Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L19: Convection and Buoyancy-Driven Flows: Confined Spaces and Geometric Effects |
Hide Abstracts |
Chair: Gregory Chini, University of New Hampshire Room: 2006 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L19.00001: 2D stratified cavity flow under harmonic forcing Bruno Welfert, Juan Lopez, Stephanie Taylor Turbulence at the boundary of a stably stratified fluid region can penetrate deep into the region provided the turbulence contains sufficient energy at frequencies less than the buoyancy frequency. This phenomenon manifests itself in the form of internal waves at angles which depend on the perturbation frequencies. Here we consider a 2D lid-driven cavity flow with an imposed stable linear temperature gradient on the sidewalls and constant cold temperature on the bottom and constant hot temperature on the driven lid. In particular, we determine numerically the response to harmonic oscillations of the lid over a range of frequencies and identify resonances and the dynamics associated with their saturation. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L19.00002: Large-scale-circulation dynamics of turbulent Rayleigh-Benard convection in a cubic container Kunlun Bai, Dandan Ji, Eric Brown We present measurements of the large-scale-circulation (LSC) in turbulent Rayleigh-Benard convection in a cubic container. The experiments cover the Rayleigh number ranging from $0.5\times10^9$ to $3\times10^9$ at Prandtl number 6.4. Using three rows of thermistors at different height of the container, the large-scale-circulation (LSC) can then be identified. It is found that the LSC prefers to lock at the corners of the container, and switches between them stochastically. The strength of LSC keeps as a constant during the switching which suggests those switching correspond to fluctuation driven crossings of a potential barrier in $\theta$ due to the side wall geometry as predicted by Brown and Ahlers (Phys. Fluids, 2008). The switching frequency is found to decrease as Ra increases. The measured LSC orientation and its switching will be compared to the model which predicts the effects of container geometry on large-scale coherent structure dynamics for arbitrary geometry. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L19.00003: Influence of container shape on scaling of turbulent fluctuations in convection Najmeh Foroozani, Joseph J. Niemela, Vincenzo Armenio, Katepalli R. Sreenivasan We perform large-eddy simulations of turbulent convection in a cubic enclosure for Rayleigh numbers $1\times10^6\leq Ra \leq 1\times10^{10}$ and molecular Prandtl number, $Pr=0.7$. The simulations were carried out using a second-order-accurate finite-difference method in which subgrid-scale fluxes of momentum and heat were parametrized using a Lagrangian dynamic Smagorinsky model. The scalings of root-mean-square fluctuations of density and velocity in the cell center with $Ra$ differ significantly from those in cylindrical containers, and are in agreement with laboratory observations by Daya and Ecke [{\it Phys.\ Rev. Lett.} {\bf 87}, 184501 (2001)], also using a cell with square cross-section. We find that the {\it time-averaged} spatial distributions of the local heat flux and temperature fluctuations are inhomogeneous in the horizontal plane, associated with the forced orientation of the mean wind along either one or the other diagonal. Larger values of the steady-state density (temperature) gradients occur at the mid-plane corners of the diagonal opposite to that of the mean wind, due to the presence of strong counter-rotating circulations. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L19.00004: Experimental and numerical results for CO$_{2}$ concentration and temperature profiles in an occupied room Aline Cotel, Lars Junghans, Xiaoxiang Wang In recent years, a recognition of the scope of the negative environmental impact of existing buildings has spurred academic and industrial interest in transforming existing building design practices and disciplinary knowledge. For example, buildings alone consume 72{\%} of the electricity produced annually in the United States; this share is expected to rise to 75{\%} by 2025 (EPA, 2009). Significant reductions in overall building energy consumption can be achieved using green building methods such as natural ventilation. An office was instrumented on campus to acquire CO$_{2}$ concentrations and temperature profiles at multiple locations while a single occupant was present. Using openFOAM, numerical calculations were performed to allow for comparisons of the CO$_{2}$ concentration and temperature profiles for different ventilation strategies. Ultimately, these results will be the inputs into a real time feedback control system that can adjust actuators for indoor ventilation and utilize green design strategies. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L19.00005: How time-varying heating of a wall changes the stratification in a room Rachael Bonnebaigt, C.P. Caulfield, Paul Linden Building interiors often experience time-dependent heating of vertical surfaces, for example, through sunlight falling on walls. How do these heated surfaces change the temperature stratification in a room? We consider a vertically distributed source of buoyancy, in a sealed insulated space, that provides a linearly-varying-in-time (with slope $a$) buoyancy flux. This source drives a time-dependent flow: a plume rising up the wall, and return flow in the ambient. We solve the governing equations numerically, using Germeles's method (1975 {\it J. Fluid Mech.} {\bf 71} 601-623), but we allow the plume to detrain. We find that at small times, the ambient stratification profiles for rates of decrease of source buoyancy flux that are slower than a critical rate, $a_c < a < 0$, are qualitatively similar to those with $a > 0$, with the profiles getting steeper near the ceiling, while the profiles for $a < a_c < 0$ are qualitatively different, with the profiles getting shallower near the ceiling. We compare these predictions with analogue laboratory experiments. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L19.00006: Transient Flows and Stratification of an Enclosure Containing Both a Localised and Distributed Source of Buoyancy Jamie Partridge, Paul Linden We examine the transient flow and stratification in a naturally ventilated enclosure containing both a localised and distributed source of buoyancy. Both sources of buoyancy are located at the base of the enclosure to represent a building where there is a distributed heat flux from the floor, for example from a sun patch, that competes with a localised heat source within the space. The steady conditions of the space are controlled purely by the geometry of the enclosure and the ratio of the distributed and localised buoyancy fluxes $\Psi$ and are independent of the order buoyancy fluxes are introduced into the space. However, the order sources are introduced into the space, such as delaying the introduction of a localised source, alter the transients significantly. To investigate this problem, small-scale experiments were conducted and compared to a `perfect-mixing' model of the transients. How the stratification evolves in time, in particular how long it takes to reach steady conditions, is key to understanding what can be expected in real buildings. The transient evolution of the interior stratification is reported here and compared to the theoretical model. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L19.00007: Small scale properties of Rayleigh Benard convection in confined space Kai-Leong Chong, Matthias Kaczorowski, Ke-Qing Xia We report a direct numerical simulation (DNS) study on small scale properties of turbulent Rayleigh Benard convection (RBC) in highly confined configurations. Our simulations span a wide range of Rayleigh number (from 10$^{7}$ to 10$^{10})$ at Pr$=$0.7 and Pr$=$4.38. It is found that the cell's smallest dimension, characterized by the aspect ratio, introduces a cut off for the local Bolgiano length scale (evaluated in the bulk of the cell). This result may provide an opportunity for studying the cascade processes in the RBC system through a simply geometrical confinement. Another finding of the study is that the change in flow topology induced by confinement (decreasing aspect ratio) leads to more plumes entering the bulk, thus increasing the velocity and temperature fluctuations in the bulk until the merging of viscous and thermal boundary layers from sidewalls. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L19.00008: Aspect-Ratio-Dependent Upper Bounds for Two-Dimensional Rayleigh--B\'{e}nard Convection between Stress-Free Isothermal Boundaries Gregory Chini, Baole Wen, Charles Doering One of the central challenges in studies of Rayleigh--B\'{e}nard convection is the determination of the heat transport enhancement factor, i.e. the Nusselt number $Nu$, as a function of the Rayleigh number $Ra$, Prandtl number $Pr$, and domain aspect ratio $L$. Although the functional relation between $Nu$, $Pr$ and $Ra$ is usually presumed to be $Nu \sim Pr^\alpha Ra^\beta$ in the ``ultimate" high-$Ra$ regime, experiments and simulations have yielded different scaling exponents. Here, we investigate this scaling relationship for two-dimensional Rayleigh--B\'{e}nard convection between stress-free isothermal boundaries by computing rigorous upper bounds on the heat transport in domains of varying aspect ratio. Using a novel two-step algorithm (Wen et al. PLA 2013), we numerically solve the full ``background field" variational problem arising from the upper bound analysis of Whitehead \& Doering (PRL 2011) to obtain the optimal bound for $Ra \le 10^{10}$ as a function of $L$. Our results show that $Nu \le 0.106Ra^{5/12}$ at fixed $L = 2\sqrt{2}$ uniformly in $Pr$, confirming that molecular transport \emph{cannot} be neglected even at extreme values of $Ra$. Moreover, for large $Ra$, the aspect ratio has little impact on the bounds until the domain becomes sufficiently small. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L19.00009: Experimental study of free convection in a slender cell using PIV Fernando Aragon, Juan Casillas, Salomon Peralta, Mario Sanchez, Abraham Medina An experimental study of the steady free convection flow induced by a cylindrical heat source immersed in a slender cell for several values of Richardson and Raleigh numbers is undertaken using PIV in order to determine the corresponding velocity fields. The flow is set in motion under the action of a temperature differential (dT) that is induced between the heat source and the surrounding fluid. In the case in which such differential is positive (i.e. hot source) vertical ascending flow occurs, while in the case of a negative value of dT vertical descending flow takes place. Stream or close path trajectories occur, depending on the value of Ri and Ra numbers. Velocity fields and streamlines are presented for several values of Ri and Ra. Said results are compared with numerical models. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L19.00010: The effect of cell tilting on turbulent thermal convection in a rectangular cell Sheng-Qi Zhou, Shuang-Xi Guo, Xian-Rong Cen, Ling Qu, Yuan-Zheng Lu, Liang Sun, Xiao-Dong Shang In the study, the influence of cell tilting on flow dynamics is explored experimentally in a rectangular cell (aspect ratios $\Gamma_x = 1$ and $\Gamma_y = 0.25$). The measurements are carried out in a wide range of tilt angles ($0 \le \beta \le \pi/2$ rad) at Prandtl number ($Pr \simeq 6.3$) and Rayleigh number ($Ra \simeq 4.42 \times {10^9}$). With the velocity measurements, the large-scale circulation (LSC) is found to be sensitive to the symmetry of the system. In the level case, the LSC is at about quarter width of the cell. As the cell is slightly tilted ($\beta \simeq 0.04$ rad), the LSC moves quickly towards the boundary. With increasing $\beta$, the LSC changes gradually from oblique ellipse-like to square-like, and to more complicated patterns. Oscillation has been found for almost all $\beta$ and it is the strongest at around $\beta \simeq 0.48$ rad. With increasing $\beta$, the Reynolds number ($Re$) first increases till it reaches its maximum at the transition angle \(\beta = 0.15 \) rad, then it gradually decreases. A simple energy model is proposed to interpret the cell tilting on flow dynamics. It is predicted that the spatial distribution of the boundary layer affects the flow dynamics by varing the potential energy of system. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L19.00011: Buoyancy and Pressure Induced Flow of Hot Gases in Vertical Shafts with Natural and Forced Ventilation Yogesh Jaluria, Gunnar Olavi Tamm An experimental investigation was conducted to study buoyancy and pressure induced flow of hot gases in vertical shafts to model smoke propagation in elevator and ventilation shafts of high rise building fires. Various configurations were tested with regard to natural and forced ventilation imposed at the upper and lower surfaces of the vertical shaft. The aspect ratio was taken at a typical value of 6. From a lower vent, the inlet conditions for smoke and hot gases were varied in terms of the Reynolds and Grashof numbers. The forced ventilation at the upper or lower boundary was of the same order as the bulk shaft flow. Measurements were taken within the shaft to allow a detailed study of the steady state flow and thermal fields established for various shaft configurations and inlet conditions, from which optimal means for smoke alleviation in high rise building fires may be developed. Results indicated a wall plume as the primary transport mechanism for smoke propagating from the inlet towards the exhaust region. Recirculation and entrainment dominated at high inlet Grashof number flows, while increased inlet Reynolds numbers allowed greater mixing in the shaft. The development and stability of these flow patterns and their effects on the smoke behavior were assessed for several shaft configurations with different inlet conditions. The comparisons indicated that the fastest smoke removal and lowest overall shaft temperatures occur for a configuration with natural ventilation at the top surface and forced ventilation up from the shaft bottom. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L19.00012: Instability Of An Advective Flow In An Inclined Fluid Layer With Perfectly Heat-Conducting Boundaries Albert Sharifulin, Rafil Sagitov The interest to studying flows in infinite layers induced by the longitudinal temperature gradient is inspired by numerous geophysical and engineering applications (horizontal advective flows in the atmosphere and ocean, convection in vertical and inclined mines and oil wells, etc.). The first investigations of this kind has been done by Ostroumov [1]. He formulated the problem about plane parallel convectional flow in an inclined plane parallel layer with perfectly heat-conducting boundaries caused by the presence of a transverse temperature gradient and the longitudinal temperature gradient. Ostroumov obtained exact analytical solution for the case where the axis of inclination was horizontal and the longitudinal and transverse temperature gradients were perpendicular to this axis. Stability of various limiting cases of the problem (plane layer heated from below, vertical layer heated from the side with and without the longitudinal temperature gradient, and inclined layer between isothermal plates) were studied by many authors. The present paper describes the results of numerical studying of short-wave instability of a plane parallel convective flow in an inclined plane layer with perfectly heat-conducting boundaries under the action of the longitudinal temperature gradient (limited case of Ostroumov problem). [1] G.A. Ostroumov Svobodnaia konvektsiia v usloviakh vnutrennei zadachi. Gostekhizdat (1952). English translation: GA Ostroumov Free convection under conditions of the internal problem. NACA Tech. Memo. 1407(1958). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700