Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J10: Multiphase Flows: Bubbly Flows (8:00am  8:45am CST)Interactive On Demand

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J10.00001: Numerical study on the interaction of two unequalsized bubbles rising inline Vedabit Saha, Varun Jadon, Kazuyasu Sugiyama, Shu Takagi In this study, the interactions of two unequalsized bubbles rising inline in quiescent fluid have been investigated through direct numerical simulations. Threedimensional simulations are conducted based on modified VOF method with the MTHINC scheme to study the twobubble interaction phenomena. Here, we discuss the effects of small difference in bubble volume as they rise inline. The bubble radii ratio was varied between 0.851.15. In general, for equalsized bubbles rising in line, a trailing bubble has larger rising velocity and comes closer to leading bubble. For unequalsized bubbles, this interaction process can change drastically depending on the difference of bubble volume. Especially, there are some interesting phenomena observed in the condition that leading bubble is slightly larger than the trailing bubble. More details will be given in the presentation. [Preview Abstract] 

J10.00002: LES of cavitation behind a backstep Filipe Brandao, Krishnan Mahesh Cavitation in reattaching shear flows is numerically investigated using a backwardfacing step configuration, with inflow turbulent boundarylayer of $Re_{\tau}=1500$. This turbulent boundarylayer is generated using the recyclerescale method of Lund et al. (1998). Different cavitation regimes are investigated: from inception to developed cavitation. For inception, we use a novel method that assumes vapor as a passive scalar in an incompressible liquid. In the more developed stages of cavitation, the compressible homogeneous vaporgasliquid mixture approach of Brandao et al. (2020) is employed. It is observed that inception starts at the low pressure streamwise vortical structures usually located in the axial position of $0.5 < x/Lr < 0.8$, where Lr is the reattachment length, agreeing well with experimental data. For lower values of cavitation numbers, the cavity shedding process is dominated by the passage of a bubbly shock. [Preview Abstract] 

J10.00003: Experimental Investigation of BubbleInduced Turbulence Modulation Xu Xu, Ashik Masuk, Ashwanth Salibindla, Shiyong Tan, Rui Ni Bubbleinduced turbulence was often studied in the context of a cluster of bubbles rising in an otherwise quiescent medium, rather than in flows that are already turbulent. In this talk, I will present our new experimental investigations on bubbles interacting with preexisting intense turbulence, where bubbles can be deformed by surrounding turbulence and also feed kinetic energy back. The unique experimental facility allows us to simultaneously track the 3D deformation dynamics of finitesized bubbles with the modulation of surrounding turbulence. In particular, we identified two possible turbulence generation mechanisms, one through returning surface energy stored on bubble interface back and the other one through wake dynamics. We will also show the results as a function of different relaxing rate, i.e. how quickly bubbles change their aspect ratios. The results will shed new light on the bubbleinduced turbulence modulation and paves the foundation to our understanding of how the deformability affects the twophase couplings. [Preview Abstract] 

J10.00004: Computational modeling for breakup and coalescence of bubbles in a turbulent bubbly flow Jaehee Chang, Kiyoung Kim, Haecheon Choi The phase interface in bubbly flows can be simulated with interfacecapturing methods such as volumeoffluid and levelset methods. Although these methods can effectively capture largescale interface structures, they are limited to grid resolution when resolving smallscale structures. When bubbles breakup or coalesce, small structures such as thin films or ligaments occur, and consequently, interfaces experience nonphysical breakup, where the bubble volume and interface structure are lost during simulation. We present a modified levelset method to prevent numerical breakups of thin structures, and to properly simulate the breakup and coalescence of bubbles. The present method distributes additional levelset functions in the regions where interfaces are close to each other, and accurately compute the surface tension and volumes of thin structures. A filmdrainage model is used to provide a criterion for the bubble breakup/coalescence, i.e., whether to keep the redistributed levelset function or not. The present method is applied to single bubble breakup and coaxial bubble coalescence, successfully capturing breakup/coalescence events. The method is further applied to a turbulent bubbly flow to investigate the effect of bubble breakup and coalescence. [Preview Abstract] 

J10.00005: Analysis of flow characteristics in turbulent bubbly channel flow through the velocity decomposition InKoo Lee, Jaehee Chang, Haecheon Choi The instantaneous velocity in a turbulent bubbly flow has been so far decomposed by the mean and fluctuation velocities. In our study, we decompose the velocity into the mean velocity without bubbles (U$_{\mathrm{0}})$, mean velocity deviated from U$_{\mathrm{0}}$ by bubbles ($\Delta $U), and the fluctuating velocity. In this decomposition, we use an ensemble averaging on the velocity fields containing bubbles with respect to their wallnormal positions. We apply this decomposition to turbulent bubbly channel flow, using direct numerical simulation. $\Delta $U contains a horseshoe vortex behind each bubble located close to the wall, which significantly affects the Reynolds stress distribution there. From turbulent fluctuations, counterrotating vortices behind bubbles are mainly observed irrespective of their wallnormal locations. Quasistreamwise vortices occurring in a singlephase flow is suppressed by the dynamic motion of nearwall bubbles. [Preview Abstract] 

J10.00006: Skinfriction drag reduction in buoyant bubbly plane Couette flow Chong Shen Ng, Kai Leong Chong, Haoran Liu, Naoki Hori, Roberto Verzicco, Detlef Lohse In surface flows, bubbles or air layers can reduce the effective density of the liquid. With a reduced effective density, the surface drag can be reduced, which is why the addition of bubbles or an air layer are attractive methods to achieve drag reduction. However, the role of buoyancy of the air on bubbly drag reduction are unknown. Here we show that highly buoyant bubbles in a plane Couette flow tend to form air layers and lead to skinfriction drag reduction of up to 20\%. The datasets are obtained from direct numerical simulations of bubbly flow using the phase field method, with over two decades of Froude numbers at a friction Reynolds number of 180. We found that by gradually increasing buoyancy relative to fluid inertia, that is, by reducing the Froude number, bubbles will preferentially concentrate to the surface away from gravity. At a critical Froude value, the bubbly flow catastrophically converts to an airlayer flow. At this point, skinfriction drag at the airlayer side is reduced by 1020\%. Remarkably, the skinfriction drag at the waterlayer side remains largely unaffected. Our results demonstrate that favorable bubbly drag reduction occurs when an ideal ratio between buoyancy and inertia is met in plane Couette flow. [Preview Abstract] 

J10.00007: PseudoTurbulence in BuoyancyDriven Bubbly Flows Vikash Pandey, Rashmi Ramadugu, Prasad Perlekar Bubbly flows occur in a variety of natural and industrial processes. We present a direct numerical simulation (DNS) study of pseudoturbulence in buoyancydriven bubbly flows for a range of Reynolds ($150 < {\rm Re} < 546$) and Atwood ($0.04 < {\rm At} < 0.9$) numbers. We study the probability distribution function of the horizontal and vertical liquid velocity fluctuations and find them to be in quantitative agreement with the experiments. The energy spectrum shows a $k^{3}$ scaling at high Re and becomes steeper on reducing Re. To investigate spectral transfers in the flow, we derive the scalebyscale energy budget equation. Our analysis shows that, for scales smaller than the bubble diameter, the net transfer because of the surface tension and the kinetic energy flux balances viscous dissipation to give $k^{3}$ scaling of the energy spectrum for both low and high At. [Preview Abstract] 

J10.00008: Numerical studies of three phase gasliquidsolid mixtures Lei Zeng, Jiacai Lu, Gretar Tryggvason Three phase gasliquidsolid mixtures are encountered in several important industrial processes. Those included bubbles rising through slurries, such as in the Fischer–Tropsch process where the particles are metal catalysts, in bubble columns where large particles are sometimes inserted to enhance mass transfer, and in mineral processes where bubbles are used to capture hydrophobic particles and float them out of a slurry. Here, we examine the dynamics of three phase flow using numerical simulations of many bubbles and drops, using a front tracking/finite volume method. The particles, which are assumed to be much smaller than the bubbles, are modeled as very viscous drops and kept nearly spherical by high surface tension. We examine the dynamics of the flow for a range of governing parameters and quantify how the dynamics of one phase is changed by the presence of another phase by looking at pair probability, correlation lengths, energy balance and other quantities, as well as comparing with results where only one of the phases is present. Preliminary simulation of froth flotation, where the drops stick to a bubble and accumulate in a foam at the top are also shown. [Preview Abstract] 

J10.00009: Bubble migration and coalescence in rotating flows Wen Liu, Jiacai Lu, Gretar Tryggvason The collective buoyant motion of bubbles in a fluid rotating around a vertical axis is studied by numerical simulations, using a front tracking method to capture the interface between the gas and the liquid. The fully threedimensional flow is resolved using stretched grids, allowing us to simulate large domains that are well resolved near the rotation axis. The computational domain rotates with the fluid, giving rise to fictitious forces (centrifugal and Coriolis) due to a noninertial frame, and is periodic in the vertical direction. We verify the numerical setup and the numerical approach by comparisons with published studies of single bubbles and drops. The bubbles are driven to the axis of rotation at a rate that depends on the problem parameters, sometimes forming a gas column if they coalesce. The bubbles also rise due to buoyancy and we examine how the relative strength of rotation and buoyancy affects the dynamics. We identify how the dynamics depends of the governing numbers, identify the transition boundaries, and propose a flow pattern map. [Preview Abstract] 

J10.00010: Effects of Weber number on the interaction of single and multiple bubbles with a vortex ring Subhajit Biswas, Raghuraman N Govardhan Bubbly turbulent flows are ubiquitous in nature and in such flows, bubbles interact with vortical structures in the flow making the interaction complex. Driven by the motivation to understand these complex interactions, we experimentally study an idealization of this, namely, the interaction of a vortex ring in water with single and multiple air bubble(s). We are interested in the bubble dynamics and in modifications to the ring core’s vorticity due to the bubble, the former measured using high speed imaging, and the latter with timeresolved PIV. An important parameter governing this interaction is a Weber number ($We$), which is defined using the ring’s circulation and the bubble diameter. Prior results show a significant fragmentation of the vortex core at low $We$ (Narsing \& Govardhan, APS 2015). In the present work, we take a step closer to the bubbly turbulent flow case, by investigating the interaction of the ring with more than one bubble. In this multiple bubbles case, the interactions with the ring shows richer dynamics, as seen in the capture of the bubble by the ring, and the bubble’s subsequent elongation within the ring. The details of these interactions, including both differences and similarities with the single bubble case, will be presented at the conference. [Preview Abstract] 

J10.00011: Bubble Trapping in TwoPhase Wakes from Upward Adiabatic LiquidGas Flow Around a Cylinder Dohwan Kim, Matthew Rau The twophase wakes that form downstream of a cylinder in crossflow with a liquidgas mixture contain complex distributions of singlephase and concentrated bubbly flow. We conducted a highspeed visualization experiment to characterize these wakes using a waterair mixture and liquid Reynolds numbers (\textit{Re}) ranging from 100 to 3,000, based on a cylinder diameter of 9.5 mm, and air superficial velocities ranging from 0.061 to 0.614 m/s. By introducing a low concentration of isopropyl alcohol into the water, we altered the surface tension of the solution to create mean air bubble diameters of 4, 1, and 0.5 mm. We calculated the mean bubble diameter and bubble velocity using image processing methods and Particle Tracking Velocimetry (PTV). The mean shadow fraction and timeaveraged PTV results showed that the singlephase wake region decreased in size with increasing \textit{Re}. The same results suggested that bubbles were trapped in the wake as \textit{Re} exceeded 2,000; however, bubble Stokes numbers did not consistently predict the trapping phenomenon. Instead, the lifttodrag ratio, which compares a ratio of inertial to buoyancy forces and a ratio of trapping velocities, consistently predicted the occurrence of bubble trapping throughout the experimental parameter ranges. [Preview Abstract] 

J10.00012: A Discrete Element Method for Rectified Bubble Motion Mark Ferraro, Timothy Koehler, Scott Roberts, John Torczynski A bubble in a vibrating liquidfilled housing experiences a Bjerknes force that can create a net (rectified) downward motion of the bubble against gravity. Here, we consider a simplified discrete element method which treats each bubble as a compressible, spherical particle with a diameter determined by the local pressure. The Bjerknes force is computed from the oscillating pressure field. Bubble motion is determined from the balance of the Bjerknes, buoyancy, and drag forces (here, Stokes' Law). We show results of both singlebubble and multibubble simulations for various frequencies, forcing amplitudes, bubble size distributions, and gas volume fractions. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DENA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. [Preview Abstract] 

J10.00013: An experimental study on the unsteady characteristics of a bubble plume Hyunseok Kim, Hyungmin Park Despite increasing importance of the unsteady flow characteristics to understand bubble plume dynamics, most of the previous studies have mainly focused on averaged parameters and 1D integral plume model. In the present study, we measured velocities for both gas and liquid phases at high volume flow rate and high volume void fraction (\textgreater 5{\%}) using laser doppler anemometry and digital image processing, and investigated the turbulence characteristics of bubble plume. We considered two types of bubble plume: regular bubble plume and irregularly pulsating bubble plume (same volume flow rate with regular bubble plume but shows more dynamic motion). In the conventional 1D plume model, bubble plume motion was treated as being only determined by gas flow rate. However, we found that at small injection area, bubble plume shows more irregular and dynamic motion and it determines the induced liquid flow. With more irregular bubble plume motion (higher fluctuation velocities of gas phase), liquid flow shows a slower development and higher turbulence kinetic energy. Also, we examined two important spatial length scales, radius of bubble plume and induced liquid plume. While mean and rootmeansquared velocities were scaled by induced liquid plume radius, integral time scale and tendency of velocity spectra described readily by scaling with bubble plume radius. [Preview Abstract] 

J10.00014: Experimental analysis of supercritical CO$_{\mathrm{2}}$ assisted atomization Shadi Shariatnia, Dorrin Jarrahbashi Dissolution of supercritical fluids in liquids introduces gas bubbles upon depressurization due to injection and the expansion and burst of these bubbles facilitate the atomization. We experimentally study the atomization behavior of water with dissolved CO$_{\mathrm{2}}$ above its critical pressure and temperature injected into ambient condition. To elucidate the effects of gas solubility, interfacial tension and injection parameters on the promotion of dissolved supercritical fluid atomization, we repeat the experiments with injection of water and dissolved N$_{\mathrm{2}}$. Highspeed imaging and laser diffraction are used to understand the effects of flow parameters: injection pressure, temperature, gastoliquid ratio and axial distance from the injector on jet breakup and droplet sizes. The CO$_{\mathrm{2}}$ atomized droplets are smaller and distributed over a narrow span compared to that of N$_{\mathrm{2}}$. Two combined phenomena explain the enhanced atomization of waterCO$_{\mathrm{2}}$: the solubility of N$_{\mathrm{2}}$ in water above its supercritical condition is substantially lower than CO$_{\mathrm{2}}$ and the interfacial tension of CO$_{\mathrm{2}}$water is much lower compared to N$_{\mathrm{2}}$water at the same condition. A novel predictive model of droplet sizes is developed for a wide range of flow conditions. [Preview Abstract] 

J10.00015: Bubble induced destratification in unconfined fluids Maathangi Ganesh, Sadegh Dabiri Rising motion of bubbles in density stratified fluids leads to destratification effects. In order to study this, DNS of hundreds of bubbles in unbounded stratified fluids is carried out using the finite volume/fronttracking method. Bubble dynamics, including the bubble rise velocities, bubble velocity fluctuations, temporal correlations and bubble dispersion is studied. Importance of bubble Reynolds number and deformability is presented. Bubble motion is seen to cause bubbleinduced turbulence (BIT) in the liquid, which is the driving mechanism for mixing and subsequent destratification. BIT is quantified by examining the liquid fluctuations and turbulent kinetic energy. The dependence of mixing efficiency on the voidfraction, stratification strength, Eotvos and Reynolds numbers is also presented. Highly deformable, high Reynolds number bubbles undergo path instabilities and give rise to higher levels of mixing. An increase in buoyancy flux across pycnoclines is observed as void fraction increases. A similar increase in vertical mass flux is observed for a decrease in stratification strength. Flow pattern around the bubbles and the bubble microstructure are studied. It is shown that the extent of mixing is heavily dependent on the bubble dynamics. [Preview Abstract] 
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