Catálogo de publicaciones - libros

In this paper, we present a new definition of liquid—vapor interface at the molecular level which can capture the local and instantaneous structure of the interface. The new definition is not a thermodynamic definition of the interface, such as the equimolar surface, but is based on the instantaneous particle density distribution of molecules. Applying the new definition of the interface to the MD result of the liquid—vapor interface, we found that our definition of the interface was able to capture the microscopic fluctuation caused by molecular motion. Furthermore, we confirmed that on the longtime average our definition of the interface shows good agreement with the equimolar surface.

Large-Eddy Simulation is used for the investigation of the breaking of steep water waves on a beach of constant bed slope. The method is built within a multi-fluid flow solver, in which the free surface is tracked using a Volume-of-Fluid method featuring piecewise planar interface reconstructions on a twice-as-fine mesh. The Smagorinsky sub-grid scale model is used for explicit under-resolved turbulence closure, coupled with a new scheme for turbulence decay treatment on the air-side of massively deformable free surfaces. The simulations were conducted for shear Reynolds numbers ≈≈400, based on the mean water depth. The Large-Eddy Simulation formulation in the interface tracking, single-fluid formulation is introduced for this purpose. The approach is demonstrated as a powerful tool for exploring large-scale, interfacial turbulent flows. The discussion focuses on coherent structures formation, the free surface flow effects at breaking, and form drag evolution with the surface.

The lateral migration of a single spherical particle in tube Poiseuille flow is simulated by ALE scheme, along with the study of the movement of a circular particle in plane Poiseuille flow with consistent dimensionless parameters. These particles are rigid and neutrally buoyant. A lift law = (Ω−Ω) analogous to =Γ is validated in both two dimensions and three dimensions here; and Ω are slip velocity and angular slip velocity, Ω is the angular slip velocity at equilibrium. A method of constrained simulation is used to generate data which is processed for correlation formulas for the lift force, slip velocity, and equilibrium position. Our formulas predict the change of sign of the lift force which is necessary in the Segré—Silberberg effect. Correlation formulas are compared between tube and plane Poiseuille flows by fixing the dimensionless size of particle and the Reynolds number. Our work provides a valuable reference for a better understanding of the migration of particle in Poiseuille flows and the Segré—Silberberg effect.

The force-coupling method (FCM) provides an efficient tool for computing particle motion and the flow in the surrounding fluid both in confined microflow systems and in larger scale suspensions. Here we present results for the interaction of individual particles in a shear flow showing that FCM captures reliably the changes in lift and drag forces. We note too the extension from spherical to non-spherical particles and comment on the use of FCM to analyze flow systems, bridging the gap between simulation data and macroscopic descriptions of dispersed two-phase flows.

To investigate the two-way interaction between solid particles and fluid turbulence, a homogeneous flow field including more than 2000 spherical particles was directly simulated. Since flow around each particle is approximately resolved, no models were used for particle motion or fluid turbulence. A particle settles under gravity with the Reynolds number ranging from 50 to 300, based on diameter and slip velocity. When particle clusters are formed due to the wake attraction, the average settling velocity increases. Thus particular attention was focused on the distribution of particles. The influence of Reynolds number and loading ratio are assessed. It is found that the rotation of particle dominates the cluster dynamics.

We discuss two sets of non-trivial effects affecting the motion of high-Reynolds number gas bubbles rising in still liquid which have been significantly clarified thanks to direct numerical simulations making use of a suitable boundary-fitted technique.We first summarize some features of the interaction between two spherical bubbles rising side by side in a viscous liquid. Then we briefly discuss some aspects of the path instability of a spheroidal bubble rising in a low-viscosity liquid.

The status of direct numerical simulations of bubbly flows is reviewd and a few recent results are presented. The development of numerical methods based on the one-field formulation has made it possible to follow the evolution of a large number of bubbles for a sufficiently long time so that converged statistics for the averaged properties of the flow can be obtained. In addition to extensive studies of homogeneous bubbly flows, recent investigations have helped give insight into drag reduction due to the injection of bubbles into turbulent flows and two-fluid modeling of laminar multiphase flows in channels.

Large-Eddy Simulation is used for the investigation of the breaking of steep water waves on a beach of constant bed slope. The method is built within a multi-fluid flow solver, in which the free surface is tracked using a Volume-of-Fluid method featuring piecewise planar interface reconstructions on a twice-as-fine mesh. The Smagorinsky sub-grid scale model is used for explicit under-resolved turbulence closure, coupled with a new scheme for turbulence decay treatment on the air-side of massively deformable free surfaces. The simulations were conducted for shear Reynolds numbers ≈≈400, based on the mean water depth. The Large-Eddy Simulation formulation in the interface tracking, single-fluid formulation is introduced for this purpose. The approach is demonstrated as a powerful tool for exploring large-scale, interfacial turbulent flows. The discussion focuses on coherent structures formation, the free surface flow effects at breaking, and form drag evolution with the surface.

The lateral migration of a single spherical particle in tube Poiseuille flow is simulated by ALE scheme, along with the study of the movement of a circular particle in plane Poiseuille flow with consistent dimensionless parameters. These particles are rigid and neutrally buoyant. A lift law = (Ω−Ω) analogous to =Γ is validated in both two dimensions and three dimensions here; and Ω are slip velocity and angular slip velocity, Ω is the angular slip velocity at equilibrium. A method of constrained simulation is used to generate data which is processed for correlation formulas for the lift force, slip velocity, and equilibrium position. Our formulas predict the change of sign of the lift force which is necessary in the Segré—Silberberg effect. Correlation formulas are compared between tube and plane Poiseuille flows by fixing the dimensionless size of particle and the Reynolds number. Our work provides a valuable reference for a better understanding of the migration of particle in Poiseuille flows and the Segré—Silberberg effect.

A finite-volume/front-tracking (FV/FT) method is developed for computations of multiphase flows in complex geomtries. The front-tracking methodology is combined with a dual time-stepping based FV method. The interface between phases is represented by connected Lagrangian marker points. An efficient algorithm is developed to keep track of the marker Points in curvilinear grids. The method is implemented to solve two-dimensional (plane or axisymmetric) dispersed multiphase flows and is validated for the motion of buoyancy-driven drops in periodically constricted tube with cases where drop breakup occurs.