21,694 research outputs found
Black Rings in (Anti)-deSitter space
We construct solutions for thin black rings in Anti-deSitter and deSitter
spacetimes using approximate methods. Black rings in AdS exist with arbitrarily
large radius and satisfy a bound |J| \leq LM, which they saturate as their
radius becomes infinitely large. For angular momentum near the maximum, they
have larger area than rotating AdS black holes. Thin black rings also exist in
deSitter space, with rotation velocities varying between zero and a maximum,
and with a radius that is always strictly below the Hubble radius. Our general
analysis allows us to include black Saturns as well, which we discuss briefly.
We present a simple physical argument why supersymmetric AdS black rings must
not be expected: they do not possess the necessary pressure to balance the AdS
potential. We discuss the possible existence or absence of `large AdS black
rings' and their implications for a dual hydrodynamic description. An analysis
of the physical properties of rotating AdS black holes is also included.Comment: 38 pages, 6 figures. v2: changes in terminology, refs added. v3:
minor improvements, refs added, published versio
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
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
The Jurassic pleurotomarioidean gastropod Laevitomaria and its palaeobiogeographical history
The genus Laevitomaria is reviewed and its palaeobiogeographical history is reconstructed based on the re-examination of its type species L. problematica, the study of material stored at the National Natural History Museum of Luxembourg, and an extensive review of the literature. The systematic study allows ascribing to Laevitomaria a number of Jurassic species from the western European region formerly included in other pleurotomariid genera. The following new combinations are proposed: Laevitomaria allionta, Laevitomaria amyntas, Laevitomaria angulba, Laevitomaria asurai, Laevitomaria daityai, Laevitomaria fasciata, Laevitomaria gyroplata, Laevitomaria isarensis, Laevitomaria joannis, Laevitomaria repeliniana, Laevitomaria stoddarti, Laevitomaria subplatyspira, and Laevitomaria zonata. The genus, which was once considered as endemic of the central part of the western Tethys, shows an evolutionary and palaeogeographical history considerably more complex than previously assumed. It first appeared in the Late Sinemurian in the northern belt of the central western Tethys involved in the Neotethyan rifting, where it experienced a first radiation followed by an abrupt decline of diversity in the Toarcian. Species diversity increased again during Toarcian\u2013Aalenian times in the southernmost part of western European shelf and a major radiation occurred during the Middle Aalenian to Early Bajocian in the northern Paris Basin and southern England. After a latest Bajocian collapse of diversity, Laevitomaria disappeared from both the central part of western Tethys and the European shelf. In the Bathonian, the genus appeared in the south-eastern margin of the Tethys where it lasted until the Oxfordian
Joint estimation and localization in sensor networks
This paper addresses the problem of collaborative tracking of dynamic targets in wireless sensor networks. A novel distributed linear estimator, which is a version of a distributed Kalman filter, is derived. We prove that the filter is mean square consistent in the case of static target estimation. When large sensor networks are deployed, it is common that the sensors do not have good knowledge of their locations, which affects the target estimation procedure. Unlike most existing approaches for target tracking, we investigate the performance of our filter when the sensor poses need to be estimated by an auxiliary localization procedure. The sensors are localized via a distributed Jacobi algorithm from noisy relative measurements. We prove strong convergence guarantees for the localization method and in turn for the joint localization and target estimation approach. The performance of our algorithms is demonstrated in simulation on environmental monitoring and target tracking tasks
Joint Estimation and Localization in Sensor Networks
This paper addresses the problem of collaborative tracking of dynamic targets
in wireless sensor networks. A novel distributed linear estimator, which is a
version of a distributed Kalman filter, is derived. We prove that the filter is
mean square consistent in the case of static target estimation. When large
sensor networks are deployed, it is common that the sensors do not have good
knowledge of their locations, which affects the target estimation procedure.
Unlike most existing approaches for target tracking, we investigate the
performance of our filter when the sensor poses need to be estimated by an
auxiliary localization procedure. The sensors are localized via a distributed
Jacobi algorithm from noisy relative measurements. We prove strong convergence
guarantees for the localization method and in turn for the joint localization
and target estimation approach. The performance of our algorithms is
demonstrated in simulation on environmental monitoring and target tracking
tasks.Comment: 9 pages (two-column); 5 figures; Manuscript submitted to the 2014
IEEE Conference on Decision and Control (CDC
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
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