Archivio della ricerca di Triestehttps://arts.units.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Wed, 12 May 2021 23:01:40 GMT2021-05-12T23:01:40Z10981- An investigation of strong backflow events at the interface of air–water systems using large-eddy simulationhttp://hdl.handle.net/11368/2928847Titolo: An investigation of strong backflow events at the interface of air–water systems using large-eddy simulation
Abstract: A large-eddy simulation of a counter-current gas–liquid flow is
performed. At the flat interface where the different fluids meet,
continuity of momentum and momentum fluxes are enforced following
the work of Lombardi et al. [Direct numerical simulation of
near-interface turbulence in coupled gas-liquid flow. Phys Fluids.
1996;8(6):1643–1665]. The increase in vertical vorticity fluctuations
near the interface increases mixing, reducing the thickness of the
inner region of the boundary layer. Such increase reduces shear while
allowing for more frequent backflow motions in the inner region,
being this phenomenon stronger on water. Due to the higher inertia
of water these backflow motions are ultimately responsible for
the streaky structure of shear stresses seen along the interface. The
present study shows that such bimodality in the streamwise velocities
is also seen in the angle distribution of vorticity relative to the
interface, where such angles are linked to the presence of interfaceconnected
and quasi-streamwise vortex cores. Finally, it is shown
that backflow events on the interface shear stresses correlate with
coupled ‘strong’ ejections in the near interface region despite the disparagingly
different near-interface streamwise velocity distributions
on the near interface boundary layers.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29288472018-01-01T00:00:00Z
- On the Stability of Antisymmetric Motions of a Ship Equipped with Passive Antirolling Tankshttp://hdl.handle.net/11368/2548939Titolo: On the Stability of Antisymmetric Motions of a Ship Equipped with Passive Antirolling Tanks
Abstract: Until now, the effectiveness of passive antirolling devices, such as tanks, has been checked against the build-up of a large amplitude synchronized rolling motion in monochromatic or narrow band sea. Recently, we have shown that this effectiveness extends also in a general stochastic and short crested sea from any direction. Through the use of a perturbation method, we have also proved that it extends to the so called parametric rolling, i.e. to subharmonic rolling that can be excited as a result of the coupling of heave and roll in longitudinal sea from the stern. The effect of the tanks is to increase the threshold for the onset of subharmonic rolling at least in the first region of instability that was investigated in a second order approximate analysis. Numerical simulation confirms this trend. Recent approaches to stability assessment seem to pay an increasing importance to the simultaneous occurrence of instability in the antisymmetric motions sway, yaw and roll, and synchronism with the external excitation. The coupling among these motions could explain phenomena such as capsizing, loss of control in waves and, probably, broaching-to. In this paper, the stability boundaries as regards the antisymmetric motions are computed for a ship considering the eigenvalues of the system of equations describing the coupled sway, yaw, roll, tanks motions. The results indicate that passive tanks properly adjusted to avoid large amplitude rolling in a beam sea, can play an important role also in reducing the range of instability of these motions.
Mon, 01 Jan 1990 00:00:00 GMThttp://hdl.handle.net/11368/25489391990-01-01T00:00:00Z
- Numerical simulation of hot smoke plumes from funnelshttp://hdl.handle.net/11368/2936907Titolo: Numerical simulation of hot smoke plumes from funnels
Abstract: The flow around ship over-structures is characterized by both separation phenomena with recirculation regions at high Reynolds number, and the fast ejection of hot smokes from chimneys. These two features are of the utmost importance for two linked goals in the ship design, namely the aerodynamic drag reduction and the preservations of passengers comfort. In particular, the latter depends on different flow aspects, among the others: the turbulent fluctuations intensity, the smoke temperature and pollutant dispersion and the flow induced noise. In this work, we present an open-source solver developed in OpenFOAMR○ that is especially suited for the analysis of ship over-structures. It adopts Large-eddy simulation approach and implements an in-house version of the dynamic Lagrangian Sub-grid Scale LES model along with an equilibrium stress wall function in order to deal with high Reynolds number simulations. Furthermore, a synthetic turbulent inflow generation has been developed to provide a more realistic condition. The hot smoke plumes are reproduced considering the buoyancy effect through the Boussinesq approximations. The model has been validated on different benchmark cases and an analysis of a real ship over-structure is presented.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29369072018-01-01T00:00:00Z
- Axisymmetric three-dimensional gravity currents generated by lock exchangehttp://hdl.handle.net/11368/2928841Titolo: Axisymmetric three-dimensional gravity currents generated by lock exchange
Abstract: 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.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29288412018-01-01T00:00:00Z
- Large eddy simulation of turbulent buoyant flow in a confined cavity with conjugate heat transferhttp://hdl.handle.net/11368/2870026Titolo: Large eddy simulation of turbulent buoyant flow in a confined cavity with conjugate heat transfer
Abstract: Turbulent natural convection in enclosure is a paradigmatic case for wide class of
processes of great interest for industrial and environmental problems.The solid-fluid
thermal interaction, the anisotropy of the turbulence intensity in the flow field along
with the transient nature of heat transfer processes, pose challenges regarding the
numerical modeling. The case of a square cavity with differently heated vertical
walls and two horizontal conductive plates is studied at Ra = 1.58 × 109. The study
is carried out numerically, using large-eddy simulation together with a dynamic
Lagrangian turbulence model and a conjugate heat transfer method to take into
account heat transfer at the solid surfaces. First, validation is carried out against the
literature experimental and numerical data. The results of validation tests evidence the
limitations of using the adiabatic conditions as a model for reproducing an insulator.
In fact, the adiabatic condition represents the asymptotic behavior which is often
difficult to reach in real conditions. Successively, the model is used to investigate the
effect on the flow field of different materials composing the horizontal walls. Initial
conditions representative of physical experiment are used. In order to reduce the
computational time required for a simulation with insulating materials at the walls,
a four-step temperature advancement strategy is proposed, based on the artificial
reduction-first and recover-later of the specific heat coefficient Cp of the materials
at different stages of the simulation. The conductivity of the solid media is found
to influence the flow configuration since heat transfer at the solid walls substantially
modifies the turbulent field and makes the flow field less homogeneous along the horizontal
direction.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11368/28700262015-01-01T00:00:00Z
- Dynamic Effects of Liquids on Board on the Stability of a Fishing Vesselhttp://hdl.handle.net/11368/2549422Titolo: Dynamic Effects of Liquids on Board on the Stability of a Fishing Vessel
Abstract: In this paper the problem of the rolling motion of ships with liquids on board is investigated. Different approaches to the phenomenon are described, highlighting their own advantages and limits. In the presented results, the fully hydrodynamic approach, which deals with the actual behaviour of the shipped liquid, is employed. A good agreement is obtained by comparison between numerical results and experimental data. Some comments on the dynamic coupling between ship and liquid motion are reported.
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/11368/25494221993-01-01T00:00:00Z
- Large eddy simulation (LES) of suspended sediment transport (SST) at a laboratory scalehttp://hdl.handle.net/11368/2895821Titolo: Large eddy simulation (LES) of suspended sediment transport (SST) at a laboratory scale
Abstract: Wall resolving large eddy simulation (LES) with the dynamic Smagorinsky model (DSM) is implemented under the single phase Euler-Euler framework to investigate the effects of suspending noncohesive sediment on the flow characteristics in a turbulent open channel flow. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11368/28958212016-01-01T00:00:00Z
- Entrainment and mixing in unsteady gravity currentshttp://hdl.handle.net/11368/2895815Titolo: Entrainment and mixing in unsteady gravity currents
Abstract: 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.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11368/28958152016-01-01T00:00:00Z
- Numerical simulation of conjugate heat transfer and surface radiative heat transfer using the P1 thermal radiation model: Parametric study in benchmark caseshttp://hdl.handle.net/11368/2895806Titolo: Numerical simulation of conjugate heat transfer and surface radiative heat transfer using the P1 thermal radiation model: Parametric study in benchmark cases
Abstract: A parametric investigation of radiative heat transfer is carried out, including the effects of conjugate heat transfer between fluid and solid media. The thermal radiation is simulated using the P1-model. The numerical model and the thermal coupling strategy, suitable for a transient solver, is described. Such numerical coupling requires that the radiative equation is solved several times at each iteration; hence, the computational cost of the radiative model is a crucial issue. The P1-model is adopted because of its particularly fast computation. First, a collection of benchmark cases is presented and used to carefully validate the radiation model against literature results and to analyse the model prediction limits. Despite the simplicity of the model, it satisfactorily reproduces the thermal radiation effects. Some lack of accuracy is identified in particular cases. Second, a number of benchmark cases are described and adopted to investigate fluid–solid thermal interaction in the presence of radiation. Three cases are designed, to couple radiation with: pure conduction, conduction and forced convection, conduction and natural convection. In all the cases, the surface radiative heat transfer strongly influences the system thermodynamics, leading to a significant increase of the fluid–solid interface temperature. The main non-dimensional numbers, related to the mutual influence of the different heat transfer modes, are introduced and employed in the analyses. A new conduction-radiation parameter is derived in order to study the conductive boundary layer in absence of convective heat transfer.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11368/28958062017-01-01T00:00:00Z
- Large-eddy simulation of thin film evaporation and condensation from a hot plate in enclosure: First order statisticshttp://hdl.handle.net/11368/2895810Titolo: Large-eddy simulation of thin film evaporation and condensation from a hot plate in enclosure: First order statistics
Abstract: Numerical investigation of water evaporation and condensation in buoyancy driven flow, along with the thermal coupling between fluid and surrounding solids, is interesting for many industrial applications. The physical complexity of the evaporation and condensation processes, the mutual thermal influence of water change of phase and solid-fluid heat transfer, the anisotropy of turbulence quantities are challenging problems from numerical and theoretical side. The archetypal case of a vertical hot plate inside a cold square enclosure, filled with humid air, is studied. The solid surfaces are wetted by a thin water film. The plate cooling process due to film evaporation is analysed. Numerical simulation adopts the large-eddy methodology along with the dynamic Lagrangian sub-grid scale model. The conjugate heat transfer technique accounts for the solid-fluid thermal coupling, while the water phase is modelled under the thin film assumption. First, a preliminary case with isothermal solid boundaries and fixed film thickness is reproduced at Ra=5×108. The absence of surface heat transfer leads to analogous distribution of temperature and humidity. The cavity is characterised by strong stratification that confines the motion in the upper part. The same setting is used to simulate the case of dry air. It is found that the presence of vapour increases the velocity by a maximum of 20%. Also, the heat transfer generated by the water change of phase, in case of humid air, overcomes the other heat transfer modes. Successively, conjugate heat transfer and water film model are activated, and three cases are studied changing the plate material. The specific heat ρCp of materials is the parameter controlling the plate cooling process, that is mainly due to evaporation and the evolution of the thermodynamic field within the enclosure. An analysis of the dew-point temperature suggests that recondensation onto the plate surface cannot occur, even for materials that are rapidly cooled.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11368/28958102016-01-01T00:00:00Z