1,720,969 research outputs found
Hydrodynamic noise from a propeller in open sea condition
No full textIn the present work a hybrid methodology is used to evaluate the hydrodynamic noise generated by a marine propeller in open sea condition. The hydrodynamic field is computed using Large Eddy simulation under the assumption of incompressible flow field; the acoustic field is reconstructed by applying the advec-tive Ffowcs Williams and Hawkings equation. For the hydrodynamics, we use the dynamic Lagrangian model for the closure of the subgrid-scale stresses and a wall-layer model to skip the resolution of the viscous sub-layer. We consider a propeller well studied in literature for a single value of the advance ratio. A grid of about 6x106 cells is used for reproducing accurately both the stresses over the propeller and the wake, the latter responsible of quadrupole noise. The equations are solved in a fixed-to-the-body frame of reference. The different noise generation mechanisms are investigated separately. Thickness and loading terms related to the propeller shape and velocity, provide significant pressure disturbance in the near field. The quadrupole noise component is obtained by integrating over an external permeable surface. Its contribution is investigated in relation to the presence of vortex persisting in the wak
The effect of Coriolis force on oil slick transport and spreading at sea
No full textIn the present paper we propose an improved Nihoul's model which accounts for the main forces acting on an oil slick and ruling its spreading and transport in the first tens of hours after the spill. In the original model, Nihoul considered the Coriolis force to be negligible; we re-formulate Nihoul's analysis including the Coriolis force in the mathematical model. Coriolis force is found to act on both the oil slick transport and spreading processes in a non-trivial way and depends on several parameters. We numerically assess the importance of the Coriolis force on oil slick transport at different latitudes and spill conditions. Simulations carried out considering different model conditions show that the use of empirical parametrizations of the drift angle, when applied together with hydrodynamic models, leads to incorrect prediction of the slick trajectory. Finally we propose an empirical, simplified formulation which describes the trajectory of the oil slick's centre of mass considering the Coriolis deviation
Numerical model for thin liquid film with evaporation and condensation on solid surfaces in systems with conjugated heat transfer
No full textCondensation and evaporation processes from wetted surfaces are of utmost importance in many technological or industrial applications.
In many devices such as home-appliances and air conditioning systems just to name a few, condensation and evaporation processes greatly impact their performance and energy efficiency; The physics of these processes is quite complex, involving conjugate heat transfer among solid-liquid film-gaseous phase together with the change of phase associated with evaporation and/or condensation. In spite of their own importance in technological applications, most physical aspects of this class of multiphase flows still need to be addressed.
So far, many studies have been performed focusing on individual issues of full process, and, to the authors knowledge, their mutual influences was not addressed as yet. The aim of the present study is to supplement this field with a reliable numerical model for evaporation and condensation of thin liquid films in buoyant, conjugated heat transfer systems with vapor transport. The model is presented, validated on a simple case and successively applied to a literature relevant case.
Validation is carried out considering a simple, insulated, 2-D geometry consisting of a fluid box connected to a solid. This allows to validate the model in a tightly controlled environment.
Successively we reproduce the work of~\nocite{laaroussi09} N.~Laaroussi, G.~Lauriat, G.~Desrayaud, Effects of variable density for film
evaporation on laminar mixed convection in vertical channel, Int. J. Heat and Mass Transfer 52 (2009) 151--164, and we highlight the importance of considering the thermal characteristics of the solid part for the accurate reproduction of the complex physics associated to evaporation/condensation processes
High resolution oil spill model for harbour and coastal areas
No full textA novel, state of-art, numerical model accounting for the main physical processes governing oil dispersion at sea is here formulated and discussed. The underground hydrodynamics is resolved using LES-COAST, a high definition numerical model suited for coastal or harbour areas. Oil dispersion is modelled considering the main physical features of the process. After spilling, the oil may form the so-called tars or it may spread over the sea surface as thin film, depending on the oil pour point with respect to the ambient temperature. We adopt two different approaches for the two different conditions respectively. In the former, oil tars are modelled as Lagrangian particles of characteristic diameter and density. In the latter the Nihoul's model (Nihoul, 1984) is considered, which accounts for the main forces acting on the oil film, namely gravity, sea current and wind stresses. In practical short-term studies (simulation of oil dispersion over few hours) forces as surface tension and the inertia can be neglected. Also, the relevant short-term weathering processes (mainly emulsification and evaporation) occurring in coastal and harbour regions, are taken into account through established literature models. We validate the model on standard test cases and we apply it to a real case scenario in the Barcelona Bay
Evaporation/condensation around a wetted cylinder confined between two parallel walls
No full textMixed convection flows in presence of condensation and evaporation phenomena have crucial role in several natural and technological processes, in particular in those involving drying and wetting of solid surfaces. Although such flows are quite common in engineering applications, their knowledge is far from being complete.
The complex physics involved can be briefly sketched as follows: wetted solid bodies exchange heat with the liquid laying on their surfaces; the liquid phase exchange mass and heat with the surrounding gas through change of phase; the consequent diffusion of temperature and vapor concentration results in density variations that greatly impact the gaseous flow introducing buoyancy forces. In the present study we will focus on the liquid-gas interaction and we study how evaporation and condensation over solid wetted surfaces take place in an archetypal problem.
To the best of our knowledge, in literature this problem has been mostly faced, through numerical modelling (see for example [1], [2]), since uncertainties in experimental approaches may come from the difficulties to control the parameters ruling the process. The mathematical model usually adopted consists of the incompressible Navier-
Stokes equations where the buoyancy forces are taken into account by means of the Boussinesq approximation; at the wetted wall the thin liquid film is modelled as semi-permeable boundary condition which prescribes a Dirichlet condition for temperature and consequently for the saturated vapor concentration. This condition permits
evaporation/condensation of the liquid/vapor phase through the evaluation of the Stefan flow at the liquid-gas interface. The set of equations and the boundary conditions are implemented within an unsteady incompressible Navier-Stokes solver developed using the openFOAM library. The new solver has been validated against literature
numerical results of [3] in the case of laminar plane channel flow. Recent literature studies have been focused on the study of the flow moving within a straight channel with different conditions at wetted walls (among the others see [1]). In the present study we consider a more complex
situation. The flow develops in a straight channel of length L and width H. A cylinder of diameter D = 0.2H is placed inside the channel at a distance equal to L/10 from the inflow section. In order to enlighten the different interaction mechanism between the walls and the bluff body two different distances of the cylinder from the bottom wall are investigated, respectively d = H/2 and d = H/4. At the inlet uniform temperature T0 and vapor
concentration C0 are prescribed along with a parabolic velocity profile with a mean velocity U0 such that the Reynolds number Re = U0 H/ν is 500, being ν the kinematic viscosity. At the outlet a zero gradient condition is imposed together with a sponge region where fluid viscosity, thermal and concentration diffusivity are artificially
increased according to an exponential law. Moreover in the sponge region the concentration is treated as a passive scalar. The liquid film interface on the wetted cylinder has temperature Tc and concentration Cc such to allow liquid evaporation. The channel walls can be either adiabatic and impermeable or wetted with fixed temperature
Tw and concentration Cw , the latter allowing vapor condensation.
Preliminary results are here very briefly discussed and more comprehensive results will be shown at the work-shop. For the case with adiabatic and impermeable walls, the different cylinder positions yield to small differences in the Stefan flow in the overall evaporation process. The latter appears substantially steady and the evaporation
fluxes around the surface are quite homogeneous with a defect in the rear of the body. The vapor flows in the upper part of the domain and two regions of different concentration level are clearly visible. The velocity field develops in
unsteady vortices without a clear dominant size. The wetted walls condition, on the other hand, seems to greatly impact the process and especially the heat and mass transfer one. The buoyancy force and hence the characteristic velocity is increased. Downstream the body the velocity field develops in well defined vortical structures of the size
of the channel width. Near the cylinder the flow is greatly unsteady and poorly organized making the vapor well mixed. In this condition evaporation process is non homogeneous and unsteady. On the average the whole surface of the cylinder permits higher Stefan flow rates.
This study encourages further developments in order to include the liquid thin film dynamic into the model
Turbulent structures of buoyant jet in cross-flow studied through large-eddy simulation
No full textIn the present paper we study buoyant (plume) and non-buoyant (jet) fluid injection in a neutrally stratified uniform cross-flow. Both cases are of practical importance in environmental fluid mechanics. The study is carried out numerically, using highly resolved large-eddy simulation in conjunction with the Lagrangian dynamic sub-grid scale model for both momentum and scalar transport equations. The velocity ratio is κ=8 . In the plume case, the Froude number is F=10 , such to allow the use of the Boussinesq approximation. The simulations are successfully validated against experimental data and well established semi-empirical relations. The study shows the existence of three different regions as regards the plume evolution, each of them characterised by different peculiarities: in momentum-buoyancy region the plume exhibits an almost steady cylindrical shape with relative small turbulence structures; in deflection region the plume is deviated horizontally and a high shear rate is detected; in entrainment region the vortex pair develops, along with the sausage-like turbulent structure. The comparison between the plume and the jet case shows that the latter has a higher eccentricity while its trajectory height is sensibly lower. Also, the sausage-like structures are not present. Finally, an empirical formula for the jet trajectory is given, although its full validation will require additional studies
Large eddy simulation model of the ABL with thermal and humid stratification effects
No full textPrediction of pollutant dispersion in urban area is of crucial importance to assess quality of life of populations. At urban scales, local phenomena take place, governed by the building canopy, together with large scale motion driven by vertical stratification, air humidity and heat sources created in the canopy. This situation requires up-to-date tools able to reproduce the physics complexities. In the present work the large-eddy simulation technique is used to model the atmospheric turbulent boundary layer at urban scales. The model, LES-AIR, has been already developed and is able to take into account geometric complexities and thermal stratification. In the present work
we show an improved version of the model where stratification and heat exchange associated to water moisture in air are considered. In particular will be considered the transport of the potential temperature Θ and of the humidity content of the air parcel. Both will be treated as active scalars and their influence on the flow will be taken into account via the Boussinesq approximation. Moreover the Lift Condensation Level (LCL) will be computed run-time in order to incorporate the effects of the change of phase of water in the limit of pseudo-adiabatic processes.
The model is tested against real scale data. The data from the meteorological station situated in Campoformido (UD), Italy, are included into the definition of the boundary condition at the surface. The Monin-Obukhov similarity theory is invoked in order to properly compute the momentum, heat and water vapor fluxes. In particular the gradient method is adopted to compute the values at the surface from the linear interpolation of the hourly available quantities. Different cases in different methodological situations are analyzed in order to enlighten the role of humidity in the atmospheric boundary layer. The results are discussed and compared to the vertical data available providing a good agreement
Numerical simulation of hot smoke plumes from funnels
No full textThe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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