Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D3: Multiphase Flows: Bubble Dynamics |
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Chair: Gretar Tryggvason, Notre Dame University Room: 23B |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D3.00001: Bubble Dynamics in Bubbly Medium J. Ma, C.-T. Hsiao, G.L. Chahine We present here a two-way coupled euler-lagrange model to study the dynamics Of a primary bubble oscillating in a bubbly mixture. The model simulates the Mixture medium by solving the N-S equations with a moving grid method to Track the motion of the primary bubble wall, while it models the surrounding Small bubbles with the R-P-K-H equation. The two-way coupling between them Is realized through the local mixture density due to the volume change and Motion of the dispersed bubbles. The simulations indicate the surrounding Bubbles absorb the energy radiated from the primary bubble thus reducing Both its maximum radius and period. The dynamics of the surrounding bubbles Result in a phase-shifting between density and pressure waves through the Medium. This is not captured by other analytical solutions assuming Homogeneous medium. Simulations considering the gravity successfully capture The interaction of the small bubbles with and their entrainment into the jet created by the primary bubble. All observations are in good agreements with Experiments of spark generated bubbles in bubbly media. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D3.00002: Numerical study of wave breaking and bubble generation in turbulent two-phase Couette flow Dokyun Kim, Ali Mani, Parviz Moin The objective of the present study is to understand the formation of bubbles due to boundary layer/free-surface interactions. Numerical simulations are performed on a turbulent two-phase Couette flow configuration. A level set method coupled to a subgrid breakup model is used to capture the wave breakup and bubble formation. The free surface is tracked by the level-set method, while small subgrid liquid drops and air bubbles produced from resolved ligaments are transferred from the level-set representation to the Lagrangian representation. The Reynolds and Weber numbers considered are 12,760 and 41,600, respectively, based on water properties. In order to investigate the effect of Froude number on the characteristics of wave breaking and bubble formation, the simulations are done for two different Froude numbers - Fr = 3.9 and 6.8. The statistical data including wave amplitude and bubble size distribution will be presented. The effect of grid resolution on the bubble size distribution will be discussed. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D3.00003: Bouncing vs penetration of a particle through fluid interface Alex Kotsch, Sungyon Lee, Sunghwan Jung Capturing small particles by air bubbles is fundamental to understanding numerous industrial processes of multiphase fluid. In the simple limit of a bubble-sphere interaction, we consider a solid sphere impinging onto a free surface inside the bath of fluid. Experimentally, we observe two main regimes of particle-interface interactions that depend on the initial particle kinetic energy: bouncing and penetration, with a clear transition point from one to the other. Specifically, we find two distinct scalings for change in Weber number versus initial Weber number for the two regimes. In this talk, we present the novel experimental findings and theoretical justifications. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D3.00004: Turbulence modulation by microbubbles in channel flow Cristian Marchioli, Dafne Molin, Alfredo Soldati In this paper we examine the mutual interactions between small non-deformable bubbles and turbulence in upward/downward vertical channel flow (at shear Reynolds number Re=150). An Eulerian-Lagrangian approach based on pseudo-spectral direct numerical simulation is used: bubbles are momentum-coupled with the fluid and are treated as pointwise spheres subject to gravity, drag, added mass, pressure gradient, Basset and lift forces. Due to local momentum exchange with the fluid and to the differences in bubble distribution, we observe significant increase (resp. decrease) of wall shear and liquid flowrate in upflow (resp. downflow). We propose a novel force scaling, which can help to judge differences in the turbulence features due to bubble presence. Two-phase flow energy spectra show that bubbles determine an enhancement (resp. attenuation) of energy at small (resp. large) flow scales, a feature already observed in homogeneous isotropic turbulence. Bubble-induced flow field modifications, in turn, alter significantly the dynamics of the bubbles and lead to different trends in preferential concentration and wall deposition. In this picture, the lift force plays a crucial role. We analyze all the observed trends emphasizing the impact that the lift force model has on the simulations. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D3.00005: The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow Daniela Narezo Guzman, Dennis P.M. van Gils, Chao Sun, Detlef Lohse Drag reduction (DR) in two-phase turbulent Taylor-Couette (TC) flow is studied for Reynolds number up to Re = 2$\times 10^6$ for pure inner cylinder (IC) rotation, thus extending the previously explored range. DR based on the global torque as a function of the global gas volume fraction ($\alpha$) over the range $0\%$ up to $4\%$ is obtained. We observe two DR regimes: moderate DR up to $7\%$ for Re $= 5.1 \times 10^5$ and stronger DR for Re $= 1.0 \times 10^6$ and $2.0 \times 10^6$, remarkably finding more than $40\%$ of DR for $\alpha = 4\%$ at Re $= 2.0 \times 10^6$. Furthermore, TC flow is locally studied in each regime (Re $= 5.1 \times 10^5$ and $1.0 \times 10^6$) at a fixed $\alpha = 3\%$: statistics of the local liquid flow azimuthal velocity and the local gas concentration are obtained. The local bubble Weber number ($We$) is computed close to the IC showing that the crossover from the moderate to the strong DR regime occurs roughly at the crossover of $We \sim 1$. We find that a larger local gas volume fraction close to the inner wall has a positive effect on the azimuthal velocity decrease, which is responsible for the observed DR. However for strong DR what is more important for the $\alpha$ values explored here is bubble deformability close to the boundary layer. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D3.00006: Volume Displacement Effects in Bubble-laden Flows Andrew Cihonski, Justin Finn, Sourabh Apte When a few bubbles are entrained in a traveling vortex ring, it has been shown that even at extremely low volume loadings, their presence can significantly affect the structure of the vortex core (Sridhar {\&} Katz 1999). A typical Euler-Lagrange point-particle model with two-way coupling for this dilute system, wherein the bubbles are assumed subgrid and momentum point-sources are used to model their effect on the flow, is shown to be unable to accurately capture the experimental trends of bubble settling location and vortex distortion for a range of bubble parameters and vortex strengths. The bubbles experience a significant amount of drag, lift, added mass, pressure, and gravity forces. However, these forces are in balance of each other, as the bubbles reach a mean settling location away from the vortex core. Accounting for fluid volume displacement due to bubble motion, using a model termed as volumetric coupling, experimental trends on vortex distortion and bubble settling location are well captured. The fluid displacement effects are studied by introducing a notion of a volumetric coupling force, the net force on the fluid due to volumetric coupling, which is found to be dominant even at the low volume loadings investigated here. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D3.00007: On the importance of the Mesler entrainment mechanism in turbulent breaking waves Milad Mortazavi, Vincent Le Chenadec, Dokyun Kim, Ali Mani Micro bubble generation due to liquid-liquid impact is observed experimentally for simple conditions such as droplet impact (Sigler and Mesler, J. Colloid and Interface Sc., 1989). This regime of air entrainment, called the Mesler regime, is active under certain range of parameters in terms of surface curvature and impact velocity. We have analyzed the importance of the Mesler regime in turbulent breaking waves by employing numerical simulation of a statistically stationary turbulent hydraulic jump at Reynolds number of 88000 and inflow Froude number of 2. A hybrid Lagrangian Eulerian volume of fluid method is used to capture the dynamics of the interface with density ratio of 831. Bubble statistics are compared against the experimental data of Murzyn et al. (Int. J. Multiphase Flow, 2005). Interface structure, curvature, and velocity statistics are analyzed. Our results indicate that impact events in the turbulent hydraulic jump are extremely likely to generate Mesler entrainment. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D3.00008: Efficient Time-Stepping Scheme for Incompressible Two-Phase Flows with Large Density Ratios Suchuan Dong We present an efficient time-stepping scheme within the phase field framework for flows of two immiscible incompressible fluids with large density ratios. The scheme has several attractive characteristics: (1) It is suitable for large density ratios, and numerical experiments with density ratios up to 1000 will be presented; (2) It involves only constant (time-independent) coefficient matrices for all flow variables, which can be pre-computed during pre-processing, so it effectively overcomes the performance bottleneck induced by variable coefficient matrices associated with variable density and variable viscosity; (3) It completely de-couples the computations for all flow variables (velocity, pressure, and phase field function). Numerical simulations will be presented for wall-bounded liquid-gas flows involving large density ratios, moving contact lines, and interfacial topology changes. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D3.00009: Experimental investigation of the motion of bubble clusters and the flow structures with the clusters Masanobu Date, Kazuki Maeda, Toshiyuki Ogasawara, Shu Takagi, Yoichiro Matsumoto In upward bubbly flows, mono-dispersed 1 mm spherical bubbles which do not coalesce in the presence of small amount of surfactants in a liquid phase migrate toward the walls due to the shear-induced lift force. Those bubbles form the bubble clusters near the walls [Takagi, S. and Matsumoto, Y., \textit{Annu. Rev. Fluid Mech. }(2011)]. In this study flow structures of the bubbly flow with the bubble clusters and the motion of the bubble clusters are investigated using scanning stereoscopic Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV), respectively. In order to focus on bubble clusters, 1 mm bubbles are injected near the one of the walls and bubble clusters are formed under some conditions of gas flow rate. From the measurement of the bubbly flows by stereoscopic PIV, it is shown that the bubbles near the wall accelerate surrounding liquids due to their buoyancy and reduce Reynolds stress with increasing a void fraction. Three-dimensional velocity fields are also measured by scanning stereoscopic PIV, and the effect of the bubble cluster on the instantaneous flow fields are analyzed. The results are discussed in the presentation. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D3.00010: DNS of Bubbly Flows in Vertical Channels Jiacai Lu, Sadegh Dabiri, Gretar Tryggvason The dynamics of bubbles in upflow, in a vertical channel, is examined using direct numerical simulations (DNS), where both the flow and the bubbles are fully resolved. Two cases are simulated. In one case all the bubbles are of the same size and sufficiently small so they remain nearly spherical. In the second case, several of the small bubbles are coalesced into one large bubble. In both cases lift forces drive small bubbles to the wall, forming a bubble rich wall-layer, removing bubbles from the channel interior until the two-phase mixture there is in hydrostatic equilibrium. The same evolution has been seen in earlier DNS of bubbly upflows, but here the friction Reynolds number is higher (Re$^{+}$=250). The results show clustering of bubbles in the wall-layer and we examine the mechanism responsible for the clustering and identify how bubbles move in and out of the wall-layer. The dynamics of the bubbles in the channel core is compared with results obtained in fully periodic domains and found to be similar. The presence of the large bubble disrupts the wall-layer slightly, but does not change the overall picture much, for the parameters examined here. [Preview Abstract] |
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