Jiacai Lu

The effect of bubbles on the wall drag in a turbulent channel flow

The effect of a few relatively large bubbles injected near the walls on the wall drag in the “minimum turbulent channel” is examined by direct numerical simulations. A front-tracking/finite-volume method is used to fully resolve all flow scales including the bubbles and the flow around them. The Reynolds number, using the friction velocity and the channel half-height, is 135 and the bubbles are 54 wall units in diameter. The results show that deformable bubbles can lead to significant reduction of the wall drag by suppression of streamwise vorticity. Less deformable bubbles, on the other hand, are slowed down by the viscous sublayer and lead to a large increase in drag.

Effect of bubble deformability in turbulent bubbly upflow in a vertical channel

As bubbles rising in a vertical channel with upflow become bigger, it is well known that the void fraction distribution changes in a fundamental way, from a wall peak for small bubbles to a maximum void fraction at the channel center for larger bubbles. Here, we use direct numerical simulations of buoyant bubbles in a turbulent flow to show that it is not the size of the bubbles that matters, but their deformability.

A comparative study of lattice Boltzmann and front-tracking finite-difference methods for bubble simulations

Abstract This communication describes the results of bubble rise simulations using a lattice Boltzmann method and a front-tracking finite-difference method. The simulations, performed in a 2-D periodic box, consider two specific examples: a steadily rising bubble and a bubble rising in an oscillatory manner with accompanying shape change. We compare the shapes and rise velocities of the steadily rising and oscillating bubbles, the oscillation frequencies and amplitudes, and the bubble shapes at different phases of the oscillations. The simulations reveal that both numerical schemes afford qualitatively similar results, which are within a few percent quantitatively.

Numerical study of turbulent bubbly downflows in a vertical channel

Direct numerical simulations are used to study turbulent bubbly downflows in a vertical channel. All flow scales, including the bubbles and the flow around them, are fully resolved using a front-tracking/finite-volume method. The turbulent bubbly channel flow is driven downward by an imposed constant pressure gradient, and the friction Reynolds number of the flow, based on the friction velocity and half-width of the channel, is 127.3, corresponding to a bulk Reynolds number of 3786 for a flow without bubbles. Three cases with several nearly spherical bubbles are examined. The bubble diameter is 31.8 wall units for all cases but the number of bubbles is varied, giving average void fractions of 1.5%, 3%, and 6%. The lift force on the bubbles drives them away from the walls until the mixture in the center of the channel is in hydrostatic equilibrium. Thus, the flow consists of a core region where the average void fraction and the mean vertical velocity are approximately constant and a bubble-free wall layer....

Transition between regimes of a vertical channel bubbly upflow due to bubble deformability

The effect of the deformability of viscous bubbles on the flow rate of bubbly upflow in a vertical channel is examined using direct numerical simulations. A sharp transition between two different flow regimes has been observed. At large bubble deformability, characterized by large Eotvos number (Eo), the flow rate is close to the single phase flow rate, with adjusted pressure gradient, and the bubbles are almost uniformly distributed in the middle of the channel. On the other hand, at low Eo the bubbles are concentrated near channel walls and flow rates are much smaller than the single phase flow. The transition from high flow rate to low flow rate occurs rather abruptly. It is found that the transition occurs when the less deformable bubbles enter the viscous sublayer due to the lateral lift force on the bubbles. This leads to an increase in the viscous dissipation near the wall which leads to a decrease in the flow rate.

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,...

Multiscale issues in DNS of multiphase flows

Abstract Direct numerical simulations (DNS) have now become a well established tool to examine complex multiphase ows. Such ows typically exhibit a large range of scales and it is generally necessary to use different descriptions of the ow depending on the scale that we are examining. Here we discuss multiphase ows from a multiscale perspective. Those include both how DNS are providing insight and understanding for modeling of scales much larger than the “dominant scale” (defined where surface tension, viscous forces or inertia are important), as well as how DNS are often limited by the need to resolve processes taking place on much smaller scales. Both problems can be cast into a language introduced for general classes of multiscale problems and reveal that while the classification may be new, the issues are not.

Using statistical learning to close two-fluid multiphase flow equations for a simple bubbly system

Direct numerical simulations of bubbly multiphase flows are used to find closure terms for a simple model of the average flow, using Neural Networks (NNs). The flow considered consists of several nearly spherical bubbles rising in a periodic domain where the initial vertical velocity and the average bubble density are homogeneous in two directions but non-uniform in one of the horizontal directions. After an initial transient motion the average void fraction and vertical velocity become approximately uniform. The NN is trained on a dataset from one simulation and then used to simulate the evolution of other initial conditions. Overall, the resulting model predicts the evolution of the various initial conditions reasonably well.

Direct Numerical Simulations of Nucleate Boiling

Direct Numerical Simulations (DNS) of multiphase flows, where all continuum length and time scales are fully resolved have progressed enormously in the last few years. Increases in computer power and new algorithms now make it possible to follow the unsteady motion of several hundred particles (drops, bubbles and solids) for long enough times so that meaningful averages for the fluid mixture can be calculated. However, most progress has so far been made for disperse flow of two-fluid systems. See Prosperetti and Tryggvason (2007) for a review.Copyright

DNS–Assisted Modeling of Bubbly Flows in Vertical Channels

Abstract The transient motion of bubbly flows in vertical channels is studied, using direct numerical simulation (DNS) in which every continuum length and time scale is resolved. A simulation of a large number of bubbles of different sizes at a friction Reynolds number of 500 shows that small bubbles quickly migrate to the wall, but the bulk flow takes much longer to adjust to the new bubble distribution. Simulations of much smaller laminar systems with several spherical bubbles have been used to examine the full transient motion; those show a nonmonotonic evolution where all the bubbles first move toward the walls, and the liquid then slowly slows down, eventually allowing some bubbles to return to the center of the channel. Unlike the statistically steady state, where the flow structure is relatively simple and in some cases depends only on the sign of the bubble lift coefficient, the transient evolution is more sensitive to the governing parameters. Early efforts to use DNS results to provide values for the unresolved closure terms in a simple average model for the flow found by statistical learning from the data using neural networks are discussed. The prospect for using the results from simulations of large systems with bubbles of different sizes in turbulent flows for large eddy—like simulations are explored, including the simplification of the interface structure by filtering. Finally, preliminary results for flows undergoing topology changes are shown.