14,260 research outputs found
Fluctuation-induced dynamics of multiphase liquid jets with ultra-low interfacial tension
Control of fluid dynamics at the micrometer scale is essential to emulsion
science and materials design, which is ubiquitous in everyday life and is
frequently encountered in industrial applications. Most studies on multiphase
flow focus on oil-water systems with substantial interfacial tension. Advances
in microfluidics have enabled the study of multiphase flow with more complex
dynamics. Here, we show that the evolution of the interface in a jet surrounded
by a co-flowing continuous phase with an ultra-low interfacial tension presents
new opportunities to the control of flow morphologies. The introduction of a
harmonic perturbation to the dispersed phase leads to the formation of
interfaces with unique shapes. The periodic structures can be tuned by
controlling the fluid flow rates and the input perturbation; this demonstrates
the importance of the inertial effects in flow control at ultra-low interfacial
tension. Our work provides new insights into microfluidic flows at ultra-low
interfacial tension and their potential applications
Comparison of multiphase SPH and LBM approaches for the simulation of intermittent flows
Smoothed Particle Hydrodynamics (SPH) and Lattice Boltzmann Method (LBM) are
increasingly popular and attractive methods that propose efficient multiphase
formulations, each one with its own strengths and weaknesses. In this context,
when it comes to study a given multi-fluid problem, it is helpful to rely on a
quantitative comparison to decide which approach should be used and in which
context. In particular, the simulation of intermittent two-phase flows in pipes
such as slug flows is a complex problem involving moving and intersecting
interfaces for which both SPH and LBM could be considered. It is a problem of
interest in petroleum applications since the formation of slug flows that can
occur in submarine pipelines connecting the wells to the production facility
can cause undesired behaviors with hazardous consequences. In this work, we
compare SPH and LBM multiphase formulations where surface tension effects are
modeled respectively using the continuum surface force and the color gradient
approaches on a collection of standard test cases, and on the simulation of
intermittent flows in 2D. This paper aims to highlight the contributions and
limitations of SPH and LBM when applied to these problems. First, we compare
our implementations on static bubble problems with different density and
viscosity ratios. Then, we focus on gravity driven simulations of slug flows in
pipes for several Reynolds numbers. Finally, we conclude with simulations of
slug flows with inlet/outlet boundary conditions. According to the results
presented in this study, we confirm that the SPH approach is more robust and
versatile whereas the LBM formulation is more accurate and faster
Stability and flow fields structure for interfacial dynamics with interfacial mass flux
We analyze from a far field the evolution of an interface that separates
ideal incompressible fluids of different densities and has an interfacial mass
flux. We develop and apply the general matrix method to rigorously solve the
boundary value problem involving the governing equations in the fluid bulk and
the boundary conditions at the interface and at the outside boundaries of the
domain. We find the fundamental solutions for the linearized system of
equations, and analyze the interplay of interface stability with flow fields
structure, by directly linking rigorous mathematical attributes to physical
observables. New mechanisms are identified of the interface stabilization and
destabilization. We find that interfacial dynamics is stable when it conserves
the fluxes of mass, momentum and energy. The stabilization is due to inertial
effects causing small oscillations of the interface velocity. In the classic
Landau dynamics, the postulate of perfect constancy of the interface velocity
leads to the development of the Landau-Darrieus instability. This
destabilization is also associated with the imbalance of the perturbed energy
at the interface, in full consistency with the classic results. We identify
extreme sensitivity of the interface dynamics to the interfacial boundary
conditions, including formal properties of fundamental solutions and
qualitative and quantitative properties of the flow fields. This provides new
opportunities for studies, diagnostics, and control of multiphase flows in a
broad range of processes in nature and technology
Corrugated interfaces in multiphase core-annular flow
Microfluidic devices can be used to produce highly controlled and
monodisperse double or multiple emulsions. The presence of inner drops inside a
jet of the middle phase introduces deformations in the jet, which leads to
breakup into monodisperse double emulsions. However, the ability to generate
double emulsions can be compromised when the interfacial tension between the
middle and outer phases is low, leading to flow with high capillary and Weber
numbers. In this case, the interface between the fluids is initially deformed
by the inner drops but the jet does not break into drops. Instead, the jet
becomes highly corrugated, which prevents formation of controlled double
emulsions. We show using numerical calculations that the corrugations are
caused by the inner drops perturbing the interface and the perturbations are
then advected by the flow into complex shapes
Phase-field modeling droplet dynamics with soluble surfactants
Using lattice Boltzmann approach, a phase-field model is proposed for simulating droplet motion with soluble surfactants. The model can recover the Langmuir and Frumkin adsorption isotherms in equilibrium. From the equilibrium equation of state, we can determine the interfacial tension lowering scale according to the interface surfactant concentration. The model is able to capture short-time and long-time adsorption dynamics of surfactants. We apply the model to examine the effect of soluble surfactants on droplet deformation, breakup and coalescence. The increase of surfactant concentration and attractive lateral interaction can enhance droplet deformation, promote droplet breakup, and inhibit droplet coalescence. We also demonstrate that the Marangoni stresses can reduce the interface mobility and slow down the film drainage process, thus acting as an additional repulsive force to prevent the droplet coalescence
A momentum-conserving, consistent, Volume-of-Fluid method for incompressible flow on staggered grids
The computation of flows with large density contrasts is notoriously
difficult. To alleviate the difficulty we consider a consistent mass and
momentum-conserving discretization of the Navier-Stokes equation.
Incompressible flow with capillary forces is modelled and the discretization is
performed on a staggered grid of Marker and Cell type. The Volume-of-Fluid
method is used to track the interface and a Height-Function method is used to
compute surface tension. The advection of the volume fraction is performed
using either the Lagrangian-Explicit / CIAM (Calcul d'Interface Affine par
Morceaux) method or the Weymouth and Yue (WY) Eulerian-Implicit method. The WY
method conserves fluid mass to machine accuracy provided incompressiblity is
satisfied which leads to a method that is both momentum and mass-conserving. To
improve the stability of these methods momentum fluxes are advected in a manner
"consistent" with the volume-fraction fluxes, that is a discontinuity of the
momentum is advected at the same speed as a discontinuity of the density. To
find the density on the staggered cells on which the velocity is centered, an
auxiliary reconstruction of the density is performed. The method is tested for
a droplet without surface tension in uniform flow, for a droplet suddenly
accelerated in a carrying gas at rest at very large density ratio without
viscosity or surface tension, for the Kelvin-Helmholtz instability, for a
falling raindrop and for an atomizing flow in air-water conditions
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