Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G28: Bubbly Flow I |
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Chair: Alison Hoxie, University of Minnesota Room: 610 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G28.00001: Bubble induced mixing in stratified fluids within a confined domain Maathangi Ganesh, Sang Kyu Kim, Sadegh Dabiri A numerical study of mixing in temperature stratified fluids induced by the motion of monodispersed swarm of bubbles in a thin gap is carried out. The confinement prevents turbulent production and mixing occurs primarily due to transport by the bubble wake. The confinement also causes the bubbles to be flattened between the solid walls, with a gap to diameter ratio of 0.31. Bubbles entrain liquid from the lower, high-density layers and release it in the low density regions, thus causing mixing between the different liquid layers. Simulations are run for void fractions between 3.35{\%} and 13.4{\%}. The strength of stratification is varied by changing the Froude number between 4.5 and 12.74. We observe a zigzag motion of the bubbles attributed to the periodic vortex shedding behind the bubbles, quantified by the bubble velocity autocorrelations. We report the formation of horizontal clusters, the sizes of which are quantified by the cluster size index, and establish a qualitative correlation between the size of the clusters and the rise velocity of the bubbles. The mixing is characterized by three parameters namely, the COX number, diapycnal eddy diffusivity and mixing efficiency. We report an increase in the buoyancy flux across the isopycnals as void fraction increases. The fraction of energy production due to the buoyancy flux increases with the strength of stratification, giving rise to a higher mixing efficiency. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G28.00002: Explaining Hydrodynamics- mass transfer interplay in a bubble column column using bubble size distribution Manas Manohar Mandalahalli, Johan Nienhuis, Luis Manuel Portela, Robert Frank Mudde Gas-liquid mass transfer in bubbly flows is strongly coupled with its hydrodynamics, an interdependency relevant for many industrial applications. Shrinking bubbles and/or presence of dissolved contaminants influence both transport processes and their interrelation; this makes understanding the interplay quite challenging. In our experimental work, we study the influence of dissolved electrolytes (up to 1M NaCl) on CO$_{2}$ mass transfer in a homogeneous rectangular bubble column, up to 7\% gas fraction. Bubble size distribution, rise velocity and gas fractions are measured by high-speed imaging and digital image processing, while the liquid CO$_{2}$ concentration by monitoring pH variation. Coalescence inhibition, due to the electrolyte, and dissolving CO$_{2}$ both lead to a bimodal bubble size distribution with a lower mean bubble diameter, when compared to an ideal N$_{2}$-water system. Besides an increment in the interfacial area for the mass transfer, the bimodal distribution leads to bubble plume oscillations and recirculation zones in the column; dynamics of the shrinking bubble plume further influences mixing in the column. Our results strongly indicate that the mass transfer can be explained by the influence of the bubble size distribution on the hydrodynamics. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G28.00003: Shaping stress fields with spreading bubble streams: applications for biofouling prevention James Bird, Mark Menesses, C. Frederik Brasz, Jesse Belden Hard fouling organisms require significant shear stresses to be removed from a ship hull. Recently it has been shown that more frequent grooming requires less shear stress to remove these fouling organisms. By introducing a stream of air bubbles beneath a submerged surface, one can prevent marine biofouling through a continuously applied average shear stress of approximately 0.01 Pa; however, recent studies have shown maximum shear stresses applied by a bubble can be on the order of 200 Pa. Additionally, it has been shown that the period between critical shear stress events rather than an average stress value may be significant in determining the recruitment of settling marine organisms. Considering these recent developments, we investigate the effect of varying aeration flow rate on the antifouling ability of bubble streams. We find that by decreasing the frequency of bubble production the extent of biofouling prevention also decreases. Using a combination of field data, multiphase PIV, and integral plume theory, we find the shape of the areas influenced by the bubble plumes is related to the lateral spreading of bubbles as they rise. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G28.00004: Interaction of a vortex ring with single and multiple air bubbles Subhajit Biswas, Raghuraman N Govardhan Bubbly turbulent flows occur in many places such as in chemical reactors and in geophysical applications, besides interest from the drag reduction perspective using injected bubbles in a (water) boundary layer. In these flows, the bubbles interact with vortical structures in the flow, this being a two-way coupled interaction. Motivated by such complex interactions of bubbles with vortical structures in turbulent flows, we experimentally study an idealization of this, namely, the interaction of a vortex ring (in water) with single and multiple air bubbles. The main parameters in this case are the ring strength, and a ratio of the bubble volume to ring core volume. In these interactions, we are interested in both the vorticity dynamics and the bubble dynamics, and these are measured using time-resolved PIV and high speed shadowgraphy, respectively. We start with the interaction of a single bubble with a single vortex ring, followed by studies with two or many bubbles (bubble swarm), the latter being closer to the case of bubbly flows. The similarities and differences from the vorticity and bubble dynamics perspectives for the different cases will be presented at the conference. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G28.00005: Direct numerical simulation on the effect of slip velocity in bubble plumes Hyunduk Seo, Sivaramakrishnan Balachandar, Kyung Chun Kim Bubbly plume is one of the examples of multi-phase flow which is not confined by the sidewall. The bubbly plume has been applied to enhance the mixing for aeration or reaction between the gas and liquid phases. Air-water plume has higher slip velocity rather than other combinations of fluids. In this study, two-phase fluids governing equations are solved by DNS using the spectral element method code Nek5000 with Eulerian-Eulerian approach. To investigate the role of slip velocity in forming a plume structure, we calculated higher-order statistics such as turbulence kinetic energy budget. Not only higher-order statistics but also plume parameters in the conventional integral framework were calculated to quantify and characterize the plume structure. From the statistics, higher slip velocity results in a sharp interface between the inner core region and the outer annular region of the plume. Plume core region with high slip velocity is confined as a column of gas-phase which cannot be dispersed from the centerline. In higher slip velocity cases, TKE production by the buoyancy perturbation happened in the smaller region rather than smaller slip velocity case, but enhanced mean shear around inner plume core has a dominant contribution on the production of TKE. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G28.00006: Numerical Simulation of Bubbly Flow and Underwater Acoustics under Breaking Waves based on a Coupled Resolved and Subgrid Scale Bubble Method Qiang Gao, Grant Deane, Lian Shen Bubbles in breaking waves play an important role in many oceanography processes, including the bubble-mediated air-sea gas transfer, production of ambient wave noise, and marine aerosol generation. To study the bubble entrainment and breaking wave acoustics generation processes, we developed a coupled resolved and subgrid scale bubble simulation method. For bubbles greater than the grid size, which are called resolved bubbles, we use a coupled level set and volume of fluid method to captured them directly. A parallel, multi-block bubble identification and tagging method is adopted to extract bubbles using both the level set and volume fraction functions. Bubbles smaller than the grid size, which are called subgrid scale bubbles, are treated with a four-way coupled polydispersed bubble model. Bubble-liquid interaction is accounted for by interfacial forces and void fraction. An underwater noise model is implemented for the generation of noise by wave breaking. It was found that our numerical method can capture the bubble size spectrum and wave noise accurately compared with experimental observations. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G28.00007: Spray Atomization Using Bubbles Generated by a Two-Phase Counterflow Mixing Layer. Alison Hoxie, Eric Johnson, Vinod Srinivasan, Peter Rohrbach, Suo Yang, Krishna Bavandla, Hongyuan Zhang In this study, we employ the well-established phenomenon of the onset of global modes and concomitant rapid breakdown of certain shear layer configurations to design an efficient two-fluid mixer. Low-density jets and countercurrent mixing layers exhibit strong global modes and elevated turbulence levels, leading to rapid mixing, which is relatively insensitive to the viscosity-mediated mean shear stresses. By arranging a liquid and an atomizing gas (air) to satisfy the requirements for the onset of global modes, we are able to demonstrate efficient atomization. It is hypothesized that the observed droplet diameter is proportional to the diameter of bubbles that are created in a two-phase mixing layer inside the atomizer, which depends on the wavelength of the unstable global mode. The droplet data are shown to be relatively insensitive to viscosity. To partially test the hypothesis, Direct Numerical Simulations (DNS) are carried out using Eulerian-Eulerian Volume of Fluid (VOF) approach. Gas and liquid are considered as compressible fluids with perfect gas and perfect fluid equations of state, respectively. Schiller Naumann interphase drag model is used to capture the dynamics of gas bubbles in liquid, and surface tension is considered. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G28.00008: Effect of Probe Inserts on the Local Void Fraction in a Bubble Column Arushi Tiwari, Thomas J. Burtnett, Theodore J. Heindel Bubble columns are found in many process industries where a gas is bubbled through a liquid to promote mixing, separation, and/or reactions. The local void fraction (also referred to a gas holdup or volumetric gas faction) is an important parameter used to quantify flow conditions, and is one measure used to validate computational fluid dynamic simulations. The local void fraction is commonly measured with an inserted probe, but the presence of a probe can also modify local flow conditions and local void fraction. This study uses X-ray computed tomography to quantify the effect different probe geometries have on the time-average local void fraction. The results indicate the local void fraction is slightly higher when probes are inserted into the bubble column, but the overall void fraction is unaffected. [Preview Abstract] |
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