1,720,982 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
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
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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