113 research outputs found
Enhanced heat transport by turbulent two-phase Rayleigh-B\'enard convection
We report measurements of turbulent heat-transport in samples of ethane
(CH) heated from below while the applied temperature difference straddled the liquid-vapor co-existance curve . When the sample
top temperature decreased below , droplet condensation occurred
and the latent heat of vaporization provided an additional heat-transport
mechanism.The effective conductivity increased linearly with
decreasing , and reached a maximum value that was an
order of magnitude larger than the single-phase . As
approached the critical pressure, increased dramatically even
though vanished. We attribute this phenomenon to an enhanced
droplet-nucleation rate as the critical point is approached.Comment: 4 gages, 6 figure
Plume motion and large-scale circulation in a cylindrical Rayleigh-B\'enard cell
We used the time correlation of shadowgraph images to determine the angle
of the horizontal component of the plume velocity above (below) the
center of the bottom (top) plate of a cylindrical Rayleigh-B\'enard cell of
aspect ratio ( is the diameter and mm
the height) in the Rayleigh-number range for a Prandtl number . We expect that gives the
direction of the large-scale circulation. It oscillates time-periodically. Near
the top and bottom plates has the same frequency but is
anti-correlated.Comment: 4 pages, 6 figure
Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.83\times 10^{12} \alt \Ra\ \alt 10^{15}\Gamma = 0.50$
We report experimental results for heat-transport measurements, in the form
of the Nusselt number \Nu, by turbulent Rayleigh-B\'enard convection in a
cylindrical sample of aspect ratio ( m is
the diameter and m the height). The measurements were made using
sulfur hexafluoride at pressures up to 19 bars as the fluid. They are for the
Rayleigh-number range 3\times 10^{12} \alt \Ra \alt 10^{15} and for Prandtl
numbers \Pra\ between 0.79 and 0.86. For \Ra < \Ra^*_1 \simeq 1.4\times
10^{13} we find \Nu = N_0 \Ra^{\gamma_{eff}} with , consistent with classical turbulent Rayleigh-B\'enard convection in a
system with laminar boundary layers below the top and above the bottom plate.
For \Ra^*_1 < \Ra < \Ra^*_2 (with \Ra^*_2 \simeq 5\times 10^{14})
gradually increases up to . We argue that above
\Ra^*_2 the system is in the ultimate state of convection where the boundary
layers, both thermal and kinetic, are also turbulent. Several previous
measurements for are re-examined and compared with the present
results.Comment: 44 pages, 18 figures, submitted to NJ
Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point
As shown in earlier work (Ahlers et al., J. Fluid Mech. 569, p.409 (2006)),
non-Oberbeck Boussinesq (NOB) corrections to the center temperature in
turbulent Rayleigh-Benard convection in water and also in glycerol are governed
by the temperature dependences of the kinematic viscosity and the thermal
diffusion coefficient. If the working fluid is ethane close to the critical
point the origin of non-Oberbeck-Boussinesq corrections is very different, as
will be shown in the present paper. Namely, the main origin of NOB corrections
then lies in the strong temperature dependence of the isobaric thermal
expansion coefficient \beta(T). More precisely, it is the nonlinear
T-dependence of the density \rho(T) in the buoyancy force which causes another
type of NOB effect. We demonstrate that through a combination of experimental,
numerical, and theoretical work, the latter in the framework of the extended
Prandtl-Blasius boundary layer theory developed in Ahlers et al., J. Fluid
Mech. 569, p.409 (2006). The latter comes to its limits, if the temperature
dependence of the thermal expension coefficient \beta(T) is significant.Comment: 18 pages, 15 figures, 3 table
Logarithmic temperature profiles in turbulent Rayleigh-B\'enard convection
We report results for the temperature profiles of turbulent Rayleigh-B\'enard
convection (RBC) in the interior of a cylindrical sample of aspect ratio
( and are the diameter and height
respectively). Results from experiment over the Rayleigh number range 4\times
10^{12} \alt Ra \alt 10^{15} for a Prandtl number \Pra \simeq 0.8 and from
direct numerical simulation (DNS) at for \Pra = 0.7
are presented. We find that the temperature varies as where
is the distance from the bottom or top plate. This is the case in the classical
as well as in the ultimate state of RBC. From DNS we find that in the
classical state decreases in the radial direction as the distance from the side
wall increases and becomes small near the sample center
Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.84\times 10^{11} \alt \Ra\ \alt 2\times10^{14}\Gamma = 1.00$
We report experimental results for heat-transport measurements by turbulent
Rayleigh-B\'enard convection in a cylindrical sample of aspect ratio ( m is the diameter and m the height).
They are for the Rayleigh-number range 4\times10^{11} \alt \Ra \alt
2\times10^{14} and for Prandtl numbers \Pra\ between 0.79 and 0.86.
For \Ra < \Ra^*_1 \simeq 2\times 10^{13} we find \Nu = N_0
\Ra^{\gamma_{eff}} with and ,
consistent with classical turbulent Rayleigh-B\'enard convection in a system
with laminar boundary layers below the top and above the bottom plate and with
the prediction of Grossmann and Lohse.
For \Ra > \Ra_1^* the data rise above the classical-state power-law and
show greater scatter. In analogy to similar behavior observed for , we interpret this observation as the onset of the transition to the
ultimate state. Within our resolution this onset occurs at nearly the same
value of \Ra_1^* as it does for . This differs from an earlier
estimate by Roche {\it et al.} which yielded a transition at \Ra_U \simeq
1.3\times 10^{11} \Gamma^{-2.5\pm 0.5}. A -independent \Ra^*_1 would
suggest that the boundary-layer shear transition is induced by fluctuations on
a scale less than the sample dimensions rather than by a global
-dependent flow mode. Within the resolution of the measurements the
heat transport above \Ra_1^* is equal for the two values, suggesting
a universal aspect of the ultimate-state transition and properties. The
enhanced scatter of \Nu\ in the transition region, which exceeds the
experimental resolution, indicates an intrinsic irreproducibility of the state
of the system.Comment: 17 pages, including 2 pages of data tables and 56 references.
Submitted to New J. Phy
Large scale dynamics in turbulent Rayleigh-Benard convection
The progress in our understanding of several aspects of turbulent
Rayleigh-Benard convection is reviewed. The focus is on the question of how the
Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the
Prandtl number Pr, and on how the thicknesses of the thermal and the kinetic
boundary layers scale with Ra and Pr. Non-Oberbeck-Boussinesq effects and the
dynamics of the large-scale convection-roll are addressed as well. The review
ends with a list of challenges for future research on the turbulent
Rayleigh-Benard system.Comment: Review article, 34 pages, 13 figures, Rev. Mod. Phys. 81, in press
(2009
A longitudial study of a neuropsychological rehabilitation program in Alzheimer's disease
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