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 H13: Convection and Buoyancy-driven Flows: General |
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Chair: Thomas Solano, Florida State University Room: 304 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H13.00001: Caf\'{e} Latte: Spontaneous layer formation in laterally heated double diffusive convection: Numerical simulations Kai Leong Chong, Rui Yang, Roberto Verzicco, Detlef Lohse The experiments of N. Xue et al., Nat. Commun., 8, 1960 (2017) have shown that the well-known layer formation in Caf\'{e} Latte is due to double diffusive convection, with the hot milk injected into the coffee cooling down to the side. Here we perform a corresponding numerical investigation of laterally cooled double diffusive convection where the fluid flow is driven by a horizontal temperature difference and stabilized by a vertical concentration difference.~ When the stabilization caused by the concentration field is too weak, the flow behaves like standard vertical convection in which there is a large-scale circulation throughout the domain. However, upon increasing stabilization, one observes the spontaneous formation of layers where there are several vertically-stacked circulations and every circulation forms a well-mixed layer. As time evolves, the two neighbouring layers can merge into one layer whenever there is strong enough flow that can break the interface in between. Finally, we analytically calculate the average initial height of the layers and find good agreement with our numerical data. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H13.00002: Effects of Prandtl number on the formation and evolution of single laminar plume. Pei-Jiang Qin, Ze-Xu Li, Lin Sun, Shi-Di Huang In this work, the formation and evolution processes of single laminar plume is studied over the Prandtl (Pr) number range of 50\textasciitilde 1000 by using shadow visualization technology. The heating power input is shut down immediately once a single plume is formed. It is found that the plume formation time increases with Pr number increasing, and the exponent of Pr-dependent power law decreases as the flux Rayleigh number Ra$_{\mathrm{F}}$ (i.e. the heating power) increases. After its formation, the plume first undergoes an accelerated ascending process, and then decelerates after reaching a maximum velocity, without a uniform ascending region as observed in previous studies. Both the maximum velocity and acceleration are decreased as Pr number increases, and their Pr number dependencies become weaker for larger Ra$_{\mathrm{F}}$ numbers. We also examine how the spanwise width of single laminar plume changes during its rising process, and the results are compared with previous studies. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H13.00003: Propagating fronts through convective flow fields with cooperative and antagonistic feedback Saikat Mukherjee, Mark Paul We numerically explore propagating fronts generated by an exothermic and autocatalytic reaction where the products and reactants can vary in density. We study fronts traveling horizontally in a long, shallow, and two-dimensional layer of fluid undergoing thermal convection. In the absence of heat generation by the reaction, any variation in density between the products and reactants results in a single solutally-driven convection roll that propagates with the front. In the presence of heat release by the reaction, a hotspot at the front is generated which leads to the formation of a pair of counter-rotating convection rolls that travel with the front. When the products are less dense than the reactants, the thermal and the solutal effects are cooperative. When the products are more dense than the reactants, the thermal and solutal effects are antagonistic. We study fronts with cooperative and antagonistic feedback that travel through counter-rotating convection rolls generated by Rayleigh-B\'{e}nard convection. In the presence of convection, the front and fluid dynamics exhibit oscillatory dynamics. We quantify the fluid dynamics, front velocity, and reaction length scale over a wide range of solutal and thermal driving. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H13.00004: Conjugate Thermal Boundaries Effect On Natural Convection Flow Structures In Enclosures Tomas Solano, Juan Ordonez, Kourosh Shoele Buoyancy-induced flow for a heated sphere in a square enclosure coupled with external forced convection cooling of the enclosure walls is investigated numerically using an immersed boundary method and conjugate thermal boundary conditions. The external forced convection cooling, on the enclosure boundaries, is seen to significantly modify the internal convection recirculatory flow and thermal stratification depending on Rayleigh number and aspect ratio. The relation between the external Reynolds number and the internal Rayleigh number is reported in terms of streamlines, isotherms, local and average Nusselt numbers. Asymmetric and reversed flow is observed as the relation between external Reynolds and internal Rayleigh is changed. The coupling between the two modes of convection is presented as a means to control the convection pattern and vertical structures in an enclosure with heated components. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H13.00005: Thermal convection in a rotating liquid sphere with radial gravity as a direct function of the radius. Victor Huitron, Ruben Avila The thermal convection of the terrestrial planets without solid inner core, has been subject of research in the last decades. Some theories claim that at the beginning of the formation of the planets, the solid inner core was absent. This research is aimed to analyze the convective patterns and the heat transfer rate in a rotating liquid sphere by solving the non-steady, three-dimensional Navier-Stokes equations. In order to avoid the singularity at the center of the sphere, the set of equations for an incompressible fluid are formulated in a Cartesian coordinate system and solved by using the mesh-based h/p Spectral Element method. The thermal convection is driven by both a uniform internal energy source and a radial gravitation field that is directly proportional to the radius of the sphere. The effect of increasing the value of the Rayleigh Ra number and the Taylor Ta number, on the convective patterns and on the velocity, temperature and vorticity fields is presented. The local and average Nusselt numbers at the surface of the whole sphere are evaluated together with the temperature, vorticity and pressure fields. The obtained results are successfully compared with numerical solutions previously published in the literature. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H13.00006: Diagnosis of turbulence radiation interactions in turbulent premixed flames under high-pressure and high-Ka conditions Bifen Wu, Xinyu Zhao Predicting thermal radiation is crucial for prediction of wall heat transfer for aeronautical combustors. Intense turbulent fluctuations result in strong fluctuations of temperature and composition, which leads to highly nonlinear behavior in the radiative source terms that is conventionally referred to as to turbulence-radiation interactions (TRI). An a priori study of the TRI is performed using datasets from Direct Numerical Simulation (DNS) of strongly turbulent premixed n-dodecane/air flames with three Karlovitz numbers (Ka) ranging from 100 to 10000, under pressurized conditions that are relevant for engine operations. Here, a line-by-line database derived from HITEMP2010 and HITRAN2008 is employed to calculate the radiative properties of radiative species. The database is extrapolated to high-pressure conditions using the Voigt profile. The influence of various interactions in the radiative emission source term is investigated and their impact is evaluated in the context of large eddy simulations (LES) through explicit filtering with different filter sizes. Sub-grid scale TRI models of different fidelities, such as no-TRI, partially-TRI, are assessed. Different levels of soot volume fractions are also artificially introduced to investigate the influence of soot on TRI. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H13.00007: Weakly nonlinear stability analysis on a chemotaxis system with deformed free surface Symphony Chakraborty, Tony Wen-Hann Sheu Chemotaxis-convection-diffusion(CCD) have significant roles in medical, industrial, and geophysical areas, that is why research effort has been performed to understand the dynamics of the bacterial motility in suspension, studies through analytical, experimental, and numerical attempts previously were only for a flat free-surface of a suspension of chemotaxis bacteria in a shallow/deep chamber. We consider now a 3D CCD flow system with a deformed free-surface to explore the nature of instability by performing detailed stability analyses. Weakly nonlinear stability analysis has been carried out to determine the relative stability of the pattern formation at the onset of instability where Rayleigh number $Ra_\tau$ is the nonlinear control parameter. Nonlinear convection terms dominated the system beyond a critical $Ra_\tau$, which also depends on the critical wavenumber $k$ and Nusselt number $Nu_\tau$ as well as other parameters. We have investigated the issue of how the critical $Ra_\tau$ in this system varies with three different sets of parameters. Using the method of multiscales, a Ginzburg-Landau equation is derived from the Lorenz model (derived under the assumption of Bossinesq approximation), the solution of which helps to quantify the energy transport through $Nu_\tau$. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H13.00008: Linear thermal convection in an oscillatory fluid layer: A Floquet analysis of a sinusoidal, time-periodic basic flow Ruben Avila The onset of thermal convection of a Boussinesq fluid confined in a plane layer with harmonic oscillation is investigated. The boundaries of the layer are parallel to the x-y plane of the Cartesian coordinate system. The temperature of the lower boundary is higher than the temperature of the upper boundary. The fluid layer oscillates around the y axis with a given amplitude and frequency. The basic velocity profile is sinusoidal and time-periodic. The flow with a linear basic temperature profile, ascends close to the hot boundary and descends close to the cold boundary. The basic harmonic flow is obtained numerically, and introduced as an analytical expression in the linearized equations of the vertical velocity, vorticity and temperature. The non-steady linear equations are formulated in terms of the parameters (the wave number, and the Rayleigh, Taylor and Prandtl numbers) that govern the system. The linear equations are solved (for a fluid with Prandtl number equal to 0.7) by a collocation method that is based on the Chebyshev polynomials. By using the Floquet theory, the thermal instability of the basic harmonic flow is studied. Curves of the critical values of the Rayleigh number and the wave number, as functions of the other parameters of the system are shown. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H13.00009: Turbulent Mixing and Entrainment in a Buoyancy Driven Continuous Thermal Plume using Large-Eddy Simulations. Sudheer Reddy Bhimireddy, Kiran Bhaganagar Understanding the effects of extreme events such as wildland fires, volcanic eruptions are some of the critical issues the scientific community is facing now. For this purpose, a high-fidelity Large-Eddy Simulation has been developed to simulate thermal plumes with subcritical Froude's number in a 4 km x 4 km x 7 km domain under idealized conditions. A fundamental fluid dynamics analysis has been performed to quantify the mean and turbulent characteristics of the thermal plumes. Additional 2-D simulations performed revealed that a well-defined head exists and travels with nearly constant velocity as long as it is attached to the plume stem. Due to intense turbulent mixing, 3-D thermal plume does not exhibit a well-defined head. To quantify the mixing and entrainment, 1st principle control volume approach based on tracking the plume interface has been used in both 2-D and 3-D cases. Using this interface we also study the plume ascent rate and half-widths based on both radial velocity and buoyancy profiles. Entrainment in 3-D thermal plumes is found to be significantly higher than in 2-D. The primary mechanism entraining the ambient fluid in 3-D is due to the instabilities at interface, whereas in 2-D, the centerline vorticity is found to be responsible for the mixing. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H13.00010: Modeling of Round Buoyancy Driven Particle Clouds. Ali Alnahit, Nigel Kaye, Abdul Khan A numerical model was developed to investigate the dynamics of round buoyancy driven particle clouds in a quiescent ambient. The developed model was validated and then applied to a range of test cases including releases of positively/negatively buoyant particles. The cloud was modeled using the standard Morton et al. (1956) entrainment assumption, and the flow field induced by the cloud was approximated as a Hill's spherical vortex. The buoyancy of the cloud was calculated as the sum of the buoyancy contributed by all particles within the cloud. Individual particles were tracked using a particle tracking equation considering the forces acting on individual particles and the computed induced velocity field. Particle--particle interactions were modeled as both elastic and inelastic collisions. The turbulent dispersion of particles was also considered and estimated using a random walk model. The model was validated by comparing simulations with the experimental and numerical results of Wang et al. 2016. The limitations of the model were then discussed. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H13.00011: Buoyancy distribution in a filling box segmented by a planar jet Nigel Kaye, Nicholas Williamson Results are presented from a theoretical and experimental investigation of the buoyancy distribution in a filling box in which the box is divided by a planar jet. The planar jet acts as a barrier that inhibits the transfer of buoyancy from the plume side to the non-plume side of the jet. Filling box theory is used to model the layer depth and total buoyancy on either side of the planar jet. Although the planar jet initially prevents the plume outflow from spreading into the non-plume side there is still some transfer of buoyancy due to entrainment of buoyant fluid into the planar jet that is then transferred to the non-plume side. The theoretical model is validated against a series of small scale salt bath experiments. The model well predicts the initial distribution of buoyancy between the plume and non-plume sides of the jet. However, the build-up of buoyancy on the plume side eventually bends the jet toward the non-plume side and the jet no longer provides a barrier. The time at which the model breaks down is well predicted by air-curtain theory. The results are discussed in the context of the potential for air curtains to inhibit smoke spread in compartment fires and potentially allowing increased evacuation times for occupants. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H13.00012: Manipulation of Small-Scale Motions Induced by Self-Oscillating Reeds for Heat Transfer Enhancement Sourabh Jha, Ari Glezer Low Reynolds (\textit{Re}) number forced convection heat transport within the fin channels of air-cooled condensers are enhanced by deliberate formation of unsteady, small-scale vortical motions using aero-elastically fluttering thin-film reeds that span the channel height.~~These vortical motions substantially increase the local heat transfer coefficient at the channel walls and mixing between the wall thermal boundary layers and the cooler core flow.~~The flow mechanisms associated with production, advection and dissipation of these small-scale motions are investigated in a modular, high aspect ratio channel using micro-PIV, video imaging of the reed motion, and hot-wire anemometry.~~The global heat transfer enhancement is investigated in a modular heat sink prototype using temperature and pressure measurements.~~It is shown that the reed-induced small scale motions increase the turbulent kinetic energy of the flow even when the base flow undergoes transition to turbulence, leading to an increase in the local and global Nusselt number (\textit{Nu}) that is sustained at higher~\textit{Re}~and a minor relative increase in losses.~~While these losses depend primarily on the reed's oscillation Strouhal number (\textit{St}$=$\textit{fL/U}) which is determined by the reed's mass ratio ($M$*) and structural rigidity ($U$*), ~because the global~\textit{Nu}~depends only weakly~on \textit{St}, the losses associated with the presence of the reed can be strongly mitigated by reducing its~\textit{St}~while maintaining the heat transfer enhancement. [Preview Abstract] |
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