4,154 research outputs found
Numerical simulations of breakage, coalescence and droplet size distribution
Full text linkThe dynamics of surfactant-laden droplets are investigated in this thesis using Direct Numerical Simulations (DNS) of turbulence coupled with a two-order-parameter Phase Field (PF) method to describe interface and surfactant dynamics. This problem is characterized by the presence of a deformable interface transported by the (eventually turbulent) flow and of a soluble surfactant. The complex interplay among flow, interface and surfactant, whose effects are deeply intertwined, is presented in detail in the following chapters and is also briefly summarized here with the help of the graphical abstract. The flow deforms the interface and advects surfactant via the shear stresses at the interface. In turn, the interface feeds back onto the flow field via capillary stresses (normal to the interface). The interface also, while deforming, breaking and merging, modifies the local surfactant concentration over the interface. Surfactant locally reduces surface tension of the interface, changing the local deformability of the interface. In addition, eventual surface tension gradients, generated by an uneven surfactant distribution, introduce stresses tangential to the interface (Marangoni stresses). Surfactant, indeed, feeds back onto the flow field via Marangoni stresses and onto the interface by locally reducing surface tension.
In the thesis, the outcome of this complex interplay is characterized, starting from simpler laminar cases (as for instance the deformation and interaction of droplets in laminar flow) and concluding with the more complex case of a swarm of surfactant-laden droplets in turbulence.The dynamics of surfactant-laden droplets are investigated in this thesis using Direct Numerical Simulations (DNS) of turbulence coupled with a two-order-parameter Phase Field (PF) method to describe interface and surfactant dynamics.
This problem is characterized by the presence of a deformable interface transported by the (eventually turbulent) flow and of a soluble surfactant.
The complex interplay among flow, interface and surfactant, whose effects are deeply intertwined, is presented in detail in the following chapters and is also briefly summarized here with the help of the graphical abstract.
The flow deforms the interface and advects surfactant via the shear stresses at the interface.
In turn, the interface feeds back onto the flow field via capillary stresses (normal to the interface).
The interface also, while deforming, breaking and merging, modifies the local surfactant concentration over the interface.
Surfactant locally reduces surface tension of the interface, changing the local deformability of the interface.
In addition, eventual surface tension gradients, generated by an uneven surfactant distribution, introduce stresses tangential to the interface (Marangoni stresses).
Surfactant, indeed, feeds back onto the flow field via Marangoni stresses and onto the interface by locally reducing surface tension.
In the thesis, the outcome of this complex interplay is characterized, starting from simpler laminar cases (as for instance the deformation and interaction of droplets in laminar flow) and concluding with the more complex case of a swarm of surfactant-laden droplets in turbulence
Mass-conservation-improved phase field methods for turbulent multiphase flow simulation
Full text linkThe phase field method has emerged as a powerful tool for the simulation of multiphase flow. The method has great potential for further developments and applications: it has a sound physical basis, and when associated with a highly refined grid, physics is accurately rendered. However, in many cases, especially when dealing with turbulent flows, the available computational resources do not allow for a complete resolution of the interfacial phenomena and some undesired effects such as shrinkage, coarsening and misrepresentation of surface tension forces and thermo-physical properties can affect the accuracy of the simulations. In this paper, we present two improved phase field method formulations (profile-corrected and flux-corrected), specifically developed to overcome the previously mentioned drawbacks, and we benchmark their performance versus the classic one. The formulations are first tested considering the rise of a bubble in a quiescent fluid and the interaction of two droplets in laminar shear flow; then, their performances are compared in the simulation of a droplet-laden turbulent flow. The aim of this work is to review and benchmark the different phase field method formulations, with the final goal of laying down useful guidelines for the accurate simulation of turbulent multiphase flow with the phase field method
Deformation of clean and surfactant-laden droplets in shear flow
Full text linkIn this work we study the deformation of
clean and surfactant-laden droplets in laminar shearflow. The simulations are based on Direct Numerical
Simulation of the Navier–Stokes equations coupled
with a Phase Field Method to describe interface
topology and surfactant concentration. Simulations
are performed considering both 2D (circular droplet)
and 3D (spherical droplet) domains. First, we focus on
clean droplets and we characterize the droplet shape
and deformation. This enables us to define the range of
parameters in which theoretical models well predict
the results obtained from 2D and 3D simulations.
Then, surfactant-laden droplets are considered; the
main factors leading to larger droplet deformation are
carefully described and quantified. Results obtained
indicate that the average surface tension reduction and
the accumulation of surfactant at the tips of the
deformed droplet have a dominant role, while tangential stresses at the interface (Marangoni stresses) have
a limited effect on the overall droplet deformation.
Finally, the distribution of surfactant over the droplet
surface is examined in relation to surface deformation
and shear stress distribution
Coalescence of surfactant-laden drops by Phase Field Method
No full textIn this work, we propose and test the validity of a modified Phase Field Method (PFM), which is specifically developed for large scale simulations of turbulent flows with large and deformable surfactant-laden droplets. The time evolution of the phase field, ϕ, and of the surfactant concentration field, ψ, are obtained from two Cahn–Hilliard-like equations together with a two-order-parameter Time-Dependent Ginzburg–Landau (TDGL) free energy functional. The modifications introduced circumvent existing limitations of current approaches based on PFM and improve the well-posedness of the model. The effect of surfactant on surface tension is modeled via an Equation Of State (EOS), further improving the flexibility of the approach. This method can efficiently handle topological changes, i.e. breakup and coalescence, and describe adsorption/desorption of surfactant. The capabilities of the proposed approach are tested in this paper against previous experimental results on the effects of surfactant on the deformation of a single droplet and on the interactions between two droplets. Finally, to appreciate the performances of the model on a large scale complex simulation, a qualitative analysis of the behavior of surfactant-laden droplets in a turbulent channel flow is presented and discussed
Influence of density and viscosity on deformation, breakage, and coalescence of bubbles in turbulence
Full text linkWe investigate the effect of density and viscosity differences on a swarm of large and deformable bubbles dispersed in a turbulent channel flow. For a given shear Reynolds number, Reτ=300, and a constant bubble volume fraction, φ≃5.4%, we perform a campaign of direct numerical simulations of turbulence coupled with a phase-field method accounting for interfacial phenomena. For each simulation, we vary the Weber number (We, ratio of inertial to surface tension forces), the density ratio (ρr, ratio of bubble density to carrier flow density) and the viscosity ratio (ηr, ratio of bubble viscosity to carrier flow viscosity). Specifically, we consider two Weber numbers, We=1.50 and We=3.00, four density ratios, from ρr=1 down to ρr=0.001, and five viscosity ratios, from ηr=0.01 up to ηr=100. Our results show that density differences have a negligible effect on breakage and coalescence phenomena, while a much stronger effect is observed when changing the viscosity of the two phases. Increasing the bubble viscosity with respect to the carrier fluid viscosity damps turbulence fluctuations, makes the bubble more rigid, and strongly prevents large deformations, thus reducing the number of breakage events. Local deformations of the interface, on the contrary, depend on both density and viscosity ratios: as the bubble density is increased, a larger number of small-scale deformations, small dimples and bumps, appear on the interface of the bubble. The opposite effect is observed for increasing bubble viscosities: the interface of the bubbles become smoother. We report that these effects are mostly visible for larger Weber numbers, where surface forces are weaker. Finally, we characterize the flow inside the bubbles; as the bubble density is increased, we observe, as expected, an increase in the turbulent kinetic energy (TKE) inside the bubble, while as the bubble viscosity is increased, we observe a mild reduction of the TKE inside the bubble and a strong suppression of turbulence
Le «buone letture». 2. Giovanni Casati
No full textIl saggio è costituito da due parti, la prima delle quali, dedicata alla fondazione della Federazione italiana delle biblioteche circolanti cattoliche, è stata pubblicata nel precedente numero dei «Nuovi Annali», XXVII (2013), pp. 137-163. In questa seconda parte viene delineata la figura intellettuale di Giovanni Casati, che diresse la «Rivista di letture» dal 1912 al 1944, trasformando il periodico della Federazione in una rivista impegnata nella divulgazione della cultura cattolica. A questo impegno militante Casati fece corrispondere un intenso programma editoriale, che trovò espressione nella pubblicazione di saggi letterari, di manuali e opere repertoriali.The study consists of two parts; the first is dedicated to the history of the Federazione italiana delle biblioteche circolanti cattoliche since its foundation (1904) up to 1912 and was published in the previous volume of the «Nuovi Annali », XXVII (2013), pp. 137-163. In this second part, the author outlines the intellectual figure of Giovanni Casati, who directed the «Rivista di letture» from 1912 to 1944, transforming the magazine of the Federation in a journal engaged in the spreading of Catholic culture. To this militant engagement Casati matched an intense publishing program, which found its expression in the publication of literary essays, manuals and reference works
Non-generic concentrations for shape-memory alloys; the case of CuZnAl
No full textAs is well known, the remarkable properties of many active crystalline materials, such as shape-memory alloys, originate from a symmetry-breaking martensitic phase transformation. A recent analysis (see Pitteri and Zanzotto, Acta mater., 1998, 46, 225) has shown that if a special condition on the strain parameters is satisfied, the twinning ability of a crystal undergoing a cubic-to-monoclinic transformation is considerably increased. As this feature may improve the memory behavior of cubic–monoclinic alloys, it is suggested that these predictions be tested experimentally. A procedure is given to determine explicitly which “non-generic” concentrations allow suitable alloys to produce the extra twins; which concentrations give a volume-preserving transformation are also determined (this condition guarantees self-accommodation of the martensite). As an example, the ternary CuZnAl system is considered, for which, based on the available experimental data, the non-generic concentrations are established. The work indicates that there is a scarcity of the necessary data on the mapping of lattice parameters vs concentrations in alloys with a potential for shape memory. A specific experimental program aiming at the systematic gathering of this information is proposed: building such a database may prove of considerable help in the search and synthesis of new materials with improved performance
The 2.5-dimensional turbulence in shear-thinning jets
Full text linkThe dimensional transition in turbulent jets of a shear-thinning fluid is studied via direct numerical simulations. Our findings reveal that under vertical confinement, the flow exhibits a unique mixed-dimensional (or 2.5-dimensional) state, where large-scale two-dimensional and small-scale three-dimensional structures coexist. This transition from three-dimensional turbulence near the inlet to two-dimensional dynamics downstream is dictated by the level of confinement: weak confinement guarantees turbulence to remain three-dimensional, whereas strong confinement forces the transition to two dimensions; the mixed-dimensional state is observed for moderate confinement and it emerges as soon as flow scales are larger than the vertical length. In this scenario, we observed that the mixed-dimensional state is an overall more energetic state, and it shows a multi-cascade process, where the direct cascade of energy at small scales and the direct cascade of enstrophy at large scales coexist. The results provide insights into the complex dynamics of confined turbulent flows, relevant in both natural and industrial settings
Comico e tragico del diritto nella novellistica italiana: il novelliere di Giovanni Sercambi
No full textNelle novelle di età tardomedievale ed umanistica, a causa del loro carattere fortemente realistico, il diritto ha un posto importante, poiché rappresenta un elemento centrale e ineliminabile della vita della comunità. Il novelliere di Giovanni Sercambi da Lucca non fa eccezione, anche se l’autore non ha una cultura giuridica né s’interessa in modo specifico al diritto. Il presente saggio vuole analizzare sotto tale profilo queste novelle per verificare quale concezione del diritto emerge da esse e come la narrazione ne risulti arricchita.In the late medieval and humanistic novellas, owing to their very realistic character, the right is an important central and inevitable element in community life. Giovanni Sercambi’s novelliere is no exception even if the author lacks a legal culture, nor is specifically interested in right. This essay intends to analyze these novellas under this profile to verify what kind of right they consider and how the stories are enriched
The association of rituximab and a thrombopoietin receptor agonist in high-risk refractory immune thrombocytopenic purpura
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