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 1238 prodotti
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6/2008 - 26/02/2008
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 - 07/02/2008
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 - 05/02/2008
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 - 30/01/2008
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 - 29/01/2008
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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1/2008 - 09/01/2008
Rosso, Riccardo: Verani, Marco
Stabilizing rôle of a curvature correction to line tension | Abstract | | We study the effects that a curvature correction to a bare value of the line tension has on the equilibrium and stability of droplets laid upon a rigid substrate. In the simple case of cylindric liquid bridges we prove that even a tiny curvature correction prevents the onset of wildly oscillating perturbations that would make the contact line unstable if a negative line tension were present alone. However, if the curvature correction is not large enough, unstable modes can persist that are not related to the classical Rayleigh instability.
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26/2007 - 12/12/2007
Colecchia, Maurizio; Nicolai, Nicola; Secchi, Piercesare; Bandieramonte, Gaetano; Paganoni, Anna M.; Sangalli , Laura M.; Piva, Luigi; Salvioni, Roberto
Penile Superficial Squamous cell Carcinoma (SCC) Submitted to CO2 Laser Excision only: Oncologic Outcome of T1 Disease in 25 Years-Long Experience | Abstract | | 1. Purpose. We retrospectively identified patients with ascertained T1 SCC pf the penis who were
submitted to laser excision only, in order to evaluate clinical and pathological characteristics
which can associate with outcomes.
2. Materials and Methods. Peniscopic magnification and 5% acetic acid application were
performed for diagnosis and prior to CO2 laser excision. Only naïve patients with no urethral (> 5
mm from the meatus) neither shaft lesions were considered. Specimens were all reviewed in
order to reassess
pathological characteristics: stage, grade, invasion depth, carcinoma in situ,
margins, tumor extension, lymphovascular invasion and HPV infection. Association between
local recurrence (LR) and prognostic factors was ascertained with Fisher exact test and ChiSquare
test for categorical variables and Wilcoxon rank sum test for continuous variables.
3. Results. A total of 56 pT1 stage patients were identified. After a median followup
of 66 months
(range: 12256),
53 patients are alive and disease free, whereas 3 died of unrelated and intercurrent
diseases. Thirteen (23.21%) had a LR, with 4 suffering multiple recurrences and 1
needing a partial amputation 30 months following primary excision. Only 2 (3.57%) patients had
inguinal nodal metastasis in one node. LR had a positive correlation with positive microsurgical
margins (onesided
Fisher p value .0019) and depth of invasion and a negative correlation with
tumor extension (onesided
Wilcoxon p value of .0028 and .0054, respectively). A logistic
regression model included margin status, depth of invasion, tumor extension and presence of CIS
as significant variables.
4. Conclusions. CO2 laser excision of selected pT1 SCC of the penis is a highly reliable procedure
which associates with excellent oncological and organsparing
long term results. Predictive
variables can be identified and a model that estimates the risk of this event could be adopted. |
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25/2007 - 10/12/2007
Micheletti, Stefano; Perotto, Simona
Space-Time Adaption for Advection-Diffusion-Reaction Problems on Anisotropic Meshes | Abstract | | We deal with the approximation of an unsteady advection-diffusion-
reaction problem by means of space-time finite elements, continuous affine
in space and piecewise constant in time. In particular, we are interested in
the advection-dominated framework. To face the trade-off between
computational cost and accuracy, we devise a space-time adaptive procedure
where both the time step and the spatial grid are adapted throughout the
simulation. Two are the key points involved: the derivation of an a
posteriori error estimator where the contributions of the spatial and of the
temporal discretization are split; a balance of these two contributions via a
proper adaptive scheme. The main novelty of the paper is the interest for
an anisotropic mesh adaption framework.
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