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 1153 products
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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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7/2008 - 03/25/2008
Vergara, Christian; Ponzini, Raffaele; Veneziani, Alessandro; Redaelli, Alberto; Neglia, Danilo; Parodi, Oberdan
Reliable CFD-based Estimation of Flow Rate in Hemodynamics Measures. Part II: Sensitivity Analysis and First Clinical Application | Abstract | | In Ponzini et al. (2006) a new approach has been proposed for estimating in a
reliable way blood flow rate from velocity Doppler measurements. In that
paper, basic features of the approach and some
in silico test
cases were furnished. Here, we give more insights of this approach by
performing a sensitivity analysis of the formulae relating blood flow rate
to blood velocity. In particular we analyze their sensitivity to the
physiological parameters in comparison with the standard formula proposed in
Doucette et al. (1992). A first glance at in vivo validation of the formulae is given too. |
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6/2008 - 02/26/2008
Fumagalli, E.; Lo Schiavo, L.; Paganoni, A. M.; Secchi, P.
Statistical analyses of exceptional events: the Italian experience | Abstract | | In the analysis of reliability performance of distribution utilities
as well as in continuity of supply regulation, criteria are needed
for separating normal operation data from exceptional events. In recent
years a number of statistical methodologies has been proposed for
this purpose. We present here the new methodology that was adopted
by the Italian regulatory authority at the beginning of 2008. The
decision is supported by a statistical analysis of the number of faults
on the MV and on the LV networks, for each 6-hour time interval in
a three year time span, for different provinces and distribution companies.
The new methodology is employed in the reward and penalty mechanisms
that regulate the SAIDI, SAIFI and MAIFI indicators and, with some
original provisions, also in the Guaranteed Standard on maximum restoration
times.
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5/2008 - 02/07/2008
Badia, Santiago; Quaini, Annalisa; Quarteroni, Alfio
Modular vs. non-modular preconditioners for fluid-structure systems with large added-mass effect | Abstract | | In this article we address the numerical simulation of fluid- structure interaction (FSI) problems featuring large added-mass effect.
We analyze different preconditioners for the coupled system matrix obtained after space-time discretization and linearization of the FSI problem. The classical Dirichlet-Neumann preconditioner has the advantage of modularity because it allows to reuse existing fluid and structure codes with minimum effort (simple interface communication). Unfortunately, its performance is very poor in case of large added-mass effects.
Alternatively, we consider two non-modular approaches. The first one consists in preconditioning the coupled system with a suitable diagonal scaling combined with an ILUT preconditioner. The system is then solved by a Krylov method. The drawback of this procedure is that the combination of fluid and structure codes to solve the coupled system is not straightforward. The second non-modular approach we consider is a splitting technique based on an inexact block-LU factorization of the linear FSI system. The resulting algorithm computes the fluid velocity separately from the coupled pressure-structure system at each iteration, reducing the computational cost.
Independently of the preconditioner, the efficiency of semi-implicit algorithms (i.e., those that treat geometric and fluid nonlinearities in an explicit way) is highlighted and their performance compared to the one of implicit algorithms. All the methods are tested on three- dimensional blood-vessel systems. The algorithm combining the non- modular ILUT preconditioner with Krylov methods proved to be the fastest.
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4/2008 - 02/05/2008
Milani, Roberto; Quarteroni, Alfio; Rozza, Gianluigi
Reduced basis method for linear elasticity problems with many parameters | Abstract | | The reduced basis (RB) methods are proposed here for the solution of parametrized equations in linear elasticity problems.
The fundamental idea underlying RB methods is to decouple the generation and projection stages (offline/online computational
procedures) of the approximation process in order to solve parametrized equations in a rapid, inexpensive and reliable way.
The method allows important computational savings with respect to the classical Galerkin-finite element method, ill suited to a repetitive environment like the parametrized contexts of optimization, many queries and sensitivity analysis. We consider different parametrization for the systems: either physical quantities --to model the materials and loads-- and geometrical parameters --to model different geometrical configurations--. Then we describe three different applications of the method in problems with isotropic and orthotropic materials working in plane stress and plane strain approximation and subject to harmonic loads.
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3/2008 - 01/30/2008
Zunino, Paolo; D'Angelo, Carlo; Petrini, Lorenza; Vergara, Christian; Capelli, Claudio; Migliavacca, Francesco
Numerical simulation of drug eluting coronary stents: mechanics, fluid dynamics and drug release | Abstract | | Mathematical models and numerical methods have emerged as fundamental tools in the investigation of life sciences. In particular, this is the case of medical devices as cardiovascular drug eluting stents where experimental/clinical evidence may often be very expensive and extremely variable.
Here we present a complete overview of mathematical models and numerical methods applied to the modelling of drug eluting stents and of their interaction with the coronary arteries.
This is a challenging task because it involves mechanics, fluid dynamics and mass transfer processes.
In particular, we will focus on the importance of the interplay between all these factors to determine the efficacy of the device.
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2/2008 - 01/29/2008
Berrone, Stefano; Verani, Marco
An Adaptive Gradient-DWR Finite Element Algorithm for an Optimal Control Constrained Problem | Abstract | | We present an adaptive finite element algorithm for the numerical approximation of distributed control constrained problems governed by second order elliptic PDEs.
The algorithm is based on a suitable co-operation between a gradient type descent numerical scheme and the dual weighted residual (DWR) method. We assess the efficiency of the algorithm on several test problems and compare its performances with the ones of the residual based adaptive algorithm introduced in [17]. |
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