2,802 research outputs found
Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions
Turbulent Rayleigh-Benard convection with phase changes in an extended layer
between two parallel impermeable planes is studied by means of
three-dimensional direct numerical simulations for Rayleigh numbers between
10^4 and 1.5\times 10^7 and for Prandtl number Pr=0.7. Two different sets of
boundary conditions of temperature and total water content are compared:
imposed constant amplitudes which translate into Dirichlet boundary conditions
for the scalar field fluctuations about the quiescent diffusive equilibrium and
constant imposed flux boundary conditions that result in Neumann boundary
conditions. Moist turbulent convection is in the conditionally unstable regime
throughout this study for which unsaturated air parcels are stably and
saturated air parcels unstably stratified. A direct comparison of both sets of
boundary conditions with the same parameters requires to start the turbulence
simulations out of differently saturated equilibrium states. Similar to dry
Rayleigh-Benard convection the differences in the turbulent velocity
fluctuations, the cloud cover and the convective buoyancy flux decrease across
the layer with increasing Rayleigh number. At the highest Rayleigh numbers the
system is found in a two-layer regime, a dry cloudless and stably stratified
layer with low turbulence level below a fully saturated and cloudy turbulent
one which equals classical Rayleigh-Benard convection layer. Both are separated
by a strong inversion that gets increasingly narrower for growing Rayleigh
number.Comment: 19 pages, 13 Postscript figures, Figures 10,11,12,13, in reduced
qualit
Dynamical mean field solution of the Bose-Hubbard model
We present the effective action and self-consistency equations for the
bosonic dynamical mean field (B-DMFT) approximation to the bosonic Hubbard
model and show that it provides remarkably accurate phase diagrams and
correlation functions. To solve the bosonic dynamical mean field equations we
use a continuous-time Monte Carlo method for bosonic impurity models based on a
diagrammatic expansion in the hybridization and condensate coupling. This
method is readily generalized to bosonic mixtures, spinful bosons, and
Bose-Fermi mixtures.Comment: 10 pages, 3 figures. includes supplementary materia
Orographic effects on convective precipitation and space-time rainfall variability: preliminary results
International audienceIn the EFFS Project, an attempt has been made to develop a general framework to study the predictability of severe convective rainfall events in the presence of orography. Convective activity is embedded in orographic rainfall and can be thought as the result of several physical mechanisms. Quantifying its variability on selected area and time scales requires choosing the best physical representation of the rainfall variability on these scales. The main goal was (i) to formulate a meaningful set of experiments to compute the oscillation of variance due to convection inside model forecasts in the presence of orography and (ii) to give a statistical measure of it that might be of value in the operational use of atmospheric data. The study has been limited to atmospheric scales that span the atmosphere from 2 to 200 km and has been focused on extreme events with deep convection. Suitable measures of the changing of convection in the presence of orography have been related to the physical properties of the rainfall environment. Preliminary results for the statistical variability of the convective field are presented
Towards a Formal Approach to Validating and Verifying Functional Design for Complex Safety Critical Systems
The quality and reliability of safety criticalsoftware systems are highly dependent on proper systemvalidation and verification. In model-driven softwaredevelopment, semi-formal notations are often used inrequirements capture. Though semi-formal notations possessadvantages, their major disadvantage is their imprecision. Atechnique to eliminate imprecision is to transform semi-formalmodels into an analyzable representation using formalspecification techniques (FSTs). With this approach to systemvalidation and verification, safety critical systems can bedeveloped more reliably. This work documents early experienceof applying FSTs on UML class diagrams as attributeconstraints, and pre- post-conditions on procedures. Thevalidation and verification of the requirements of a system tomonitor unmanned aerial vehicles in unrestricted airspace is theorigin of this work. The challenge is the development of a systemwith incomplete specifications; multiple conflicting stakeholders’interests; existence of a prototype system; the need forstandardized compliance, where validation and verification areparamount, which necessitates forward and reverse engineeringactivities
Analysis of the environments of seven Mediterranean tropical-like storms using an axisymmetric, nonhydrostatic, cloud resolving model
Tropical-like storms on the Mediterranean Sea are occasionally observed on satellite images, often with a clear eye surrounded by an axysimmetric cloud structure. These storms sometimes attain hurricane intensity and can severely affect coastal lands. A deep, cut-off, cold-core low is usually observed at mid-upper tropospheric levels in association with the development of these tropical-like systems. In this study we attempt to apply some tools previously used in studies of tropical hurricanes to characterise the environments in which seven known Mediterranean events developed. In particular, an axisymmetric, nonhydrostatic, cloud resolving model is applied to simulate the tropical-like storm genesis and evolution. Results are compared to surface observations when landfall occurred and with satellite microwave derived wind speed measurements over the sea. Finally, sensitivities of the numerical simulations to different factors (e.g. sea surface temperature, vertical humidity profile and size of the initial precursor of the storm) are examined
Complete topology of cells, grains, and bubbles in three-dimensional microstructures
We introduce a general, efficient method to completely describe the topology
of individual grains, bubbles, and cells in three-dimensional polycrystals,
foams, and other multicellular microstructures. This approach is applied to a
pair of three-dimensional microstructures that are often regarded as close
analogues in the literature: one resulting from normal grain growth (mean
curvature flow) and another resulting from a random Poisson-Voronoi
tessellation of space. Grain growth strongly favors particular grain
topologies, compared with the Poisson-Voronoi model. Moreover, the frequencies
of highly symmetric grains are orders of magnitude higher in the the grain
growth microstructure than they are in the Poisson-Voronoi one. Grain topology
statistics provide a strong, robust differentiator of different cellular
microstructures and provide hints to the processes that drive different classes
of microstructure evolution.Comment: 5 pages, 6 figures, 5 supplementary page
Distribution of Topological Types in Grain-Growth Microstructures
An open question in studying normal grain growth concerns the asymptotic
state to which microstructures converge. In particular, the distribution of
grain topologies is unknown. We introduce a thermodynamic-like theory to
explain these distributions in two- and three-dimensional systems. In
particular, a bending-like energy is associated to each grain topology
, and the probability of observing that particular topology is
proportional to , where is the order
of an associated symmetry group and is a thermodynamic-like constant.
We explain the physical origins of this approach, and provide numerical
evidence in support.Comment: 6 pages, 5 figure
Tensor Perturbations in Quantum Cosmological Backgrounds
In the description of the dynamics of tensor perturbations on a homogeneous
and isotropic background cosmological model, it is well known that a simple
Hamiltonian can be obtained if one assumes that the background metric satisfies
Einstein classical field equations. This makes it possible to analyze the
quantum evolution of the perturbations since their dynamics depends only on
this classical background. In this paper, we show that this simple Hamiltonian
can also be obtained from the Einstein-Hilbert lagrangian without making use of
any assumption about the dynamics of the background metric. In particular, it
can be used in situations where the background metric is also quantized, hence
providing a substantial simplification over the direct approach originally
developed by Halliwell and Hawking.Comment: 24 pages, JHEP forma
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