MOX Reports
The preprint collection of the Laboratory for Modeling and Scientific Computation MOX. It mainly contains works on numerical
analysis and mathematical modeling applied to engineering problems. MOX web site is mox.polimi.it
Found 1239 products
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15/2008 - 07/02/2008
Micheletti, Stefano; Perotto, Simona
An anisotropic mesh adaptation procedure for an optimal control problem of the advection-diffusion-reaction equation | Abstract | | We derive an anisotropic a posteriori error estimator for a PDE-con-
strained optimal control problem, governed by the scalar advection-diffusion-
reaction equation. With a view to the advection dominated case, a strongly
consistent symmetric stabilization is employed so that the “optimize-then-
discretize” and “discretize-then-optimize” philosophies coincide and lead
to the same discrete problem. The estimator is turned into an anisotropic
mesh adaptation procedure which allows us to approximate the cost func-
tional within to a given tolerance. Both an academic and a realistic test,
inspired by an environmental application, assess the performance of the
proposed approach. |
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14/2008 - 07/01/2008
D'Angelo, Carlo; Zunino, Paolo
A finite element method based on weighted interior penalties for heterogeneous incompressible flows | Abstract | | We propose a finite element scheme for the approximation of multidomain
heterogeneous problems arising in the general framework of linear
incompressible flows (e.g. Stokes’ and Darcy’s equations). We exploit stabilized
mixed finite elements together with Nitsche type matching conditions
that automatically adapt to the coupling of different subproblem combinations.
Optimal error estimates are derived for the coupled problem. Finally,
we propose and analyze an iterative splitting strategy for the approximation
of the multidomain solution by means of a sequence of independent
and local subproblems. Thanks to the introduction of a suitable relaxation
strategy, the iterative method turns out to be convergent for any possible
coupling between subproblems |
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13/2008 - 06/30/2008
Sangalli, Laura Maria; Secchi, Piercesare; Vantini, Simone; Vitelli, Valeria
K-means alignment for curve clustering | Abstract | | We deal with the problem of curve clustering when curves are mis- aligned. We propose a k-means alignment algorithm which jointly cluster and align the curves. We illustrate the procedure via simulation studies and applications to real data.
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12/2008 - 06/26/2008
Passerini, Tiziano; de Luca, Maria Rita; Formaggia, Luca; Quarteroni, Alfio; Veneziani, Alessandro
A 3D/1D geometrical multiscale model of cerebral vasculature | Abstract | | Geometrical multiscale modeling is a strategy advocated in computational
hemodynamics for representing in a single numerical model dynamics that
involve different space scales. This approach is particularly useful to describe complex
networks such as the circle of Willis in the cerebral vasculature. In this paper
we present a multiscale model of the cerebral circulation where a one dimensional
description of the circle of Willis, relying on the one-dimensional Euler equations, is
coupled to a fully three dimensional model of a carotid artery, based on the solution
of the incompressible Navier-Stokes equations. Even if vascular compliance is often
not relevant to the meaningfulness of 3D results, it is crucial in the multiscale model,
since it is the driving mechanism of pressure wave propagation. Unfortunately, 3D
simulations in compliant domains still demand computational costs significantly
higher than the rigid case. Appropriate matching conditions between the two models
have been devised to concentrate the effects of the compliance at the interfaces and
to obtain reliable results still solving a 3D problems on rigid vessels |
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11/2008 - 06/05/2008
Gerardo-Giorda, Luca; Mirabella, Lucia; Nobile, Fabio; Perego, Mauro; Veneziani, Alessandro
A model preconditioner for the Bidomain problem in electrocardiology | Abstract | | We introduce a preconditioner for the solution of the Bidomain system governing the propagation of action potentials in the myocardial tissue, represented by a degenerate parabolic set of nonlinear reaction-diffusion equations. The nonlinear term describes the ion flux at the cellular level. The degenerate nature of the problem results in a severe ill conditioning of its discretization. Our preconditioning strategy is based on a suitable adaptation of the Monodomain model, a simplified version of the Bidomain one, which is by far simpler to solve, nevertheless is unable to capture significant features of the action potential propagation. We prove optimality for the preconditioner with respect to the mesh size, and corroborate our results with 3D numerical simulations.
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10/2008 - 04/22/2008
Grieco, Niccolò; Corrada, Elena; Sesana, Giovanni; Fontana, Giancarlo; Lombardi, Federico; Ieva, Francesca; Paganoni, Anna Maria; Marzegalli, Maurizio
Predictors of the reduction of treatment time for ST-segment elevation myocardial infarction in a complex urban reality. The MoMi2 survey | Abstract | | Aims: To achieve a rapid and effective reperfusion of infarct related artery in a complex urban reality.
Methods and results: A net that connects the territory to 23 hospitals, by a centralized coordination of the emergency resources has been activated in the Milan urban area to obtain a real time knowledge of critical resources availability and to transmit a 12 lead ECG to the admitting Coronary Care Unit. During the survey periods, data related to 627 patients have been collected. Most of the patients (73%) were treated with primary PCI, 45 (7%) with thrombolysis, 105 (17%) didn’t receive any reperfusion therapy. 57% patients arrived to hospital with rescue units; in 23% of all cases, a 12 lead ECG was tele-transmitted to the hospital of destination before patient arrival.
The modality of hospital presentation was the most critical determinant of door-to-balloon time. The shorter one (49.5 minutes) was that of patients transported by means of Advanced Rescue Units with 12 lead ECG tele-transmission and activation of a fast track directly to the Cath Lab.
Conclusions: Pre-hospital ECG recording is the most critical factor necessary to keep door-toballoon time within suggested limits in a large urban area where most of STEMI patients are treated with PCI. |
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9/2008 - 04/21/2008
Secchi, Piercesare; Zio, Enrico; Di Maio, Francesco
Quantifying Uncertainties in the Estimation of Safety Parameters by Using Bootstrapped Artificial Neural Networks | Abstract | | For licensing purposes, safety cases of Nuclear Power Plants (NPPs) must be presented at the Regulatory Authority with the necessary confidence on the models used to describe the plant safety behavior. In principle, this requires the repetition of a large number of model runs to account for the uncertainties inherent in the model description of the true plant behavior. The present paper propounds the use of bootstrapped Artificial Neural Networks (ANNs) for performing the numerous model output calculations needed for estimating safety margins with appropriate confidence intervals. Account is given both to the uncertainties inherent in the plant model and to those introduced by the ANN regression models used for performing the repeated safety parameter evaluations. The proposed framework of analysis is first illustrated with reference to a simple analytical model and then to the estimation of the safety margin on the maximum fuel cladding temperature reached during a complete group distribution header blockage scenario in a RBMK-1500 nuclear reactor. The results are compared with those obtained by a traditional parametric approach. |
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8/2008 - 04/18/2008
Micheletti, Stefano; Perotto, Simona
Space-time adaptation for purely diffusion problems in an anisotropic framework | Abstract | | The main goal of this work is the proposal of an efficient space-time
adaptive procedure for a cGdG approximation of an unsteady diffusion
problem. We derive a suitable a posteriori error estimator where the contribution
of the spatial and of the temporal discretization is kept distinct.
In particular our interest is addressed to phenomena characterized by temporal
multiscale as well as strong spatial directionalities. On the one hand
this leads us to exploit the experience matured on handling anisotropic
grids, to enrich the analysis currently available in the literature. On the
other hand we devise a sound criterion to update the time step, able to
follow the evolution of the problem under investigation. The reliability and
the efficiency of the proposed error estimator are assessed numerically. |
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