Numerical simulations of 2-D steady and unsteady breaking waves

In this work we analyze by means of numerical simulations the features of breaking of two dimensional free surface waves induced by a body or a sloping bottom. The sample cases selected for the simulations characterize different aspects of wave breaking, thus they are supposed to represent rather widely a problem of large interest for ship hydrodynamics and ocean engineering applications. The simulations considered are: wave breaking induced by a fully submerged hydrofoil towed in calm water at constant speed; shallow water waves breaking on a sloping beach in spilling and plunging mode; regular intermediate depth waves breaking gently over a weakly submerged horizontal circular cylinder at a low Keulegan\u2013Carpenter number. Each simulated case is supported by detailed comparisons with experi- mental data in time and frequency domain. The results presented have been obtained adopting a standard RANS approach. They show a generally good reproduction of the wave breaking characteristics even though it is rather clear that there is a case dependent potential loss of accuracy in the presence of pronounced foamy flow

An investigation of particle dispersion in a tidally driven turbulent flow

2005/2006The Gulf of Trieste, subset of our investigation, is a shallow water inlet with a mean depth of 17 m (maximum 25 m) and an area of about 20 km × 25 km. It is located in the north area of the Adriatic Sea. The dynamics of the Gulf is characterized by seasonal variability of temperature and density, and the mixing and dispersion processes are strongly dependent to this context. This variability is due to the combined effects of many factors such as the presence of a strong wind (Bora) whose action is directly related to the water column instability, the input of fresh water from the river Isonzo and the water exchange due to Adriatic sea currents. For all this phenomena an extended quantity of measured data have been collected from more than one hundred years: the interest in the knowledge of these processes is due to the deep impact in the local economy (from fishing to tourism). This allows a correct formulation of the forcing acting in the dispersion problem regarding the Gulf

On the nonlinear transformation of breaking and non-breaking waves induced by a weakly submerged shelf

The interaction of regular quasi-monochromatic waves with a weakly submerged rectangular shelf is studied by means of CFD simulations. The fundamental incident wave frequency is kept constant for the full set of simulated cases, while the incident wave amplitude is made increase progressively, so that the interaction with the shelf is dominated by almost inviscid non-linear flow for the smallest and by breaking for the highest incident waves. A parameter identification (PI) procedure is used to adapt a reduced model to the highly resolved time-space matrix of wave elevations obtained from the numerical simulations, on the weather and lee side respectively. In particular the wave number and the frequency of the component waves in the reduced model are left uncoupled, thus computed by the PI independently. The comparison of simulated data with experiments generally shows a very good agreement. Free/locked, incident/reflected, first/higher order wave components are quantified accurately by the PI and the energy transfer to super-harmonics is clearly evidenced. Moreover the results of the PI show clearly a very large increase in the phase speed of the higher order free waves on the lee side of the shelf, with increasing deviation from the linear behavior with increasing incident wave amplitude

Numerical Study of Unsteady Breaking Waves Induced by a Submerged Hydrofoil at Steady Forward Speed

In this work we analyse the features of the unsteady breaking of two dimensional free surface waves induced by a submerged hydrofoil at steady forward speed. The study is conducted by means of numerical simulations with OpenFOAM: interFoam and swenseFoam solvers being used as a computational framework for the free-surface treatment with the Volume Of Fluid (VOF) and Level Set (LS) techniques respectively. Due to the strongly unsteady behaviour of the flow, the interest is here focused on the spectral content of the free surface elevation. The results from two interface-capturing methods, VOF and LS are discussed and compared with experimental data from the literature. The influence of the turbulence models on the free surface shape and spectral content is also discussed. The final goal of the entire research work is the simulation of complex two-phase flows for robust and fast engineering applications in the ship hydrodynamics and wave loads fields, including wave breaking phenomena

A NUMERICAL STUDY ON THE VISCOUS EFFECTS OF WAVES TRAVELLING PAST A WEAKLY SUBMERGED CYLINDER

The interaction of waves with a weakly submerged horizontal circular cylinder is studied numerically, solving 2-D Navier-Stokes equations with reference to former experimental sessions in a wave flume. A selection of aspects of the viscous wave-body interaction is analyzed, in particular the free surface elevation perturbed by the obstacle, the pressure at the body surface, the inertia coefficient of the Morison equation and finally the generation, persistence and merging of vorticity structures detached from the cylinder

On the wavy flow past a weakly submerged horizontal circular cylinder at low Keulegan\u2013Carpenter numbers

The interaction of regular waves with a weakly submerged horizontal circular cylinder at Keulegan\u2013Carpenter KC numbers up to 2.5 is studied by means of numerical simulations, solving the two-phase flow Navier\u2013Stokes equations with the Finite Volume method and the Volume of Fluid interface capturing method. Experimental data from laboratory tests conducted in the past by one of the authors are used as reference data. The study is focused on the surface wave transformation, on the flow field in the near region and on the loads on the cylinder. The interest is driven on the spectral content of the transmitted waves at higher order frequencies and on the non-linear features of the wave reflected by the cylinder. The flow field in the near-body region is analyzed in terms of vortex onset, development and detachment, and correlating pressure pulses with the vorticity field at the cylinder surface. Inviscid flow simulations are carried out too, to evince the origin of higher order terms either from the free surface or from the viscous terms in the momentum equations. The steady streaming around the cylinder is analyzed, including its variability along the cylinder surface and its relationship with the vortex cores generated at the body surface

Numerical and experimental investigation on the wave-wave interaction in breaking and non-breaking focusing waves

The modelling of large individual waves for the computation of loads on ships and offshore structures in extreme weather conditions is still a challenging problem. Since the early 50s the predictions of loads on fixed offshore structures and motions of compliant or sailing structures due to surface waves are commonly made by computations on the basis of the statistical/spectral description of the sea elevation and of a linearized response model. Quadratic Transfer Functions or fully non-linear methods are used only in specific cases. The linear approach is recognized to work reasonably well for the so-called operational conditions, assuming that hydrodynamic and dynamic nonlinear effects can be neglected. On the other hand, it is also recognized that the modelling of large amplitude motions and the modelling of waves in the so-called survival conditions, i.e. extreme wave conditions, cannot recast a linear approach. In these conditions the wave-wave interaction plays a fundamental role (energy transfer, down-shift, etc) in the actual deterministic or spectral representation of the wave/flow field and thus in the related loads on the structure. In the present paper the nonlinear aspects related to the behavior of steep focusing breaking and non- breaking waves are analyzed by means of numerical simulations and new experiments. The experiments are carried out at the wave flume of the Laboratory of Maritime Engineering (LABIMA) of the Dept. of Civil and Environmental Engineering of the University of Florence. The computations are carried out at Hydrodynamic and MetOcean Laboratory (HyMOLab) of the Dept. of Engineering and Architecture of the University of Trieste. The paper focuses the attention on the comparison between the results obtained with a state-of-art viscous flow simulation and laboratory experiments, with particular emphasis on the spectral energy exchange between component waves of a non-breaking and breaking focusing wave train. This study is carried out as part of the research project \u201cOpenViewSHIP Development of an integrated computational ecosystem for the hydrodynamic design of the hull-propeller system\u201d, co-financed by Friuli Venezia Giulia Region in the field of industrial application of open-source CFD and High Performance Computing

Numerical study of a cone mill for emulsion preparation: Hydrodynamics and population balance modelling

The work concerns numerical simulations of a cone mill used for emulsion preparation. Hydrodynamics, power consumption and population balance are investigated for various operating conditions at high phase volume emulsions and for different rheologies. Cone mills are usually simplified as a simple gap between rotor and stator but by increasing the complexity of the geometry till it represents the commercial device identifies a wealth of additional features such as recirculation zones above (which enhance breakage) and below (which allow for coalescence) the rotor-stator gap. Two separate sets of population balance modelling constants are required to capture all the experiment results – even with the most complex geometries. Some suggestions are made for improvements and further studies will consider other rotor-stator devices

MWM: A 35 years wind&wave high resolution hindcast dataset and an operational forecast service for the mediterranean sea

The use of reliable wind and wave data for the planning of operational activities at sea is considered of primary importance. This regards the coastal engineering, the oil&gas and recoverable energy fields, the civil protection, the design of offshore structures and ships, the planning of operations at sea and so on. DHI and HyMOLab (Hydrodynamics and Met-Ocean Laboratory of the Dept. of Engineering and Architecture of the University of Trieste) have undertaken a joint research project with the aim to develop a state-of-art wind-wave hindcast dataset for the Mediterranean Sea. The dataset consists of 35 years of hourly data for the period 1979-2013, obtained from a last-generation model chain. The meteorological model used is WRF-ARW, one of the most widely used state-of-the-art open-source non-hydrostatic model. The CFSR d093.0 hourly dataset with a spatial resolution of 0.5\ub0 provides the boundary and initial conditions. MIKE21 is used as the wave model with resolution ranging from 0.1\ub0 to 0.03\ub0 approximately. The use of a local area meteorological model guarantees higher levels of resolution and accuracy in an area such as the Mediterranean Sea where the complex orography and coastline induce short-time/smallspace weather scales. The atmospheric and wave models performance is checked against seven satellite datasets, missions Envisat, ERS-2, Geosat FO, Jason-1, Jason-2, Topex-Poseidon and CryoSat, by means of a procedure based on the moving window technique. Wave data close to coast are compared with available data from more than 20 buoys. The validation performed at the time of the preparation of this manuscript involves ten years of data (2002-2011) and is based on the significant wave height parameter. The results are summarized in probability scatter plots, which show a very good accuracy of the model for both the comparisons with satellite and fixed buoys. Taking advantage of the model set-up performed for hindcast purposes and verified with measured data, an operational wind-wave forecast service has been developed and put to use. The forecast service uses the GFS dataset to provide the boundary and initial conditions