Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L33: Convection and Buoyancy Driven Flows: Special TopicsConvection

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Chair: Ali Tohidi, University of Maryland College Park Room: 106 
Monday, November 20, 2017 4:05PM  4:18PM 
L33.00001: Effect of thermally inertial particles on heat transport in RayleighB\'{e}nard convection Kim Alards, Rudie Kunnen, Federico Toschi, Herman Clercx We track particles that experience both mechanical and thermal inertia in direct numerical simulations of RayleighB\'{e}nard convection (RBC), a fluid layer heated from below and cooled from above. Both particles and fluid exhibit thermal expansion. The particles have a larger thermal expansion coefficient than the fluid, such that particles become lighter than the fluid near the hot bottom plate and heavier than the fluid near the cold top plate. First we investigate how the dynamics of thermal expansion affect the distribution of particles in the RBC cell. We find a regime of viscous and thermal response times where the concentration of particles at the plates is enhanced. A particle deposited on a plate resuspends after a characteristic residence time, that depends on the thermal response time. Now that we found a mechanism driving particles towards the plates, while also enforcing a motion back to the bulk, we include mechanical and thermal twoway coupling and investigate how thermally responsive particles affect flow structures and heat transfer in RBC. Ultimately, we want to explore the possibility to enhance heat transfer using these thermally inertial particles. [Preview Abstract] 
Monday, November 20, 2017 4:18PM  4:31PM 
L33.00002: Numerical simulation of particle settling through a sharp density interface ChenYen Hung, YiJu Chou An EulerianLagrangian model is used to simulate sedimentation of suspended particles in stratified environments. Double diffusive convection is found when density stratification is due to heat. While suspended particles with infinitesimal settling velocities act as a slowlydiffusive agent, both the velocity and concentration profiles exhibit symmetrical patterns with respect to the density interface, as those found in the classical saltfinger problems. In the case of the saltinduced background stratification, a layer of reverse buoyancy forms, particularly when particles are coarse. This leads to RayleighTaylor instability, while the resulting rising bubbles are dissipated due to the presence of the background density interface, which acts like a solid boundary. It further results in a significant horizontal flow near the density interface, leading to the sheetlike pattern of sinking sediment plumes. [Preview Abstract] 
Monday, November 20, 2017 4:31PM  4:44PM 
L33.00003: The effect of particle inertia on buoyancydriven instabilities in particleladen flows Sara Nasab, Pascale Garaud Modeling particles in a fluid has been approached by a number of methods. One of the simpler models used treats both particles and fluid as a continuum, a singlefluid model. Particle motion is derived under the assumption that particle drag and gravity are in equilibrium, neglecting particle inertia. Applicable to dilute systems consisting of small particles, the singlefluid model is not valid in many practical settings, where particles and fluid are more weakly coupled. Here we consider a twofluid model in which particle inertia is included, by solving for fluid motion and particle motion, separately. Coupling between the two fluids is due to drag. We investigate the buoyancydriven instabilities that this twofluid model has, focusing on behavior that deviates from the singlefluid model. Our results are obtained using linear stability analysis. Direct Numerical Simulations are used to study the nonlinear saturation of the instabilities. [Preview Abstract] 
Monday, November 20, 2017 4:44PM  4:57PM 
L33.00004: Optimal conditions for particlebubble attachment in flotation: an experimental study Aaron Sanchez Yanez, Jose Federico Hernandez Sanchez, Sigurdur T. Thoroddsen Mineral flotation is a process used in the mining industry for separating solid particles of different sizes and densities. The separation is done by injecting bubbles into a slurry where the particles attach to them, forming floating aggregates. The attachment depends mainly on the bubbles and particles sizes as well as the hydrophobicity and roughness of the particles. We simplified the collective behavior in the industrial process to a single free particlebubble collision, in contrast with previous studies where one of the two was kept fixed. We experimentally investigated the collision of spherical solid particles of a fixed diameter with bubbles of different sizes. By controlling the initial relative offset of the bubble and the particle, we conducted experiments observing their interaction. Recording with two synchronized highspeed cameras, perpendicular to each other, we can reconstruct the tridimensional trajectories of the bubble, the solid particle, and the aggregate. We describe the conditions for which the attachment happens in terms of dimensionless parameters such as the Ohnesorge number, the relative particlebubble offset and the hydrophobicity of the particle surface. We furthermore investigate the role of the surface roughness in the attachment. [Preview Abstract] 
Monday, November 20, 2017 4:57PM  5:10PM 
L33.00005: Investigation of the NonIsothermal Convective Mixing of Turbulent, Round, Wall Jets Paul Kristo, Mark Kimber The wall jet has become a paradigm for geometrically bounded flows due to the intrinsically difficult nature of the advection promoted by the geometry of the jet, coupled with prompt diffusion from the adjacent wall. Previous experimental investigations have sought to characterize the hydraulic and thermal behavior of such flows, however the physics promoted by parallel coplanar round jets has received inadequate experimental attention. The current effort is comprised of three parallel, coplanar, equidistant round jets issuing vertically downward into a pseudounconfined test section. The outer diameters of the jets are placed tangentially along a smooth flat plate. Nonintrusive optical techniques are incorporated for both hydraulic and thermal observations. Preliminary tests provide accurate inlet boundary conditions for each case. Reference metrics are captured during testing to account for ambient effects and readings inside of the test section. By varying the velocity and temperature inlet parameters, insights are drawn regarding the effects on the merging point (MP) and combined point (CP) of both the flow and thermal fields. Velocity fields in the plane normal to the wall yield additional insight into the deceleration caused by dissipation from both the plate and surrounding stagnant fluid. [Preview Abstract] 
Monday, November 20, 2017 5:10PM  5:23PM 
L33.00006: Computational Study of a VortexRing Pair Interacting with a ConstantTemperature Heated Wall Hussam Jabbar, Ahmed Naguib Impinging jets are used widely in industrial and manufacturing processes because of their ability to increase the heat transfer rate from the impingement surface. The vortical structures of these jets have an important influence on the heat transfer; by affecting the thermal boundary layer (TBL) during their interaction with the wall. In order to better understand the physics of this interaction, particularly when pairing of two vortices happens near the wall, a simplified model problem of two isolated vortex rings interacting with a flat wall is investigated computationally using ANSYS FLUENT 17.1. Observations of the vorticity field, the temperature field, the wall shear stress, the TBL and the Nusselt number (\textit{Nu}) provide insight into the association of local \textit{Nu} maxima/minima with different flow features. The results provide physical understanding of the flow processes leading to enhancement/deterioration of \textit{Nu} due to vortexwall interaction. Additionally, the characteristics of the vortical structures are quantified, and possible correlations between the temporal development of these characteristics and the evolution of the maximum/minimum \textit{Nu} are investigated. The results are compared to those involving a single vortex ring in order to understand the effect of vortex pairing. [Preview Abstract] 
Monday, November 20, 2017 5:23PM  5:36PM 
L33.00007: Source and boundary condition effects on confined vertically distributed turbulent plumes Nigel Kaye, Paul Cooper Recent experiments into the behavior of an enclosed vertically distributed source of buoyancy have shown that the plume partially detrains in the stratified region of the enclosure. This detrainment is not observed for enclosed constant buoyancy flux plumes. While models have been proposed to quantify the detrainment process it is still unclear why vertically distributed buoyancy sources detrain while constant buoyancy flux plumes do not detrain in the same physical geometry. One difference between distributed and localized sources is that the impact of nonideal source conditions (i.e. where mass as well as buoyancy is added at the source) is distributed over the whole height of the enclosure for a vertically distributed source. Another difference is the presence of a solid boundary along the plume source leading to a retarding boundary shear stress. Herein the impact of nonideal source conditions on a vertically distributed plume are analyzed and it is shown that, at any height, either the plume volume flow rate is significantly influenced by the wall source volume flux or the wall source buoyancy is greater than the mean plume buoyancy creating a nonselfsimilar horizontal buoyancy distribution in the plume. The impacts of source and boundary effects on previously published experiments of vertically distributed plumes are reviewed and the possible implications for plume detrainment are discussed. [Preview Abstract] 
Monday, November 20, 2017 5:36PM  5:49PM 
L33.00008: Crossflow shearing effects on the trajectory of highly buoyant bentover plumes Ali Tohidi, Nigel Berkeley Kaye, Michael J. Gollner The dynamics of highly buoyant plumes in crossflow is ubiquitous throughout both industrial and environmental phenomena. The rise of smoke from a chimney, wastewater discharge into river currents, and dispersion of wildfire plumes are only a few instances. There have been many previous studies investigating the behavior of jets and highly buoyant plumes in crossflow. So far, however, very little attention has been paid to the role of shearing effects in the boundary layer on the plume trajectory, particularly on the rise height. Numerical simulations and dimensional analysis are conducted to characterize the near and farfield behavior of a highly buoyant plume in a boundary layer crossflow. The results show that shear in the crossflow leads to large differences in the rise height of the plume in relation to a uniform crossflow, especially at farfield. [Preview Abstract] 
Monday, November 20, 2017 5:49PM  6:02PM 
L33.00009: A Virtual Study of Grid Resolution on Experiments of a HighlyResolved Turbulent Plume Pietro M. F. Maisto, Andre W. Marshall, Michael J. Gollner An accurate representation of subgrid scale turbulent mixing is critical for modeling fire plumes and smoke transport. In this study, PLIF and PIV diagnostics are used with the saltwater modeling technique to provide highlyresolved instantaneous field measurements in unconfined turbulent plumes useful for statistical analysis, physical insight, and model validation. The effect of resolution was investigated employing a virtual interrogation window (of varying size) applied to the highresolution field measurements. Motivated by LES lowpass filtering concepts, the highresolution experimental data in this study can be analyzed within the interrogation windows (i.e. statistics at the subgrid scale) and on interrogation windows (i.e. statistics at the resolved scale). A dimensionless resolution threshold (L/D*) criterion was determined to achieve converged statistics on the filtered measurements. Such a criterion was then used to establish the relative importance between large and smallscale turbulence phenomena while investigating specific scales for the turbulent flow. First order data sets start to collapse at a resolution of 0.3D*, while for second and higher order statistical moments the interrogation window size drops down to 0.2D*. [Preview Abstract] 
Monday, November 20, 2017 6:02PM  6:15PM 
L33.00010: Laboratory Layered Latte Nan Xue, Sepideh Khodaparast, Lailai Zhu, Janine Nunes, Hyoungsoo Kim, Howard Stone Layered composite fluids are sometimes observed in confined systems of rather chaotic initial states, for example, layered lattes formed by pouring espresso into a glass of warm milk. In such configurations, pouring forces a lower density liquid (espresso) into a higher density ambient, which is similar to the fountain effects that characterize a wide range of flows driven by injecting a fluid into a second miscible phase. Although the initial state of the mixture is complex and chaotic, there are conditions where the mixture cools at room temperature and exhibits an organized layered pattern. Here we report controlled experiments injecting a fluid into a miscible phase and show that, above a critical injection velocity, layering naturally emerges over the time scale of minutes. We perform experimental and numerical analyses of the timedependent flows to observe and understand the convective circulation in the layers. We identify critical conditions to produce the layering and relate the results quantitatively to the critical Rayleigh number in doublediffusive convection, which indicates the competition between the horizontal thermal gradient and the vertical density gradient generated by the fluid injection. Based on this understanding, we show how to employ this singlestep process to produce layered structures in soft materials, where the local elastic properties as well as the local material concentration vary stepwise along the length of the material. [Preview Abstract] 
Monday, November 20, 2017 6:15PM  6:28PM 
L33.00011: Moist, Doublediffusive convection Jeffrey Oishi, Keaton Burns, Ben Brown, Daniel Lecoanet, Geoffrey Vasil Doublediffusive convection occurs when the competition between stabilizing and a destabilizing buoyancy source is mediated by a difference in the diffusivity of each source. Such convection is important in a wide variety of astrophysical and geophysical flows. However, in giant planets, doublediffusive convection occurs in regions where condensation of important components of the atmosphere occurs. Here, we present preliminary calculations of moist, doublediffusive convection using the Dedalus pseudospectral framework. Using a simple model for phase change, we verify growth rates for moist double diffusive convection from linear calculations and report on preliminary relationships between the ability to form liquid phase and the resulting Nusselt number in nonlinear simulations. [Preview Abstract] 
Monday, November 20, 2017 6:28PM  6:41PM 
L33.00012: Threedimensional doubly diffusive convectons: instability and transition to complex dynamics Edgar Knobloch, Cedric Beaume, Alain Bergeon Doubly diffusive convection in a closed vertically extended 3D container driven by competing horizontal temperature and concentration gradients is studied. Noslip boundary conditions are imposed. The buoyancy number $N=1$ to ensure the presence of a conduction state. The primary instability is subcritical and generates two families of spatially localised steady states known as convectons. The convectons bifurcate directly from the conduction state and are organized in a pair of primary branches that snake within a welldefined range of Rayleigh numbers as the convectons grow in length. Secondary instabilities generating twist result in secondary snaking branches of twisted convectons. These destabilize the primary convectons and are responsible for the absence of stable steady states, localized or otherwise, in the subcritical regime. As a result, once the Rayleigh number for the primary instability of the conduction state is exceeded, the system exhibits an abrupt transition to large amplitude spatiotemporal chaos that arises whenever the twist instability leading to collapse is faster than the nucleation time for new rolls. These numerical results are confirmed by determining the stability properties of all convecton states as well as spatially extended convection. [Preview Abstract] 
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