6,421 research outputs found
Optimal Prandtl number for heat transfer in rotating Rayleigh-Benard convection
Numerical data for the heat transfer as a function of the Prandtl (Pr) and
Rossby (Ro) numbers in turbulent rotating Rayleigh-Benard convection are
presented for Rayleigh number Ra = 10^8. When Ro is fixed the heat transfer
enhancement with respect to the non-rotating value shows a maximum as function
of Pr. This maximum is due to the reduced efficiency of Ekman pumping when Pr
becomes too small or too large. When Pr becomes small, i.e. for large thermal
diffusivity, the heat that is carried by the vertical vortices spreads out in
the middle of the cell, and Ekman pumping thus becomes less efficient. For
higher Pr the thermal boundary layers (BLs) are thinner than the kinetic BLs
and therefore the Ekman vortices do not reach the thermal BL. This means that
the fluid that is sucked into the vertical vortices is colder than for lower Pr
which limits the efficiency of the upwards heat transfer.Comment: 5 pages, 6 figure
Radial boundary layer structure and Nusselt number in Rayleigh-Benard convection
Results from direct numerical simulations for three dimensional
Rayleigh-Benard convection in a cylindrical cell of aspect ratio 1/2 and Pr=0.7
are presented. They span five decades of Ra from to . Good numerical resolution with grid spacing Kolmogorov
scale turns out to be crucial to accurately calculate the Nusselt number, which
is in good agreement with the experimental data by Niemela et al., Nature, 404,
837 (2000). In underresolved simulations the hot (cold) plumes travel further
from the bottom (top) plate than in the fully resolved case, because the
thermal dissipation close to the sidewall (where the grid cells are largest) is
insufficient. We compared the fully resolved thermal boundary layer profile
with the Prandtl-Blasius profile. We find that the boundary layer profile is
closer to the Prandtl Blasius profile at the cylinder axis than close to the
sidewall, due to rising plumes in that region.Comment: 10 pages, 6 figure
Heat transport and flow structure in rotating Rayleigh-B\'enard convection
Here we summarize the results from our direct numerical simulations (DNS) and
experimental measurements on rotating Rayleigh-B\'enard (RB) convection. Our
experiments and simulations are performed in cylindrical samples with an aspect
ratio \Gamma varying from 1/2 to 2. Here \Gamma=D/L, where D and L are the
diameter and height of the sample, respectively. When the rotation rate is
increased, while a fixed temperature difference between the hot bottom and cold
top plate is maintained, a sharp increase in the heat transfer is observed
before the heat transfer drops drastically at stronger rotation rates. Here we
focus on the question of how the heat transfer enhancement with respect to the
non-rotating case depends on the Rayleigh number Ra, the Prandtl number Pr, and
the rotation rate, indicated by the Rossby number Ro. Special attention will be
given to the influence of the aspect ratio on the rotation rate that is
required to get heat transport enhancement. In addition, we will discuss the
relation between the heat transfer and the large scale flow structures that are
formed in the different regimes of rotating RB convection and how the different
regimes can be identified in experiments and simulations.Comment: 12 pages, 10 figure
Sidewall effects in Rayleigh-B\'enard convection
We investigate the influence of the temperature boundary conditions at the
sidewall on the heat transport in Rayleigh-B\'enard (RB) convection using
direct numerical simulations. For relatively low Rayleigh numbers Ra the heat
transport is higher when the sidewall is isothermal, kept at a temperature
(where is the temperature difference between the
horizontal plates and the temperature of the cold plate), than when the
sidewall is adiabatic. The reason is that in the former case part of the heat
current avoids the thermal resistance of the fluid layer by escaping through
the sidewall that acts as a short-circuit. For higher Ra the bulk becomes more
isothermal and this reduces the heat current through the sidewall. Therefore
the heat flux in a cell with an isothermal sidewall converges to the value
obtained with an adiabatic sidewall for high enough Ra ().
However, when the sidewall temperature deviates from the heat
transport at the bottom and top plates is different from the value obtained
using an adiabatic sidewall. In this case the difference does not decrease with
increasing Ra thus indicating that the ambient temperature of the experimental
apparatus can influence the heat transfer. A similar behavior is observed when
only a very small sidewall region close to the horizontal plates is kept
isothermal, while the rest of the sidewall is adiabatic. The reason is that in
the region closest to the horizontal plates the temperature difference between
the fluid and the sidewall is highest. This suggests that one should be careful
with the placement of thermal shields outside the fluid sample to minimize
spurious heat currents.Comment: 27 pages, 16 figure
Observation of the Meissner effect with ultracold atoms in bosonic ladders
We report on the observation of the Meissner effect in bosonic flux ladders
of ultracold atoms. Using artificial gauge fields induced by laser-assisted
tunneling, we realize arrays of decoupled ladder systems that are exposed to a
uniform magnetic field. By suddenly decoupling the ladders and projecting into
isolated double wells, we are able to measure the currents on each side of the
ladder. For large coupling strengths along the rungs of the ladder, we find a
saturated maximum chiral current corresponding to a full screening of the
artificial magnetic field. For lower coupling strengths, the chiral current
decreases in good agreement with expectations of a vortex lattice phase. Our
work marks the first realization of a low-dimensional Meissner effect and,
furthermore, it opens the path to exploring interacting particles in low
dimensions exposed to a uniform magnetic field
Roughness-facilitated local 1/2 scaling does not imply the onset of the ultimate regime of thermal convection
In thermal convection, roughness is often used as a means to enhance heat
transport, expressed in Nusselt number. Yet there is no consensus on whether
the Nusselt vs. Rayleigh number scaling exponent () increases or remains unchanged. Here we numerically
investigate turbulent Rayleigh-B\'enard convection over rough plates in two
dimensions, up to . Varying the height and wavelength of
the roughness elements with over 200 combinations, we reveal the existence of
two universal regimes. In the first regime, the local effective scaling
exponent can reach up to 1/2. However, this cannot be explained as the
attainment of the so-called ultimate regime as suggested in previous studies,
because a further increase in leads to the second regime, in
which the scaling saturates back to a value close to the smooth case.
Counterintuitively, the transition from the first to the second regime
corresponds to the competition between bulk and boundary layer flow: from the
bulk-dominated regime back to the classical boundary-layer-controlled regime.
Our study clearly demonstrates that the local scaling does not signal the
onset of asymptotic ultimate thermal convection.Comment: Submitted, 11 pages, 5figur
Drop impact on superheated surfaces
At impact of a liquid droplet on a smooth surface heated above the liquid's
boiling point, the droplet either immediately boils when it contacts the
surfaces (``contact boiling''), or without any surface contact forms a
Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film
boiling''), or both forms the Leidenfrost layer and ejects tiny droplets upward
(``spraying film boiling''). We experimentally determine conditions under which
impact behaviors in each regime can be realized. We show that the dimensionless
maximum spreading of impacting droplets on the heated surfaces in both
gentle and spraying film boiling regimes shows a universal scaling with the
Weber number \We (\gamma\sim\We^{2/5}) -- regardless of surface temperature
and of liquid properties -- which is much steeper than for the impact on
non-heated (hydrophilic or hydrophobic) surfaces (\gamma\sim\We^{1/4}). We
also intereferometrically measure the vapor thickness under the droplet
Transition to the ultimate regime in two-dimensional Rayleigh-B\'enard convection
The possible transition to the so-called ultimate regime, wherein both the
bulk and the boundary layers are turbulent, has been an outstanding issue in
thermal convection, since the seminal work by Kraichnan [Phys. Fluids 5, 1374
(1962)]. Yet, when this transition takes place and how the local flow induces
it is not fully understood. Here, by performing two-dimensional simulations of
Rayleigh-B\'enard turbulence covering six decades in Rayleigh number Ra up to
for Prandtl number Pr , for the first time in numerical
simulations we find the transition to the ultimate regime, namely at
. We reveal how the emission of thermal plumes enhances
the global heat transport, leading to a steeper increase of the Nusselt number
than the classical Malkus scaling [Proc.
R. Soc. London A 225, 196 (1954)]. Beyond the transition, the mean velocity
profiles are logarithmic throughout, indicating turbulent boundary layers. In
contrast, the temperature profiles are only locally logarithmic, namely within
the regions where plumes are emitted, and where the local Nusselt number has an
effective scaling , corresponding to the
effective scaling in the ultimate regime.Comment: 6 pages, 4figure
Flow organization and heat transfer in turbulent wall sheared thermal convection
We perform direct numerical simulations of wall sheared Rayleigh-B\'enard
(RB) convection for Rayleigh numbers up to , Prandtl number unity, and
wall shear Reynolds numbers up to . Using the Monin-Obukhov length
we identify three different flow states, a buoyancy dominated regime
(; with the thermal
boundary layer thickness), a transitional regime (; with the height of the domain), and a shear dominated
regime (). In the buoyancy dominated regime the flow
dynamics are similar to that of turbulent thermal convection. The transitional
regime is characterized by rolls that are increasingly elongated with
increasing shear. The flow in the shear dominated regime consists of very
large-scale meandering rolls, similar to the ones found in conventional Couette
flow. As a consequence of these different flow regimes, for fixed and with
increasing shear, the heat transfer first decreases, due to the breakup of the
thermal rolls, and then increases at the beginning of the shear dominated
regime. For the Nusselt number effectively scales as
, with while we find
in the buoyancy dominated regime. In the transitional regime the effective
scaling exponent is , but the temperature and velocity profiles
in this regime are not logarithmic yet, thus indicating transient dynamics and
not the ultimate regime of thermal convection
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