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 A14: Convection and Buoyancy-driven Flows: Multiphase |
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Chair: Raymond Shaw, Michigan Technological University Room: 307/308 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A14.00001: Heat flux in moist and cloudy Rayleigh-B\'enard convection Raymond Shaw, Subin Thomas, Prasanth Prabhakaran, Will Cantrell Heat flux in moist and cloudy Rayleigh-B\'enard convection consists of both sensible heat flux (temperature flux) and latent heat flux (water vapor flux and condensation/evaporation growth of cloud droplets). Under cloudy conditions, condensation of water vapor away from the boundaries is a source term for temperature and a sink term for water vapor. Thus, the condensation of water vapor reduces the mean water vapor concentration in the bulk and increases the mean bulk temperature. This in turn enhances the latent heat flux and lowers the sensible heat flux passing through the system. We investigate the effect of condensation rates in a cloudy Rayleigh-B\'enard convection system, using an experimentally validated LES model. We propose a modified heat flux that remains conserved under these conditions, and we explore the influence of varying cloud properties such as cloud droplet concentration (modulated via the aerosol properties) and liquid water mixing ratio. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A14.00002: Effects of the Large-Scale Circulation on Temperature and Water Vapor Distributions in the Michigan Tech $\Pi$-Chamber Jesse Anderson, Gregory Kinney, Prasanth Prabhakaran, Subin Thomas, Raymond Shaw, Will Cantrell In Rayleigh-B\'enard convection, it is well known that within the turbulent motion a mean flow forms, commonly referred to as the large-scale circulation. We report experimental results on the nature of this circulation and its impact on the temperature and water vapor fields in Michigan Tech's $\Pi$-chamber (Aspect ratio=2 and $Pr=0.7$) under dry, moist (no injection of aerosols) and cloudy conditions. The $\Pi$-chamber is designed to study aerosol-cloud interactions in a turbulent environment. These interactions are strongly influenced by the temperature and water vapor fields because they control the growth rates of each cloud droplet. The differential growth rates between droplets could result in a broadening of the cloud droplet distribution which is important for the onset of precipitation in clouds. We report various features of the circulation - an azimuthal oscillation, a sloshing mode, and a possible compression along the axis of the of the main roll. In addition, the temperature and water vapor concentration were measured and analyzed with respect to the orientation to the large-scale circulation. The distributions were found to be positively skewed along the updraft and negatively skewed along the downdraft. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A14.00003: A Lagrangian network-based analysis of evaporative droplets in Rayleigh-Benard convection Theo MacMillan, David Richter Rayleigh-Benard convection is characterized by different scales of coherent motion, from a large-scale circulation (LSC) to transient thermal plumes. To better understand the mixing behavior of evaporative droplets in such a system, we assemble a time-evolving weighted network from the droplets' individual trajectories using direct numerical simulation (DNS) with Lagrangian droplet tracking. By varying the time over which the network is assembled, we detect both the LSC and transient thermal plumes in the spectral gap of the network's Laplacian while accurately determining their respective time scales. Using tools developed for the analysis of complex graphs, we are able to objectively characterize the limiting cases of homogeneous (slow microphysics) and heterogeneous (fast microphysics) mixing and the interplay between short time-scale coherent structures and the LSC in the dynamics of the droplet ensemble. This has implications for understanding, for example, essential mechanisms of the formation of rain, including cloud entrainment and droplet spectra broadening processes such as stochastic condensation. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A14.00004: Dynamics of air bubbles in Rayleigh-Be ìnard Convection: pair dispersion and effect of initial separation Leonardo P Chamorro, Jin-Tae Kim, Jaewook Nam, Shikun Shen, Changhoon Lee Laboratory experiments were performed to uncover the dynamics of bubbles in Rayleigh-Benard (RB) convection at Ra$=$1.1x10$^{\mathrm{10}}$, where streams of 1-mm bubbles were released at various locations from the bottom of the RB tank along the path of the roll structure. 3D particle tracking velocimetry was used to track simultaneously a relatively large number of bubbles, and to quantify the pair dispersion for various initial separations in the range of 25$\le \eta \le $225, where $\eta $ is the local Kolmogorov length scale. Numerical simulation was carried out to further study the role of the bubble's path instability. Results show that the pair dispersion underwent a transition phase similar to the ballistic-to-diffusive (t$^{\mathrm{2}}$-to-t$^{\mathrm{1}})$ regime in the vicinity of the cell center; however, it approached to a bulk behavior t$^{\mathrm{1.5}}$ in the diffusive regime as the distance away from the cell center increased. At small initial separation, the pair dispersion exhibited t$^{\mathrm{1}}$ in the diffusive regime, indicating that the convective turbulence reduced the amplitude of the bubble's path instability. At large initial separations, the pair dispersion exhibited t$^{\mathrm{2}}$, showing the effect of the roll structure. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A14.00005: Formation of vortex rings and drops at particle-laden fronts in thermally stratified environments Chen-Yen Hung, Yi-Ju Chou We conduct numerical simulations to investigate the formation of drops and vortex rings at particle-laden fronts descending in density stratified environments. We show that the temporal evolution can be divided into double diffusion, acceleration, and deceleration phases. The acceleration phase is a result of the vanishing temperature perturbation in the drop during the descent in the layer of uniform temperature. The drop decelerates because it transforms into a vortex ring, whose motion follows the similarity assumption. A theoretical drag model is presented to predict the spherical drop speed with the low drop Reynolds number. In conjunction with the similarity argument for the motion of the vortex ring, our drag model shows good agreement in predicting the drag coefficient for the drop after the drop becomes spherical. Comparison of our drag model with simulations under various bulk conditions and previous experimental results shows good model predictability for the descending speed of drops. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A14.00006: ABSTRACT WITHDRAWN |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A14.00007: Self-sustained biphasic catalytic particle turbulence Ziqi Wang, Varghese Mathai, Chao Sun Turbulence is known for its ability to vigorously mix fluid and transport heat. While over a century of research for enhancing heat transport, few have exceeded the inherent limits posed by turbulent-mixing. Here we have conceptualized a kind of ``active particle'' turbulence machine: we find that by adding a minute concentration ($\phi_{v} \sim 1\%$) of a heavy liquid~(hydrofluoroether) to a water-based turbulent convection system, remarkably, high efficient biphasic dynamics is born, which supersedes turbulent heat transport by up to 500\%. The system is unique in that it operates on a self-sustained dynamically equilibrated cycle of a ``catalyst-like'' species, and exploits several heat-carrier agents including pseudo-turbulence, latent heat and bidirectional wake capture. We find that the heat transfer enhancement is dominated by the kinematics of the active elements and their induced-agitation. The present finding opens the door towards a new class of tunable, ultra-high efficiency heat transfer/mixing devices, with potential for major improvements in biochemical, nuclear, and process technologies, as well as in energy usage. [Preview Abstract] |
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