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 1249 prodotti
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09/2016 - 19/02/2016
Rizzo, C.B.; de Barros, F.P.J.; Perotto, S.; Oldani, L.; Guadagnini, A.
Relative impact of advective and dispersive processes on the efficiency of POD-based model reduction for solute transport in porous media | Abstract | | We study the applicability of a model order reduction technique to the cost-effective solution of transport of passive scalars in porous media. Transport dynamics is modeled through the advection-dispersion equation (ADE) and we employ Proper Orthogonal Decomposition (POD) as a strategy to reduce the computational burden associated with the numerical solution of the ADE. Our application of POD relies on solving the governing ADE for selected time intervals, termed snapshots. The latter are then employed to achieve the desired model order reduction. The problem dynamics require alternating, over diverse time scales, between the solution of the full numerical transport model, as expressed by the ADE, and its reduced counterpart, constructed through the selected snapshots. We explore the way the selection of these time scales is linked to the Péclet number characterizing transport under steady-state flow conditions taking place in two-dimensional homogeneous and heterogeneous porous media. We find that the length of the time scale within which the POD-based reduced model solution provides accurate results tends to increase with decreasing Péclet number. This suggests that the effects of local scale dispersive processes facilitate the POD method to capture the salient features of the system dynamics embedded in the selected snapshots. Since the dimension of the reduced model is much lower than that of the full numerical model, the methodology we propose enables one to accurately simulate transport at a markedly reduced computational cost. |
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08/2016 - 19/02/2016
Dassi, F.; Perotto, S.; Si, H.; Streckenbach, T.
A priori anisotropic mesh adaptation driven by a higher dimensional embedding | Abstract | | We generalize the higher embedding approach proposed in B. Levy, N. Bonnell, 2012,
to generate an adapted mesh matching the intrinsic directionalities of an assigned function.
In more detail, the original embedding map between the physical (lower dimensional) and the embedded (higher dimensional) setting
is modified to include information associated with the function and with its gradient. Then, we
set an adaptive procedure driven by the embedded metric but performed in the lower dimensional setting
which results into an anisotropic adapted mesh of the physical domain.
The effectiveness of the proposed procedure is extensively investigated on several two-dimensional test cases,
involving both analytical functions and finite element approximations of differential problems.
The preliminary verification in three dimensions corroborates the robustness of the method. |
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06/2016 - 16/02/2016
Micheletti, S.; Perotto, S.; Signorini, M.
Anisotropic mesh adaptation for the generalized Ambrosio-Tortorelli functional with application to brittle fracture | Abstract | | Quasi-static crack propagation in brittle materials is modeled via the
Ambrosio-Tortorelli approximation [7]. The crack is modeled by a smooth
phase-field, defined on the whole computational domain. Since the crack
is confined to a thin layer, the employment of anisotropic adapted grids is
shown to be a really effective tool in containing computational costs. We
extend the error analysis in [3, 4, 5] to the generalized Ambrosio-Tortorelli functional introduced in [8], where a unified framework for several elasticity laws is dealt with as well as a non-convex fracture energy can be accommodated. After deriving an anisotropic a posteriori error estimator, we devise an algorithm which alternates optimization and mesh adaptation. Both anti-plane and plane-strain configurations are numerically checked.
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05/2016 - 16/02/2016
Alfio Quarteroni, A.; Lassila, T.; Rossi, S.; Ruiz-Baier, R.
Integrated Heart - Coupling multiscale and multiphysics models for the simulation of the cardiac function | Abstract | | Mathematical modelling of the human heart and its function can expand our understanding of various cardiac diseases, which remain the most common cause of death in the developed world. Like other physiological systems, the heart can be understood as a complex multiscale system involving interacting phenomena at the molecular, cellular, tissue, and organ levels. This article addresses the numerical modelling of many aspects of heart function, including the interaction of the cardiac electrophysiology system with contractile muscle tissue, the sub-cellular activation-contraction mechanisms, as well as the haemodynamics inside the heart chambers. Resolution of each of these sub-systems requires separate mathematical analysis and specially developed numerical algorithms, which we review in detail. By using specific sub-systems as examples, we also look at systemic stability, and explain for example how physiological concepts such as microscopic force generation in cardiac muscle cells, translate to coupled systems of differential equations, and how their stability properties influence the choice of numerical coupling algorithms. Several numerical examples illustrate three fundamental challenges of developing multiphysics and multiscale numerical models for simulating heart function, namely: (i) the correct upscaling from single-cell models to the entire cardiac muscle, (ii) the proper coupling of electrophysiology and tissue mechanics to simulate electromechanical feedback, and (iii) the stable simulation of ventricular haemodynamics during rapid valve opening and closure.
Key words: Integration of cardiac function, Coupling of multiphysics and multiscale models, Electrophysiology, Nonlinear elasticity, Navier-Stokes equations, Reaction-diffusion systems, Finite element methods, Stability analysis, Numerical simulation |
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04/2016 - 01/01/2016
Pettinati, V.; Ambrosi, D; Ciarletta P.; Pezzuto S.
Finite element simulations of the active stress in the imaginal disc of the Drosophila Melanogaster | Abstract | | During the larval stages of development, the imaginal disc of Drosphila Melanogaster is composed by a monolayer of epithelial cells, which undergo a strain actively produced by the cells themselves. The well organized collective contraction produces a stress field that seemingly has a double morphogenetic role: it orchestrates the cellular organization towards
the macroscopic shape emergence while simultaneously providing a local information on the organ size. Here we perform numerical simulations of such a mechanical control on morphogenesis at a continuum level, using a three-dimensional finite model that accounts for the active cell contraction. The numerical model is able to reproduce the (few) known
qualitative characteristics of the tensional patterns within the imaginal disc of the fruit fly. The computed stress components slightly deviate from planarity, thus confirming the previous theoretical assumptions of a nonlinear elastic analytical model, and enforcing the hypothesis that the mechanical stress may act as a size regulating signal that locally scales
with the global dimension of the domain. |
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03/2016 - 01/01/2016
Tarabelloni, N.; Ieva, F.
On Data Robustification in Functional Data Analysis | Abstract | | The problem of outlier detection in high dimensional settings is nowadays a crucial point for a number of statistical analysis. Outliers are often considered as an error or noise, instead, they may carry important information on the phenomenon under study. If not properly identified, they may lead to model misspecification, biased parameter estimation and incorrect results, especially in those contexts where the number of available statistical units is lower than the number of parameters (for example, Functional Data Analysis).
In this paper we introduce a robustly adjusted version of the functional boxplot, which is the most common tool adopted to perform outlier detection in Functional Data Analysis. A crucial element of the functional boxplot is the inflation factor of the fences, controlling the proportion of observations flagged as outlier. After an overview of the methods and tools currently available in the literature, we will describe a robust method to compute a data-driven value for such inflation factor. In doing so, we will make use of robust estimators of variance-covariance operators and the corresponding eigenvalues and eigenfunctions.
Two simulation studies are proposed to give direct insights into the use of the proposed functional boxplot, and test both the robustness and accuracy of robust variance-covariance estimators, together with the performances of the functional boxplot in recognising truly outlying observations. |
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02/2016 - 01/01/2016
Crivellaro, A.; Perotto, S.; Zonca, S.
Reconstruction of 3D scattered data via radial basis functions by efficient and robust techniques | Abstract | | We propose two algorithms to overcome separately two of the most constraining limitations of surface reconstruction methods in use. In particular, we focus on the large amount of data characterizing standard acquisitions by scanner and the noise intrinsically introduced by measurements. The first algorithm represents an adaptive variant of the multi-level interpolating approach proposed in [Ohtake et al., ACM Transactions on Graphics, 2003], based on an implicit surface representation via radial basis functions. The second procedure is based on a least-squares approximation to filter noisy data. An extensive numerical validation is performed to check the performances of the proposed techniques. |
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01/2016 - 01/01/2016
Domanin, M.; Buora, A.; Scardulla, F.; Guerciotti, B.; Forzenigo, L.; Biondetti, P.; Vergara, C.
Computational fluid-dynamic analysis of carotid bifurcations after endarterectomy: closure with patch graft versus direct suture | Abstract | | Objectives. This study aims at comparing by means of computational fluid-dynamics (CFD) the hemodynamics before and after carotid endarterectomy (CEA), and, after CEA, between patch graft (PG) and direct suture (DS) closures.
Design. We analyzed wall shear stress (WSS), velocities, and vorticity before and after CEA, to highlight the effect of surgery on hemodynamics, and time averaged WSS (TAWSS), oscillatory shear index (OSI) and relative residence time (RTT) after CEA to highlight the differences in terms
of risk of restenosis between PG and DS.
Methods. Thirteen cases underwent CEA for stenosis > 70% and were closed with 9 PG and 4 DS. For each of them we measured the flow with Doppler Ultrasound (DUS) and reconstructed geometries from Magnetic Resonance Angiography (MRA).
Results. The mean value of the spatial averaged TAWSS was 1.0 Pa for PG and 1.5 Pa for DS, whereas it was 0.03 for PG and 0.07 for DS in the case of OSI. The percentage areas with low TAWSS and high OSI resulted wider for PG in comparison to DS (TAWSS: 66.2% for PG and 37.2% for DS; OSI: 10.1% for PG and 3.7% for OSI). RRT values resulted higher in the PG group.
In particular, the mean of the averaged-in-space values was 4.4 1/Pa for PG and 1.6 1/Pa for DS, whereas the PA with high RRT were 22.5% and 6.5%, respectively.
Conclusions. The carotid regions with percentage area of low TAWSS, high OSI and high RRT were wider in the PG group when compared with DS. Thus, hemodynamic conditions more prone to restenosis were observed for PG. The increase of the cross-section area created by PG could induce flow disturbances with potential consequences on long term outcomes. Selective use of PG should consider specific conditions of the patients. |
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