Quaderni MOX
Pubblicazioni
del Laboratorio di Modellistica e Calcolo Scientifico MOX. I lavori riguardano prevalentemente il campo dell'analisi numerica, della statistica e della modellistica matematica applicata a problemi di interesse ingegneristico. Il sito del Laboratorio MOX è raggiungibile
all'indirizzo mox.polimi.it
Trovati 1277 prodotti
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25/2009 - 26/08/2009
Farrell, P.E.; Micheletti, Stefano; Perotto, Simona
An anisotropic Zienkiewicz-Zhu a posteriori error estimator for 3D applications | Abstract | | We extend the anisotropic Zienkiewicz-Zhu a posteriori error estimator of [1] to three dimensions. Like the standard Zienkiewicz-Zhu estimator, the proposed estimator is designed to be independent of the problem at hand, is cheap to compute and easy to implement. In contrast to the standard Zienkiewicz-Zhu estimator, the elementwise counterpart of the proposed
estimator explicitly takes into account the geometrical properties of the actual tetrahedron. Thus, in a wide variety of applications, the estimator is able to detect the anisotropic features exhibited by the solution of the
governing equations. A metric-based optimization procedure, rigorously addressed, drives the adaptation of the mesh. It is shown numerically
to yield quasi-optimal triangulations, dictating the accuracy-vs-number of elements behaviour. Despite being heuristic to some extent, in practice the overall anisotropic adaptation procedure turns out to be effective. |
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24/2009 - 25/08/2009
Di Maio, Francesco; Secchi, Piercesare; Vantini, Simone; Zio, Enrico
Optimized Fuzzy C-Means Clustering and Functional Principal Components for Post-Processing Dynamic Scenarios in the Reliability Analysis of a Nuclear System | Abstract | | This paper deals with the processing of accident scenarios generated from a dynamic reliability analysis of a Nuclear Power Plant (NPP). A large number of scenarios are simulated to account for the influence of the timing and magnitudes of fault events on the accident end states; post-simulation processing is then required for retrieving the safety-relevant information.
For classifying the final system state reached at the end of the accident scenarios, Fuzzy C-Means clustering is performed with different sets of Functional Principal Components (FPCs) of a selected relevant process variable. The approach allows capturing the characteristics of the process evolution determined by the occurrence,
timing, and magnitudes of the fault events.
An illustrative case study is considered, regarding the fault scenarios of the digital I&C
of the Lead Bismuth Eutectic eXperimental Accelerator Driven System (LBE-XADS).
The results obtained are compared with those of the Kth Nearest Neighbor (KNN) and Classification and Regression Tree (CART) classifiers. |
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23/2009 - 24/08/2009
Gerardo Giorda, Luca; Nobile, Fabio; Vergara, Christian
Analysis and optimization of Robin-Robin partitioned procedures in Fluid-Structure Interaction Problems | Abstract | | In the solution of Fluid-Structure Interaction problems, partitioned procedures are modular algorithms that involve separate fluid and structure solvers, that interact, in an iterative framework, through the exchange of suitable transmission conditions at the FS interface. In this work we study, using Fourier analysis, the convergence of partitioned algorithms based on
Robin transmission conditions. We derive, for different models of the fluid and the structure, a frequency dependent reduction factor at each iteration of the partitioned algorithm, which is minimized by choosing optimal values of the coefficients in the Robin transmission conditions. Two-dimensional numerical results are also reported, which highlight the effectiveness of the optimization procedure. |
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22/2009 - 21/08/2009
Formaggia, Luca; Minisini, Sara; Zunino, Paolo
Modeling erosion controlled drug release and transport phenomena in the arterial tissue | Abstract | | We introduce and analyze a model for simulating the release of a drug from a polymeric matrix into the arterial tissue, with the aim to describe
the processes which occur after the implantation of a cardiovascular drug eluting stent (DES). The main processes occurring in the polymeric matrix
are drug dissolution and diffusion. Moreover, surface erosion, which consists in mass loss due to the degradation of the polymeric network, is considered as well. The drug eluted from the matrix is released in the arterial wall, modelled as an homogeneous porous medium. By consequence, we assume that drug molecules are transported by diffusion and convection. Moreover, inside the tissue the reversible reaction of the drug with specific binding sites is taken into account and the coupled problem of mass transfer between
matrix and tissue is formulated. It is shown that the mass conservation principle leads to non standard boundary coupling conditions to describe
the transfer of the drug both in the solid and dissolved phases. Then, the problem at hand is solved numerically, highlighting the importance of
enforcing mass conservation and focusing on the influence of the polymer erosion on the drug release profile and drug distribution in the tissue. |
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21/2009 - 20/08/2009
Bonaventura, Luca; Castruccio, Stefano; Sangalli, Laura M.
A Bayesian approach to geostatistical interpolation with exible variogram models | Abstract | | A Bayesian approach to covariance estimation and geostatistical interpolation based on flexible variogram models is introduced. In particular,
we consider black-box kriging models. These variogram models do not require restrictive assumptions on the functional shape of the variogram; furthermore, they can handle quite naturally non isotropic random fields.
The proposed Bayesian approach does not require the computation of an empirical variogram estimator, thus avoiding the arbitrariness implied by the construction of the empirical variogram itself. Moreover, it provides a complete assessment of the uncertainty in the variogram estimation. The advantages of this approach are illustrated via an extensive simulation study and by application to a well known benchmark dataset. |
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20/2009 - 25/07/2009
Accoto, Nadia; Rydén, Tobias; secchi, Piercesare
Bayesian hidden Markov models for performance-based regulation of continuity of electricity supply | Abstract | | A fundamental aspect in the regulation of continuity of electricity supply is the identification of faults that could be caused by an exceptional event and therefore that are outside the utility control and responsibility.
Different methods have been proposed during the years; the interpretation of the observed faults as a signal of an underlying system naturally leads to the analysis of the problem by means of an hidden Markov model. These models, in fact, are widely used for introducing dependence in data and/or for modelling observed phenomena depending on hidden processes. The application of this method shows that the model is able to identify exceptional events; moreover the study of the estimated model parameters gives rise to reality-linked considerations. |
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19/2009 - 24/07/2009
Formaggia, Luca; Mola, Andrea; Parolini, Nicola; Pischiutta, Matteo
A three-dimensional model for the dynamics and hydrodynamics of rowing boats | Abstract | | This paper proposes a new model describing the dynamics of a rowing boat for general three dimensional motions. The complex interaction among the different components of the rowers/oars/boat system is analysed and reduced to a set of ordinary differential equations governing the rigid motion along the six degrees of freedom. To treat the unstable nature of the physical problem, a rather simple (but effective) control model is included, which mimics the main active control techniques adopted by the rowers during their action. |
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18/2009 - 23/07/2009
Silva Soares, Joao; Zunino, Paolo
A mathematical model for water uptake, degradation, erosion and drug release from degradable polydisperse polymeric networks | Abstract | | We introduce a general class of mixture models to study water uptake, degradation, erosion, and drug release from degradable polydisperse polymeric matrices.
The mathematical model is based on a finite number of constituents describing the polydisperse polymeric system, i.e. each representing collection of chains whose size
belongs to a finite interval of degree of polymerization. In order to model water uptake
and drug release, two additional constituents (water and drug) constitute the mixture.
Constituents diffuse individually accordingly to Fick’s first law and balances of mass of constituents yield partial differential equations that govern the reaction-diffusion system. Hydrolysis, a chemical reaction that breaks down larger chains into smaller ones, is accounted with reactions terms quantifying sources and sinks of polymeric chains and a sink of water. Hydrolysis couples the system of equations and nonlinearities appear through constitutive specification of the diffusivities of constituents on the current network, reaction rates, and boundary conditions.
The mathematical model is independent of the number of constituents describing the polydisperse polymeric system and hydrolysis kinetics describe with accuracy the overall decrease in molecular weight distribution and satisfies a monomer conservation principle. A shift between two different types of solutions of the system of partial differential equations, each identified to surface or bulk erosion, is obtained with the variation of a single non-dimensional number, the Thiele modulus, which measures the relative importance of the mechanisms of reaction and diffusion. Results of drug release confirm that drug release from bulk eroding matrices is diffusion-controlled, whereas for surface eroding polymers, drug release is enhanced in an erosion-controlled process. |
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