull’intero Mar Mediterraneo

Entrainment and mixing in unsteady gravity currents

Entrainment and mixing in lock-exchange gravity currents are investigated by large eddy simulations. Nine cases are analysed, varying the initial excess density driving the motion and the aspect ratio r of the initial water depth to the lock length. Laboratory experiments are also performed and a fair agreement between numerical simulations and measurements is found. Mixing between the gravity current and the ambient fluid, in both the slumping and self-similar phases, is studied for a range of entrainment parameters, gravity current fractional area and using an energy budget method. The entrainment is found to increase as r decreases. The occurrence of irreversible mixing is detected during the entire development of the flow, i.e. both in the slumping and self-similar phases. A higher amount of mixing is observed as r decreases and the initial excess density increases

HF radar for wind waves measurements in the Malta-Sicily Channel

The CALYPSO HF radar network is a permanent and fully operational observing system currently composed of four CODAR SeaSonde stations. The system is providing real-time hourly maps of sea surface currents and waves data in the Malta-Sicily Channel. The present work aims to compare significant wave height measurements by HF Radar to wave data from numerical models and satellite altimeter. This is the first time that this set of wave data are analysed since the four HF radars were installed between 2012 and 2015. Results suggest that CODAR HF Radar wave data are a reliable source of wave information even in the case of extreme events, providing an avenue to improve and complete the offer of services deriving from the CALYPSO system. Comparisons of HF radar data with both numerical sea wave model and satellite altimeter data confirm agreement, in particular for radar measurements in the annular sectors within the central range-cells which are also characterized by a more reliable and homogeneous temporal behaviour

Atlante delle onde nei mari italiani

Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 , Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Mapping seabed habitats over large areas: prospects and limits

Since its inception, in 2009, EMODnet Seabed Habitats has brought together a European consortium of specialists in benthic ecology and seabed habitat mapping to develop a transnational broad-scale seabed habitat map, named EUSeaMap. EUSeaMap is the only pan-European cartographic product that provides a standardised trans-boundary overview of the spatial distribution of seabed habitats across Europe. As such, it has been extensively used in various applications such as Marine Protected Area evaluation or cumulative impact of stressors on habitats, and it is likely to be used again in the future in various marine ecosystem assessments. It is therefore important to continue to update it regularly when significant improvements to the data products that constitute its basis, i.e. the seabed substrate, bathymetry or environmental variables, are published. In addition to EUSeaMap, it would be desirable to provide stakeholders with products on the spatial distribution of targeted habitats/biotopes such as those of conservation interest (e.g. kelp forest, seagrass meadows, coral reefs). Some techniques, hereafter referred to as "SDMs", are acknowledged to be effective in mapping these habitats. We argue that a program that would use these techniques to map these key habitats/biotopes in European waters would be invaluable, but such a program can only be achieved if there is a significant improvement in the spatial resolution of environmental variables. An important message of this report is therefore that the EU should consider funding a project that would develop spatially explicit high-resolution (at least 500 m) data products on key variables (light availability, hydrodynamics, wave exposure, temperature, oxygenation, chlorophyll-a, phosphate, nitrate, etc.) that would spatially cover all European waters

EUSeaMap. A European broad-scale seabed habitat map

In order to most benefit from the potential offered by the European marine basins in terms of growth and employment (Blue Growth), and to protect the marine environment, we need to know more about the seafloor. European Directives, such as the MSFD, but also the Horizon 2020 roadmap explicitly called for a multi-resolution full coverage of all European seas including bathymetry, geology and habitats. The present work, following on a suite of past initiatives, has made a big step forward in this direction. It has first boosted the collation of existing maps from surveys by setting up a framework and a procedure to encourage people to submit their maps and data. This resulted in a more attractive EMODnet seabed habitat portal and a snowball effect with more and more people willing to join. However, collation will eventually come to an end and as new creations of seabed habitat maps are so complex and time-consuming, a cost-efficient way to meet the need for a full-coverage habitat map was found to be low-resolution maps and models to predict seafloor habitat types. The broad-scale map referred to as EUSeaMap has been created by this project and after the first two phases it now covers all European basins from the Barents Sea to Macaronesia and to the Black Sea. By harmonising mapping procedures - based on the EUNIS classification - and fostering a common understanding among seabed mappers in Europe, EUSeaMap provides today the community with a comprehensive, free and ready-to-use map that can find applications at regional scale for management and conservation issues. Tables and maps for all basins can be found in section 3 “Results and disciussions”. The project has played a key role in giving feedback to other EMODnet communities dealing with bathymetry, geology and biology, all essential data sources for the broad-scale map. It has also improved the understanding of the EUNIS habitat classification - with a focus on the Adriatic and the Black Sea - by better specifying transitions between classes based on benthic ground-truth data. It has fostered the development of oceanographic variables such as light, waves and currents that have a strong bearing on habitats. Finally it has also been instrumental in developing map confidence assessment methods that account for the broad spatial variation in data sources quality and for uncertain boundaries between habitat classes. The EUSeaMap methods are repeatable and ensure that the predictive maps can continue to be improved in the future, as a result either of EUNIS enhancements or increase in resolution. From today’s 250m resolution it is likely that new deliveries of enhanced source layers due to steady progress in oceanography and geophysics will enable constant refinement of the maps over time