1,720,999 research outputs found
Direct numerical simulation of turbulence-interface interactions
No full textIn this thesis, the interactions between a deformable interface and turbu- lence have been investigated using Direct Numerical Simulations (DNS). The interface and the surfactant concentration are tracked using a Phase Field Method (PFM). The turbulence-interface interactions have been anal- ysed in two different flow configurations, a dispersed and a stratified flow. First, a dispersed flow is considered, a swarm of large deformable drops is re- leased in a turbulent channel flow. The coalescence and breakup rates have been characterised for different values of the surface tension and viscosity ratios. Results show that the drop size, determined by the equilibrium be- tween coalescence and breakup, is influenced either by the surface tension, either by the internal viscosity. In particular, for small values of the surface tension values, the internal viscosity enhances the stability of the interface and prevent drop breakup.
Second, a viscosity stratified configuration is considered. This setup mimics a core annular flow; a low viscosity fluid is interposed between the core and the walls to decrease the pressure drop. Results show that the interface is able to damp the near-wall turbulence, an increase of the core flow rate is observed. For the range of viscosity ratios analysed, the turbulence-interface interactions play a key role for obtaining Drag Reduction (DR). The DR performance is slighty affected by the viscosity ratio.In this thesis, the interactions between a deformable interface and turbu- lence have been investigated using Direct Numerical Simulations (DNS). The interface and the surfactant concentration are tracked using a Phase Field Method (PFM). The turbulence-interface interactions have been anal- ysed in two different flow configurations, a dispersed and a stratified flow. First, a dispersed flow is considered, a swarm of large deformable drops is re- leased in a turbulent channel flow. The coalescence and breakup rates have been characterised for different values of the surface tension and viscosity ratios. Results show that the drop size, determined by the equilibrium be- tween coalescence and breakup, is influenced either by the surface tension, either by the internal viscosity. In particular, for small values of the surface tension values, the internal viscosity enhances the stability of the interface and prevent drop breakup.
Second, a viscosity stratified configuration is considered. This setup mimics a core annular flow; a low viscosity fluid is interposed between the core and the walls to decrease the pressure drop. Results show that the interface is able to damp the near-wall turbulence, an increase of the core flow rate is observed. For the range of viscosity ratios analysed, the turbulence-interface interactions play a key role for obtaining Drag Reduction (DR). The DR performance is slighty affected by the viscosity ratio
Turbulent drag reduction in water-lubricated channel flow of highly viscous oil
No full textWe study the problem of drag reduction (DR) in a lubricated conduit, in which a thin layer of low-viscosity (e.g., water) fluid is injected in the near-wall region and facilitates the transport of a core of high-viscosity fluid (e.g., oil). In the present investigation, the flow instance is a channel flow, and consequently we have one thin layer of low-viscosity fluid lubricating each wall. We run direct numerical simulations of this flow instance, respecting the protocol of the constant power input approach. This approach prescribes that the flow rate is adjusted according to current pressure gradient, so to keep constant the power in- jected into the flow, it mimics closely real transport pipelines. A phase-field method is used to describe the dynamics of the liquid-liquid interface. As this technique is tailored toward the transport of very viscous fluids like oils, we study the drag reduction performance of the system by keeping fixed the lubricating fluid properties (e.g., water) and by considering two different types of oil characterized by different viscosities, 10 and 100 times more viscous than water, respectively. As in real instances the presence of impurities and surfactants— which act by locally reducing the local value of the surface tension—is inevitable, we consider, for each type of transported oil, a clean and a surfactant-laden interface. For all four tested configurations, we unambiguously show that significant DR can be achieved. Reportedly, compared to the single-phase case, we observe a reduction of the mean pressure gradient down to px/px,sp = 0.25 for the largest viscosity oil. By analyzing the features of turbulence in the lubricating layer, and the close interaction with the perturbations induced by the oil-water interface deformation, we elucidate the physical mechanisms leading to DR and we underline the effects of viscosity ratios and of surfactants
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
Turbulent drag reduction by compliant lubricating layer
No full textWe propose a physically sound explanation for the drag reduction mechanism in a lubricated channel, a flow configuration in which an interface separates a thin layer of less-viscous fluid (viscosity η₁) from a main layer of a more-viscous fluid (viscosity η₂). To single out the effect of surface tension, we focus initially on two fluids having the same density and the same viscosity ( λ=η₁/η₂=1), and we lower the viscosity of the lubricating layer down to λ=η₁/η₂=0.25, which corresponds to a physically realizable experimental set-up consisting of light oil and water. A database comprising original direct numerical simulations of two-phase flow channel turbulence is used to study the physical mechanisms driving drag reduction, which we report between 20 and 30 percent. The maximum drag reduction occurs when the two fluids have the same viscosity ( λ=1 ), and corresponds to the relaminarization of the lubricating layer. Decreasing the viscosity of the lubricating layer ( λ<1 ) induces a marginally decreased drag reduction, but also helps sustaining strong turbulence in the lubricating layer. This led us to infer two different mechanisms for the two drag-reduced systems, each of which is ultimately controlled by the outcome of the competition between viscous, inertial and surface tension forces
Turbulent drag reduction in channel flow with viscosity stratified fluids
No full textIn this work we use Direct Numerical Simulation (DNS) to study the turbulent Poiseuille flow of two immiscible liquid layers inside a rectangular channel. A thin liquid layer (fluid 1) flows on top of a thick liquid layer (fluid 2), such that their thickness ratio is . The two liquid layers have the same density but different viscosities (viscosity-stratified fluids). In particular, we consider three different values of the viscosity ratio : and . Numerical Simulations are based on a Phase Field method to describe the interaction between the two liquid layers. Although a small viscosity ratio is assumed, this physical setup aims at mimicking the situation where water (less viscous fluid) is used to favour the transport of oil (large viscous fluid) inside pipelines. Compared with the case of a single phase flow, the presence of a liquid-liquid interface produces a remarkable turbulence modulation inside the channel, since a significant proportion of the kinetic energy is subtracted from the mean flow and converted into work to deform the interface. This induces a strong turbulence reduction in the proximity of the interface and causes a substantial increase of the volume-flowrate. These effects become more pronounced with decreasing λ
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
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
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
Viscosity-modulated breakup and coalescence of large drops in bounded turbulence
No full textIn this work, we examine the influence of viscosity on breakup and coalescence of a swarm of large drops in a wall-bounded turbulent flow. We consider several values of surface tension and a wide range of drops to fluid viscosity ratios λ=ηd/ηc (with ηd the viscosity of the drops and ηc the viscosity of the carrier fluid), from λ=0.01 to λ=100, while we maintain the same density for drops and carrier fluids. Drops can coalesce and break following a complex dynamics that is primarily controlled by the interplay between turbulence fluctuations (measured by Reynolds number, Reτ), surface tension (measured by Weber number, We), and λ. We use direct numerical simulation of turbulence coupled with a phase field method to describe the drops dynamics. We consider three different values of We (which is the inverse of the surface tension): We=0.75, 1.5, and 3. For each value of We, we assume five values of λ: λ=0.01, 0.1, 1, 10, and 100. We observe a consistent action of increasing λ, which, especially for the larger Weber numbers, decreases significantly the breakup rate of the drops. Qualitatively, an increase of drop viscosity decreases the breakup rate, very much like an increase of surface tension does. The mechanism by which drop viscosity acts is a modulation of turbulence fluctuations inside the drop, which reduces the work surface tension has to do to preserve drop integrity. We believe that this may give important indications in many industrial applications to control drop coalescence and fragmentation via the ratio of drop to fluid viscosity. © 2017 American Physical Society
Turbulent Drag Reduction by a Near Wall Surface Tension Active Interface
Full text linkIn this work we study the turbulence modulation in a viscosity-stratified two-phase flow using Direct Numerical Simulation (DNS) of turbulence and the Phase FieldMethod (PFM) to simulate the interfacial phenomena. Specifically we consider the caseof two immiscible fluid layers driven in a closed rectangular channel by an imposedmean pressure gradient. The present problem, which may mimic the behaviour of anoil flowing under a thin layer of different oil, thickness ratioh2/h1=9, is describedby three main flow parameters: the shear Reynolds numberReτ(which quantifies theimportance of inertia compared to viscous effects), the Weber numberWe(which quan-tifies surface tension effects) and the viscosity ratioλ=ν1/ν2between the two fluids.For this first study, the density ratio of the two fluid layers is the same (ρ2=ρ1),we keepReτandWeconstant, but we consider three different values for the viscosityratio:λ=1,λ=0.875 andλ=0.75. Compared to a single phase flow at the sameshear Reynolds number (Reτ=100), in the two phase flow case we observe a decreaseof the wall-shear stress and a strong turbulence modulation in particular in the proxim-ity of the interface. Interestingly, we observe that the modulation of turbulence by theliquid-liquid interface extends up to the top wall (i.e. the closest to the interface) and pro-duces local shear stress inversions and flow recirculation regions. The observed resultsdepend primarily on the interface deformability and on the viscosity ratio between the twofluids (λ)
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