Bahman Aboulhasanzadeh

Multiscale computations of mass transfer from buoyant bubbles

In the computation of multiphase flow with mass transfer, the large disparity between the length and time scale of the mass transfer and the fluid flow demand excessive grid resolution for fully resolved simulation of such flow. We have developed a subscale description for the mass transfer in bubbly flow to alleviate the grid requirement needed at the interface where the mass gets transferred from one side to the other. In this fluid dynamics video, a simulation of the mass transfer from buoyant bubbles is done using a Front Tracking method for the tracking of interface and a subscale description for the transfer of mass from the bubble into the domain. After the mass is transferred from the bubble into the domain, mass is followed by solving an advection-diffusion equation on a relatively coarse Cartesian grid. More detail about the method can be found in our paper[1]. This simulation shows 13 moving bubbles in a periodic domain, 3db × 3db × 48db, where db is the bubble diameter. The grid resolution is 64×64×1024, which results in about 21 cell across one bubble diameter. The flow non-dimensional governing parameters are Eo = 2.81 and Mo = 4.5 × 10−7 with density and viscosity ratio of 0.1 and for the mass transfer we have Sc = 60. In the movie, bubbles are colored to show the mass boundary layer thickness, with blue showing a close to zero value and red showing the maximum value. Mass concentration inside the domain is colored from transparent blue for low value, 0, to solid red for high value, 1. Time is non-dimensionalized with √ db/g. ar X iv :1 21 0. 35 73 v2 [ ph ys ic s. fl udy n] 1 7 O ct 2 01 2

Multiscale considerations in direct numerical simulations of multiphase flowsa)

Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models,...

An observable regularization of compressible two-phase flow

Abstract Many fluid flow problems involving turbulence, shocks, and material interfaces create a common issue high wave number irregularity. The non-linear advection term in the governing equations for all of these problems keep generating higher wave modes as k goes to infinity. In this work, we present an inviscid regularization technique, called observable regularization, for the simulation of two-phase compressible flow. In this technique, we use observable divergence theorem to derive an observable equation for tracking material interface (volume fraction). In some one-dimensional test cases, first we show that this method preserves pressure equilibrium at material interface, then we compare our results to exact Euler solutions. At the end we demonstrate a two-dimensional simulation of shock-bubble interaction showing good agreement with available experimental data from literature.

Capturing Subgrid Physics in DNS of Multiphase Flows

Mass transfer in the liquid phase of gas-liquid multiphase flows usually takes place at a considerably slower rate than the transfer of momentum, so mass flux boundary layers are much thinner than momentum boundary layers. In Direct Numerical Simulations (DNS) the resolution requirement for flows where the Schmidt number (kinematic viscosity divided by mass diffusion) is high are therefore significantly higher than for flow without mass transfer. While it is, in principle, possible to capture the mass transfer using adaptive grid refinement, the structure of the boundary layer is relatively simple and well understood. Here we discuss a multi-scale approach to compute the mass transfer from buoyant bubbles, using a boundary-layer approximation next to the bubble and a relatively coarse grid for the rest of the flow.Copyright

Multiscale Issues in DNS of Multiphase Flows

In direct numerical simulations (DNS) of multiphase flows it is frequently found that features much smaller than the “dominant” flow scales emerge. Those features consist of thin films, filaments, drops, and boundary layers, and usually surface tension is strong so the geometry is simple. Inertia effects are also small so the flow is simple and often there is a clear separation of scales between those features and the rest of the flow. Thus it is often possible to describe the evolution of this flow by analytical models. Here we discuss two examples of the use of analytical models to account for small-scale features in DNS of multiphase flows. For the flow in the film beneath a drop sliding down a sloping wall we capture the evolution of films that are too thin to be accurately resolved using a grid that is sufficient for the rest of the flow by a thin film model. The other example is the mass transfer from a gas bubbly rising in a liquid. Since diffusion of mass is much slower than the diffusion of momentum, the mass transfer boundary layer is very thin and can be captured by a simple boundary layer model.Copyright