Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KJ: Convection and Buoyancy-Driven Flows III |
Hide Abstracts |
Chair: Mike Schatz, Georgia Institute of Technology Room: Salt Palace Convention Center 250 D |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KJ.00001: Dynamics of the large-scale circulation of turbulent Rayleigh-B\'enard convection at low Rayleigh numbers. Matthew Schreiner, Eric Brown, Guenter Ahlers Measurements of the large-scale circulation (LSC) of turbulent Rayleigh-B\'enard convection (RBC) in cylindrical samples of aspect ratio $\Gamma = 1$ (= diameter/height) are reported. They covered the Rayleigh-number range $9 \times 10^5 < R < 2 \times 10^{9}$ and the Prandtl-number range $4.4 < \sigma < 29$. Using observations of the mid-plane temperature-profile around the side wall of the sample and successive shadowgraph images taken from above, we determined that the amplitude $\delta$ of the thermal imprint by the LSC at the side wall, divided by the applied vertical temperature difference $\Delta T$, increased with decreasing $R$, approximately as $\delta/\Delta T \simeq R^{-0.33}$ when $R > 10^8$. For smaller $R$ the $R$-dependence became more complicated, but $\delta/\Delta T$ remained finite. This suggests that the LSC persisted for small R. For $R < 10^6$ the azimuthal temperature profile could no longer be described by a simple cosine function. This profile is as yet unexplained. We continued to observe diffusive meandering of the LSC orientation as well as spontaneous cessations and reorientations of the flow at $R \simeq 10^6$. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KJ.00002: Non-Oberbeck-Boussinesq effects on flow structure in Rayleigh-B\'{e}nard convection in water Kazuyasu Sugiyama, Enrico Calzavarini, Siegfried Grossmann, Detlef Lohse Non-Oberbeck-Boussinesq (NOB) effects on the flow organization in two-dimensional Rayleigh-B\'{e}nard convection are numerically analyzed for water. The Rayleigh number $Ra$ and the temperature difference $\Delta(=T_{b}-T_{t})$ between top and bottom plates are varied up to $10^{8}$ and $60$K, respectively. The simulation captures broken symmetry features, i.e., the center temperature $T_{c}$ shift and the Nusselt number deviation, which are observed in the experiment of Ahlers et al. (2006, J. Fluid Mech., {\bf 569}). Beyond $Ra\approx 10^{6}$ the flow consists of a large diagonal center convection roll and two smaller rolls in the upper and lower corner, respectively. In the NOB case the center convection roll is still characterized by only one velocity scale, while the top and bottom corner flows are of different strength. We find the total energy based Reynolds number $Re^{E}=\langle{\bf u}^{2}/2\rangle^{1/2}L/\nu$ scales as $Re_{NOB}^{E}/Re_{OB}^{E}\sim (\beta(T_{c})/\beta(T_ {m}))^{1/2}$, with $\beta$ the thermal expansion coefficient and $T_{m}(=(T_{b}+T_{t})/2)$ the arithmetic mean temperature, corresponding to the ratio of the free fall velocities. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KJ.00003: Scaling of Structure Functions in Turbulent Rayleigh-B\'{e}nard Convection R.P.J. Kunnen, H.J.H. Clercx, B.J. Geurts Turbulent convection is actively driven by buoyancy effects, i.e., temperature is an active scalar. Hence a considerable influence of buoyancy on the velocity and temperature structure function is expected. Bolgiano and Obukhov (BO) derived scaling laws for this regime that are different from the classical Kolmogorov (K41) result. The BO scaling is valid at length scales larger than the so-called Bolgiano length, while for smaller scales K41 is recovered. Whether a BO scaling regime can be found in Rayleigh-B\'{e}nard convection is an ongoing debate. We present numerical and experimental evidence for BO scaling. Numerical simulations provide insight in the local turbulent length scales. These are found to behave very differently from the global estimates that follow directly from the Navier-Stokes equations. Besides temporal also spatial calculations of structure functions confirm BO scaling. In measurements using stereoscopic particle image velocimetry the BO scaling was also found, in agreement with the numerical data. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KJ.00004: Experimental study of turbulent Rayleigh-B\'enard convection of air Anna Maystrenko, Ronald du Puits, Christian Resagk, Andre Thess We have studied turbulent Rayleigh-B\'enard convection in air (Pr
= 0.71) in two large-scale experimental facilities, one
cylindrical cell of 7.15 m diameter and 6.30 m hight and one
rectangular box of 2.50 m length, 0.50 m width and 0.50 m height.
Profiles of the mean temperature $\Theta(z)$, rms temperature
fluctuations $\sigma(z)$ as well as higher moments have been
measured simultaneously on the cooling and heating plates in the
rectangular box for Rayleigh numbers between $Ra=6\times10^7$ and
$Ra=6\times10^{8}$.
The structure of the temperature profiles has been analysed with
the results that three different behaviors in the temperature
profile has been proved: linear behavior $\Theta\sim z$ in the
very thin viscous sublayer directly at the wall, power law
$\Theta\sim z^A$ in the boundary layer and a logarithmic behavior
$\Theta\sim \ln z$ in the overlap layer between the boundary
layer and outer flow.
The symmetry of the plume advection from the cooling and heating
plates as well as their temporal correlations have been
investigated. The local heat flux on the plates
surfaces has been measured and compared to the global heat flux
in the cell.
In the large cylindrical cell the profiles of the horizontal mean
velocity $v(z)$ have been studied in the highly turbulent regime
- $10^{11} |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KJ.00005: Temperature fluctuations in Rayleigh-Benard convection Mohammad Emran, Joerg Schumacher Statistical properties of the temperature field in Rayleigh-Benard convection are studied numerically for Rayleigh numbers $Ra=10^7,10^8$ and $10^9$ and aspect ratios $\Gamma=$1, 3 and 5. Direct numerical simulations of the Boussinesq equations in a cylindrical configuration are therefore conducted. The statistics of the scalar dissipation rate is found to deviate from the lognormal distribution in the far tails, in contrast to recent experiments. The height dependence of the statistics of the temperature fluctuations as well as their gradients are also studied. It is found that the probability density function of the temperature field becomes increasingly symmetric towards the center. The Nusselt number and thermal boundary layer thickness as a function of the Rayleigh number agree well with other experiments and simulations in that parameter range. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KJ.00006: 3-D Space-Time Topology of Spiral Defect Chaos in Rayleigh Benard Convection Huseyin Kurtuldu, Michael Schatz, Marcio Gameiro, Konstantin Mischaikow An algebraic topological method, computational homology, is used to characterize the evolution in space-time of spiral defect chaos in Rayleigh-Benard convection experiments. The convective flow is described by a time-sequential collection of 2-D shadowgraph images that forms a 3-D representation (2-D space, 1-D time) known as a space-time block. Topological analysis of a space-time block yields Betti numbers quantifying the components, holes and cavities within the block. We present results that show how the dynamics of spiral defect chaos are captured by analyzing a time series of space-time blocks [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KJ.00007: Spatial forcing in Thermal Convection Experiments Stephan Weiss, Gabriel Seiden, Eberhard Bodenschatz We present experimental results on topological and optical forcing of large aspect ratio Rayleigh-Benard and inclined layer convection of a fluid with Prandtl number 1. For three different forcing patterns (homogeneous internal heating, striped and hexagonal) the observed flow patterns are presented and compared to theory where available. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KJ.00008: Optical Actuation of Rayleigh-B\'{e}nard Convection Adam Perkins, Michael Schatz We report on a new optical approach to manipulate convective flow. The working fluid in the convection apparatus is a binary mixture of gases that strongly absorbs light at selected optical wavelengths. Laser light absorption results in localized heating, thereby altering the fluid flow. Rapid scanning of laser light allows actuation at multiple spatial points nearly simultaneously. By directing the light from above or below, the heating can be either locally stabilizing or destabilizing. Preliminary results on imposing and controlling complex convection patterns will be discussed. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KJ.00009: The nature of oscillatory modes in turbulent Rayliegh-B{\'e}nard convection Eric Brown, Guenter Ahlers Measurements of oscillatory modes of the large-scale circulation (LSC) of turbulent Rayleigh-B\'enard convection in water-filled cylindrical containers of equal height and diameter are presented. To observe and distinguish different modes, temperatures were measured simultaneously around the side wall at 8 azimuthal angles and 3 heights. A previously observed intrinsic mode consists of an azimuthal twist of the LSC circulation-plane around the orientation at mid-height, with the top and bottom oscillating {\em out of phase} by half a cycle. The oscillation amplitude varied irregularly in time, yielding a Gaussian probability distribution for the difference in LSC orientation at different heights. This is reproduced by the equation of motion of a stochastically-driven damped harmonic oscillator. This mode differs from a previously unknown {\em planar} oscillation around a fixed orientation due to an asymmetry, in which the top and bottom of the LSC oscillate {\em in phase}. The planar mode was observed in a container tilted relative to gravity, and predicted using a model that yields an LSC orientation given by the equation of a stochastically-driven damped harmonic oscillator in which the tilt-modified buoyancy of the thermal boundary layers provides the restoring force. [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