32,623 research outputs found
A variational Bayesian method for inverse problems with impulsive noise
We propose a novel numerical method for solving inverse problems subject to
impulsive noises which possibly contain a large number of outliers. The
approach is of Bayesian type, and it exploits a heavy-tailed t distribution for
data noise to achieve robustness with respect to outliers. A hierarchical model
with all hyper-parameters automatically determined from the given data is
described. An algorithm of variational type by minimizing the Kullback-Leibler
divergence between the true posteriori distribution and a separable
approximation is developed. The numerical method is illustrated on several one-
and two-dimensional linear and nonlinear inverse problems arising from heat
conduction, including estimating boundary temperature, heat flux and heat
transfer coefficient. The results show its robustness to outliers and the fast
and steady convergence of the algorithm.Comment: 20 pages, to appear in J. Comput. Phy
Review of Summation-by-parts schemes for initial-boundary-value problems
High-order finite difference methods are efficient, easy to program, scales
well in multiple dimensions and can be modified locally for various reasons
(such as shock treatment for example). The main drawback have been the
complicated and sometimes even mysterious stability treatment at boundaries and
interfaces required for a stable scheme. The research on summation-by-parts
operators and weak boundary conditions during the last 20 years have removed
this drawback and now reached a mature state. It is now possible to construct
stable and high order accurate multi-block finite difference schemes in a
systematic building-block-like manner. In this paper we will review this
development, point out the main contributions and speculate about the next
lines of research in this area
Coupling problem in thermal systems simulations
Building energy simulation is playing a key role in building design in order to reduce the energy
consumption and, consequently, the CO2 emissions. An object-oriented tool called NEST
is used to simulate all the phenomena that appear in a building. In the case of energy and momentum
conservation and species transport, the current solver behaves well, but in the case of
mass conservation it takes a lot of time to reach a solution. For this reason, in this work, instead
of solving the continuity equations explicitly, an implicit method based on the Trust Region algorithm
is proposed. Previously, a study of the properties of the model used by NEST-Building
software has been done in order to simplify the requirements of the solver. For a building with
only 9 rooms the new solver is a thousand times faster than the current method
Linear instability of Poiseuille flows with highly non-ideal fluids
The objective of this work is to investigate linear modal and algebraic
instability in Poiseuille flows with fluids close to their vapour-liquid
critical point. Close to this critical point, the ideal gas assumption does not
hold and large non-ideal fluid behaviours occur. As a representative non-ideal
fluid, we consider supercritical carbon dioxide (CO) at pressure of 80 bar,
which is above its critical pressure of 73.9 bar. The Poiseuille flow is
characterized by the Reynolds number
(), the product of Prandtl
() and Eckert number
(), and the wall temperature that in
addition to pressure determines the thermodynamic reference condition. For low
Eckert numbers, the flow is essentially isothermal and no difference with the
well-known stability behaviour of incompressible flows is observed. However, if
the Eckert number increases, the viscous heating causes gradients of
thermodynamic and transport properties, and non-ideal gas effects become
significant. Three regimes of the laminar base flow can be considered,
subcritical (temperature in the channel is entirely below its pseudo-critical
value), transcritical, and supercritical temperature regime. If compared to the
linear stability of an ideal gas Poiseuille flow, we show that the base flow is
more unstable in the subcritical regime, inviscid unstable in the transcritical
regime, while significantly more stable in the supercritical regime. Following
the corresponding states principle, we expect that qualitatively similar
results will be obtained for other fluids at equivalent thermodynamic states.Comment: 34 pages, 22 figure
The Gravothermal Instability at all scales: from Turnaround Radius to Supernovae
The gravitational instability, responsible for the formation of the structure
of the Universe, occurs below energy thresholds and above spatial scales of a
self-gravitating expanding region, when thermal energy can no longer
counterbalance self-gravity. I argue that at sufficiently-large scales, dark
energy may restore thermal stability. This stability re-entrance of an
isothermal sphere defines a turnaround radius, which dictates the maximum
allowed size of any structure generated by gravitational instability. On the
opposite limit of high energies and small scales, I will show that an ideal,
quantum or classical, self-gravitating gas is subject to a high-energy
relativistic gravothermal instability. It occurs at sufficiently-high energy
and small radii, when thermal energy cannot support its own gravitational
attraction. Applications of the phenomenon include neutron stars and
core-collapse supernovae. I also extend the original Oppenheimer--Volkov
calculation of the maximum mass limit of ideal neutron cores to the non-zero
temperature regime, relevant to the whole cooling stage from a hot
proto-neutron star down to the final cold state.Comment: Minor amendments to match published versio
Standing and travelling waves in cylindrical Rayleigh-Benard convection
The Boussinesq equations for Rayleigh-Benard convection are simulated for a
cylindrical container with an aspect ratio near 1.5. The transition from an
axisymmetric stationary flow to time-dependent flows is studied using nonlinear
simulations, linear stability analysis and bifurcation theory. At a Rayleigh
number near 25,000, the axisymmetric flow becomes unstable to standing or
travelling azimuthal waves. The standing waves are slightly unstable to
travelling waves. This scenario is identified as a Hopf bifurcation in a system
with O(2) symmetry
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