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 1251 prodotti
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26/2012 - 28/05/2012
Tamellini, L.; Le Maitre, O.; Nouy, A.
Model reduction based on Proper Generalized Decomposition for the Stochastic steady incompressible Navier-Stokes equations | Abstract | | In this paper we consider a Proper Generalized Decomposition method to solve the steady incompressible Navier-Stokes equations with random Reynolds number and forcing term. The aim of such technique is to compute a low-cost reduced basis approximation of the full Stochastic Galerkin solution of the problem at hand. A particular algorithm, inspired by the Arnoldi method for solving eigenproblems, is proposed for an efficient greedy construction of a deterministic reduced basis approximation. This algorithm decouples the computation of the deterministic and stochastic components of the solution, thus allowing reuse of pre-existing deterministic Navier-Stokes solvers. It has the remarkable property of only requiring the solution of m deterministic problems for the construction of a m-dimensional reduced basis. |
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25/2012 - 17/05/2012
Manfredini, F.; Pucci, P.; Secchi, P.; Tagliolato, P.; Vantini, S.; Vitelli, V.
Treelet decomposition of mobile phone data for deriving city usage and mobility pattern in the Milan urban region | Abstract | | The paper presents a novel geo-statistical unsupervised learning technique aimed at identifying useful information on hidden patterns of mobile phone use. These hidden patterns regard dierent usages of the city in time and in space which are related to individual mobility, outlining the potential of this technology for the urban planning community. The methodology allows to obtain a reference basis that reports the speci c eect of some activities on the Erlang data recorded and a set of maps showing the contribution of each activity to the local Erlang signal. We selected some results as signi cant for explaining speci c mobility and city usages patterns (commuting, nightly activities, distribution of residences, non systematic mobility) and tested their signi cance and their interpretation from an urban analysis and planning perspective at the Milan urban region scale. |
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24/2012 - 16/05/2012
Antonietti, P.F.; Giani, S.; Houston, P.
hp–Version Composite Discontinuous Galerkin Methods for Elliptic Problems on Complicated Domains | Abstract | | In this paper we introduce the hp-version discontinuous Galerkin composite finite element method for the discretization of second–order elliptic partial differential equations. This class of methods allows for the approximation of problems posed on computational domains which may contain a huge number of local geometrical features, or micro-structures. While standard numerical methods can be devised for such problems, the computational effort may be extremely high, as the minimal number of elements needed to represent the underlying domain can be very large. In contrast, the minimal dimension of the underlying composite finite element space is independent of the number of geometric features. The key idea in the construction of this latter class of methods is that the computational domain $ Omega$ is no longer resolved by the mesh; instead, the finite element basis (or shape) functions are adapted to the geometric details present in $ Omega$. In this article, we extend these ideas to the discontinuous Galerkin setting, based on employing the hp-version of the finite element method. Numerical experiments highlighting the practical application of the proposed numerical scheme will be presented. |
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23/2012 - 15/05/2012
Fabio Nobile, Christian Vergara
Partitioned algorithms for fluid-structure interaction problems in haemodynamics | Abstract | | We consider the fluid-structure interaction problem arising in
haemodynamic applications. The finite elasticity equations
for the vessel are written in Lagrangian form,
while the Navier-Stokes equations for the blood in Arbitrary
Lagrangian Eulerian form. The resulting three fields problem
(fluid/ structure/ fluid domain) is formalized via the introduction of
three Lagrange multipliers and consistently discretized by p-th order backward differentiation
formulae (BDFp).
We focus on partitioned algorithms for its numerical solution, which
consist in the successive solution of the three subproblems. We review
several strategies that all rely on the exchange of Robin interface
conditions and review their performances reported recently in the literature.
We also analyze the stability of explicit partitioned procedures and
convergence of iterative implicit partitioned procedures on a simple
linear FSI problem for a general BDFp temporal discretizations. |
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22/2012 - 11/05/2012
Ettinger, B.; Passerini, T.;Perotto, S.; Sangalli, L.M.
Regression models for data distributed over non-planar domains | Abstract | | We consider the problem of surface estimation and spatial smoothing over non-planar domains. In particular, we deal with the case where the data or signals occur on a domain that is a surface in a three-dimensional space. The application driving our research is the modeling of hemodynamic data, such as the shear stress and the pressure exerted by blood flow on the wall of a carotid artery. The regression model we propose consists of two key phases. First, we conformally map the surface domain to a region in the plane. Then, we apply existing regression methods for planar domains, suitably modified to respect the geometry of the original surface
domain. |
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21/2012 - 01/05/2012
Guglielmi, A.; Ieva, F.; Paganoni, A.M.; Ruggeri, F.; Soriano, J.
Semiparametric Bayesian models for clustering and classification in presence of unbalanced in-hospital survival | Abstract | | In this work, Bayesian semiparametric logit models are fitted to grouped data related to in-hospital survival outcome of patients hospitalised with ST-segment Elevation Myocardial Infarction diagnosis. Dependent Dirichlet Process priors are considered for modelling the random-effects distribution of the grouping factor (hospital of admission), in order to provide a cluster analysis of the hospitals. The clustering structure is highlighted through the optimal random partition that minimises the posterior expected value of a suitable loss function. Two are the main goals of the work: to provide model-based clustering and ranking of the providers according to the similarity of their effect on patients’ outcome, and to make reliable predictions on the survival outcome at patient’s level, even when the survival rate itself is strongly unbalanced. The study is within a project, named Strategic Program of Regione Lombardia, and is aimed at supporting decisions in healthcare policies. |
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20/2012 - 24/04/2012
Ieva, F.; Paganoni, A. M.; Zanini, P.
Statistical models for detecting Atrial Fibrillation events | Abstract | | Atrial Fibrillation is the most common cardiac arrhythmia that naturally tends to become a chronic condition and chronic Atrial Fibrillation leads to an increase in the risk of death. The study of time series of time intervals between an R peak in the electrocardiogram and the following one is an effective way to investigate the presence of Atrial Fibrillation and to detect when a single event starts and ends. This work presents a new statistical method to deal with identification of Atrial Fibrillation events. Some simulations in order to assess the performances of the proposed method are detailed and the results
obtained applying this method to real data concerning patients affected by Atrial Fibrillation are discussed. |
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19/2012 - 15/04/2012
Faggiano, E.; Antiga, L.; Puppini, G.; Quarteroni, A.; Luciani G.B.; Vergara, C.
Helical Flows and Asymmetry of Blood Jet in Dilated Ascending Aorta with Normally Functioning Bicuspid Valve | Abstract | | Bicuspid aortic valve (BAV) is associated with aortic dilatation and aneurysm.
Several studies evidenced an eccentric systolic flow in ascending aorta associated to increased wall shear stresses (WSS) and the occurrence of an helical systolic flow.
This study seeks to elucidate the connections between jet asymmetry and helical flow in patients with normally functioning BAV and dilated ascending aorta.
We performed a computational parametric study by varying, for a patient-specific geometry,
the valve area and the flow rate entering the aorta and drawing also a tricuspid valve (TAV).
We considered also phase contrast magnetic resonance imaging of four BAV and TAV patients.
Measurement of normalized flow asymmetry (NFA) index,
systolic WSS and of a new index (positive helic fraction, PHF) quantifying the presence of a single helical flow, were performed.
In our computation, BAV cases featured higher values of all indices with respect to TAV in both numerical and imaged-based results.
Moreover, all indices increased with decreasing valve area and/or with increasing flow rate.
This allows us to identify two groups for BAV: the former with valve/root area ratio r=0.29 and systolic flow rate F>18 l/min, or for r<0.29, featuring high indices values (NFA>0.45 and PHF>0.8), and the latter featuring intermediate values (0.2 |
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