Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session GE: Multiphase and Particle-Laden Flows III |
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Chair: Gretar Tryggvason, Worcester Polytechnic Institute Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 123 |
Monday, November 20, 2006 10:30AM - 10:43AM |
GE.00001: Studies of bubble dispersion Souvik Biswas, Gretar Tryggvason Considerable effort has been devoted to the study of the turbulent dispersion of small bubbles and particles by turbulent flows. While the bubbles and particles may modify the flow somewhat, the primary effect is a one-way coupling from the fluid to the particles. When the response time of the particles is comparable to the time scales of the fluid motion the effect of the bubbles on the fluid cannot be ignored. Here we examine, using direct numerical simulations, the dispersion of buoyant bubbles rising in quiescent flow, or simple horizontal shear, where every continuum length and time scale are fully resolved. The fluid motion disperses the bubbles, but the bubbles are responsible for the fluid motion, as they rise. At the moment most of our results are for two-dimensional flows and we examine the dispersion by following the motion of several tens of bubbles as they rise across a horizontal channel. The results show that the dispersion is reasonably insensitive to the initial distribution of the bubbles and the length of the channel. Weak shear has essentially no effect on the dispersion. [Preview Abstract] |
Monday, November 20, 2006 10:43AM - 10:56AM |
GE.00002: Microbubbly Taylor-Couette Flow Kazuyasu Sugiyama, Enrico Calzavarini, Detlef Lohse Direct numerical simulations of the microbubbly Taylor-Couette flow are presented, employing the point-particle approach and two-way coupling. The Reynolds number is between 600 and 2500. The addition of bubbles results in a strong torque and therefore drag reduction. The simulation results are consistent with the experimental data of Murai et al. (2005, J. Phys. Conf. Ser., {\bf 14}). To reveal the physical mechanism of the drag reduction, we investigate the energy transport balance among the energy dissipation and the energy inputs via the torque and the rising bubbles. Our analysis suggests that the torque reduction is caused by the appearance of an axial flow, induced by rising bubbles, that is able to break the energy-dissipative vortices. Although this effect is in principle similar to that produced by an axial driving force in a single phase flow, we demonstrate that the bubble addition is more efficient in reducing the torque. [Preview Abstract] |
Monday, November 20, 2006 10:56AM - 11:09AM |
GE.00003: Microbubble clustering in turbulent flow Enrico Calzavarini, Thomas H. van den Berg, Stefan Luther, Federico Toschi, Detlef Lohse Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of bubble preferential accumulation. We show that statistical analysis of the bubble-probe colliding-times series can give a robust method for investigation of clustering in the bulk regions of a turbulent flow where, due to the opacity of the flow, no imaging technique can be employed. We demonstrate that micro-bubbles ($R_0 \simeq100\ \mu m$) in a developed turbulent flow, where the Kolmogorov length-scale is $\eta \simeq R_0$, display preferential concentration in small scale structures. In particular, it is found that the clustering process is enhanced by increasing the turbulence intensity. A comparison with Eulerian-Lagrangian numerical simulations has also been performed and arising similarities and differences will be discussed. [Preview Abstract] |
Monday, November 20, 2006 11:09AM - 11:22AM |
GE.00004: DNS of turbulent bubbly flows in vertical channels Jiacai Lu, Gretar Tryggvason Direct numerical simulations are used to examine turbulent bubbly flows in vertical channels. For nearly spherical bubbles the behavior is similar to what has been observed for laminar flows. The lateral migration of the bubbles due to lift resulted in two regions: A core where the void fraction is such that the weight of the liquid/bubble mixture balances the imposed pressure gradient (and the velocity is therefore constant) and a wall-layer that is free of bubbles for downflow and bubble-rich for upflow. For spherical bubbles in a turbulent downflow the results show that the size of the bubbles plays a relatively minor role in determining the liquid velocity, as long as the bubbles remain nearly spherical. As the void fraction is changed, we do find, however, the boundaries between the core and the wall-layer for turbulent bubbly downflows are not as sharp as for laminar downflows. For upflow, nearly spherical bubbles move to the walls as for laminar upflow, but deformable bubbles stay in the middle of the channel and have relatively little effect on the liquid flow, for the same total pressure gradient. [Preview Abstract] |
Monday, November 20, 2006 11:22AM - 11:35AM |
GE.00005: Simulation \& modeling of bubbles in vortex--dominated flows Michael Mattson, Pradeep Babu, Krishnan Mahesh Bubbles in vortex--dominated flows occur widely; e.g. turbulent shear layers, free jets, and tip vortex cavitation in propellers. Two problems are considered here: (i) bubbles entrained into the core of a line vortex and (ii) bubbles entrained into the core of a ring vortex. Two different computational approaches are used: (i) Lagrangian models for the bubble dynamics coupled with direct numerical simulations of the flow field, and (ii) direct simulation of two--phase gas/water bubbles using a front tracking method coupled with the Navier--Stokes equation for the external flow. The Euler--Lagrange simulations assume one--way coupling with non--cavitation nuclei. The talk will discuss the process of entrainment of the bubbles by the vortices, and use the front--tracking simulation results to discuss validity of the model equations used in the Euler--Lagrange simulations. [Preview Abstract] |
Monday, November 20, 2006 11:35AM - 11:48AM |
GE.00006: On the accuracy of the two-fluid formulation in DNS of bubble-laden turbulent boundary layers A. Ferrante, S. Elghobashi The objective of the present paper is to examine the accuracy of the two-fluid (TF) formulation in DNS of a microbubble-laden spatially developing turbulent boundary layer (SDTBL) over a flat plate by comparing the results with those of the Eulerian-Lagrangian (EL) formulation (Ferrante \& Elghobashi, J. Fluid Mech. 2004 \& 2005). Our results show that DNS with TF (TFDNS) does not reproduce the physical mechanisms responsible for drag reduction observed in the EL results. The reason is that TFDNS does not produce accurate instantaneous local bubble concentration $C(\mbox{\boldmath$x$},t)$ gradients which are responsible for the generation of a positive $\left\langle \nabla\cdot \mbox{\boldmath$U$} \right\rangle$ that is essential for the drag reduction mechanism. The inaccuracy of the TFDNS in computing $C(\mbox{\boldmath$x$},t)$ is due to the invalidity of the bubble-phase continuity equation in regions where the continuum assumption for the bubble-phase breaks down. It is recommended that if the real (experimental or DNS) instantaneous spatial distribution of bubble (or particle) concentration is discontinuous, and if this concentration discontinuity is crucial for the realization of the physical phenomenon of interest, then DNS should use the EL formulation. We propose a {\em Knudsen number} criterion for the validity of the two-fluid formulation in DNS of dispersed two-phase flows with strong unsteady preferential concentration. [Preview Abstract] |
Monday, November 20, 2006 11:48AM - 12:01PM |
GE.00007: A discussion on the effect of bubble induced liquid velocity on the mass transfer performance of bubbles in bubble plumes. Xiaobo Gong, Shu Takagi, Yoichiro Matsumoto The effect of the bubble induced liquid velocity on the mass transfer performance of bubbles in bubble plumes has been studied numerically. A two-way coupling Euler-Lagrange method was adopted for the modeling of bubble plumes with mass transfer. The dissolution of air (nitrogen and oxygen, mainly considered) in bubble plumes with micro bubbles, 100$\mu $m$\le d_{0}\le $1mm, was simulated. The results show that in the plume with 1mm bubbles, the ratio of the bubble induced liquid velocity to bubble's terminal velocity is less than 1 and the averaged residence time of bubbles does not change much compared with a single bubble's rising period; but in the plume with 100$\mu $m bubbles, the ratio is over 10 and the averaged residence time of bubbles is around 10{\%} of the single bubble's rising period. The result suggests that under the same gas supply rate, the benefits of using smaller bubbles for high mass transfer efficiency will be overestimated without considering the reduce of the residence time of bubbles due to the effect of the bubble induced liquid velocity. The present simulations show that the dissolution efficiency of oxygen in the air bubble plume with 100$\mu $m bubbles is only half of that in a single bubble. [Preview Abstract] |
Monday, November 20, 2006 12:01PM - 12:14PM |
GE.00008: Multiscale simulations of nucleate boiling Gretar Tryggvason, Damir Juric During the last decade, direct numerical simulations of multiphase flow have emerged as a major research tool. The systems examined so far are, however, still very simple compared to those systems routinely encountered in engineering applications and numerical simulations of more complex flows, such as boiling, are emerging as the next frontier in numerical studies of multiphase flows. Here we describe a method that has been developed to study the nucleate boiling from many interacting nucleation sites in forced convection boiling. The method is based on the ``one-fluid'' formulation of the governing equations which are solved on a fixed grid and where the phase boundary is explicitly represented by connected marker points. The method has already been used to examine film boiling, but the challenge here is the accurate representation of the nucleation sites and the small-scale motion near the wall. To capture the evaporation of the microlayer left behind as the base of the bubble expands we use a semi-analytical model that is solved concurrently with the rest of the simulations. Preliminary results for the forced convective nucleate boiling in a channel are shown. [Preview Abstract] |
Monday, November 20, 2006 12:14PM - 12:27PM |
GE.00009: Numerical Simulation of the Primary Atomization of a Turbulent Coaxial Liquid Jet using a Conservative Level Set/Ghost Fluid Method Olivier Desjardins, Vincent Moureau, Edward Knudsen, Marcus Herrmann, Heinz Pitsch The atomization of liquid flows plays an important role in many engineering applications, and yet the numerical simulation of the turbulent break-up process remains an outstanding challenge. The large density ratio between the liquid and the gas, as well as the large range of scales involved in the atomization phenomenon, render this problem especially difficult. Level set methods have often been used to represent the liquid-gas interface. However, these techniques usually suffer from poor conservation properties. Moreover, the interface typically has to be artificially spread over several computational mesh points. Here, a new method is presented in which a conservative level set approach is combined with a ghost fluid formulation. With this method, a sharp representation of the interface becomes possible, and very good conservation properties are achieved. This approach is validated on a range of test cases with realistic water-air conditions and topology changes. This technique is then used to simulate the turbulent atomization of a water round jet studied experimentally by Marmottant and Villermaux (2004). The main features of the liquid jet are compared to the experiments, and the properties of the numerical approach are discussed. [Preview Abstract] |
Monday, November 20, 2006 12:27PM - 12:40PM |
GE.00010: Eulerian Models for Dilute Sprays using Quadrature Methods Rodney O. Fox, Olivier Desjardins, Philippe Villedieu, Heinz Pitsch Dilute sprays occur in many technical applications and can be described by a kinetic equation for the number density function of the droplet velocity as well as other internal variables such as size or composition. The numerical methods for simulating the kinetic equation fall into two broad categories: Lagrangian tracking techniques and Eulerian multi-fluid models. While more straightforward to implement and generally more accurate in capturing segregation due to finite Stokes number, Lagrangian methods are computationally expensive. In contrast, multi-fluid models provide multi-point statistics as well as a natural extension to dense sprays. However, Eulerian models based on number density and mean droplet velocity cannot reproduce the velocity moments obtained from Lagrangian simulations of canonical flows that involve droplet trajectory crossings. Such flows include impinging jets, elastic rebounds off walls, and turbulent flows where the droplet velocity can be locally multi-valued. In Eulerian models based on quadrature, the kinetic equation is closed at higher-order moments using weights and abscissas that are uniquely determined from transported lower-order moments. The applicability of this method is demonstrated by applying it to the aforementioned canonical flows. The moments obtained from the Eulerian model are validated against Lagrangian results. [Preview Abstract] |
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