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 1287 prodotti
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11/2020 - 16/02/2020
Antonietti, P.F.; Facciola', C.; Houston, P.; Mazzieri, I.; Pennes, G.; Verani, M.
High-order discontinuous Galerkin methods on polyhedral grids for geophysical applications: seismic wave propagation and fractured reservoir simulations
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10/2020 - 16/02/2020
Bonaventura, L.; Carlini, E.; Calzola, E.; Ferretti, R.
Second order fully semi-Lagrangian discretizations of advection--diffusion--reaction systems | Abstract | | We propose a second order, fully semi-Lagrangian method for the numerical solution of systems of advection--diffusion--reaction equations, which employs a semi-Lagrangian approach to approximate in time both the advective and the diffusive terms. Standard interpolation procedures are used for the space discretization on structured and unstructured meshes.
The proposed method allows for large time steps, while avoiding the solution of large linear systems, which would be required by an implicit time discretization technique. Numerical experiments demonstrate the effectiveness of the proposed approach and its superior efficiency with respect to more conventional explicit and implicit time discretizations. |
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09/2020 - 16/02/2020
Rea, F.; Ieva, F.,; Pastorino, U.; Apolone, G.; Barni, S.; Merlino, L.; Franchi, M.; Corrao, G.
Number of lung resections performed and long-term mortality rates of patients after lung cancer surgery: evidence from an Italian investigation | Abstract | | OBJECTIVES: Although it has been postulated that patients might benefit from the centralization of high-volume specialized centres, conflicting results have been reported on the relationship between the number of lung resections performed and the long-term, all-cause mortality rates among patients who underwent surgery for lung cancer. A population-based observational study was performed to contribute to the ongoing debate.
METHODS: The 2613 patients, all residents of the Lombardy region (Italy), who underwent lung resection for lung cancer from 2012 to 2014 were entered into the cohort and were followed until 2018. The hospitals were classified according to the annual number of pulmonary resections performed. Three categories of lung resection cases were identified: low (<30), intermediate (31–95) and high (>95). The outcome of interest was all-cause death. A frailty model was used to estimate the death risk associated with the categories of numbers of lung resections performed, taking into account the multilevel structure of the data. A set of sensitivity analyses was performed to account
for sources of systematic uncertainty.
RESULTS: The 1-year and 5-year survival rates of cohort members were 90% and 63%. Patients operated on in high-volume centres were on average younger and more often women. Compared to patients operated on in a low-volume centre, the mortality risk exhibited a significant,progressive reduction as the numbers of lung resections performed increased to intermediate (-13%; 95% confidence interval +10% to -31%) and high (-26%; 0% to -45%). Sensitivity analyses revealed that the association was consistent.
CONCLUSIONS: Further evidence that the volume of lung resection cases performed strongly affects the long-term survival of lung cancer patients has been supplied. |
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08/2020 - 16/02/2020
Antonietti, P. F.; Facciolà, C.; Verani, M.
Polytopic Discontinuous Galerkin methods for the numerical modelling of flow in porous media with networks of intersecting fractures | Abstract | | We present a numerical approximation of Darcy's flow through a porous medium that incorporates networks of fractures with non empty intersection. Our scheme employs PolyDG methods, i.e. discontinuous Galerkin methods on general polygonal and polyhedral (polytopic, for short) grids, featuring elements with edges/faces that may be in arbitrary number (potentially unlimited) and whose measure may be arbitrarily small. Our approach is then very well suited to tame the geometrical complexity featured by most of applications in the computational geoscience field. From the modelling point of view, we adopt a reduction strategy that treats fractures as manifolds of codimension one and we employ the primal version of Darcy's law to describe the flow in both the bulk and in the fracture network. In addition, some physically consistent conditions couple the two problems, allowing for jump of pressure at their interface, and they as well prescribe the behaviour of the fluid along the intersections, imposing pressure continuity and flux conservation.
Both the bulk and fracture discretizations are obtained employing the Symmetric Interior Penalty DG method extended to the polytopic setting. The key instrument to obtain a polyDG approximation
of the problem in the fracture network is the generalization of the concepts of jump and average at the intersection, so that the contribution from all the fractures is taken into account. We prove
the well-posedness of the discrete formulation and perform an error analysis obtaining a priori hp-error estimates. All our theoretical results are validated performing preliminary numerical
tests with known analytical solution. |
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07/2020 - 22/01/2020
Fumagalli, A.; Scotti, A.
Reactive flow in fractured porous media | Abstract | | In this work we present a model reduction procedure to derive a hybrid-dimensional framework for the mathematical modeling of reactive transport in fractured porous media. Fractures are essential pathways in the underground which allow fast circulation of the fluids present in the rock matrix, often characterized by low permeability. However, due to infilling processes fractures may change their hydraulic properties and become barriers for the flow creating impervious blocks. The geometrical as well as the physical properties of the fractures require a special treatment to allow the subsequent numerical discretization to be affordable and accurate. The aim of this work is to introduce a simple yet complete mathematical model to account for such diagenetic effects where chemical reactions will occlude or empty portions of the porous media and, in particular, fractures. |
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06/2020 - 22/01/2020
Domanin, M.; Piazzoli, G.; Trimarchi, S.; Vergara, C.
Image-based displacements analysis and computational blood dynamics after endovascular aneurysm repair | Abstract | | Purpose. To examine intra heartbeat displacements (IHD) and geometrical changes over years, defined as follow-up displacements (FUD), of the endograft for abdominal aortic aneurysm repair, and to correlate them with computational fluid dynamics (CFD). Despite the widespread diffusion of endovascular aneurysm repair (EVAR) we still do not know very much about endograft behavior after deployment.
Methods. Two cases, treated with expanded polytetrafluoroethylene (PTFE) on nitinol stent frame (PI) and with woven polyester fabric sutured to stainless steel Z-stent skeleton (PII), respectively, were submitted to dynamic computed tomography angiography (CTA) at 1, 12 and 60 months. After segmentation, IHD were computed as displacements of the reconstructed surface with respect to the diastolic instant. Similarly, FUD were studied using imaging techniques that align temporal successive segmentations. At last, numerical simulations for blood dynamics were performed to compute viscous forces i.e. Wall Shear Stress (WSS) and Time Average WSS (TAWSS).
Results. IHD analysis showed slight translations without deformation for PI endograft with respect to the stiffer stainless steel endograft behaviour of PII. FUD showed in PI motion of the metallic struts mainly focused in the distal main body and in overlapping zone with iliac branches while in PII we observed a huge FUD in the middle and inferior-anterior regions of the main body. CFD analysis revealed changes of velocity patterns associated to remodelling of the iliac zone for PI and of the main body region for PII, where flow impinges the lumen wall and progressively provokes deformation of the endograft wires. TAWSS exhibits flow disturbances in the enlarged region coherently with displacements analysis.
Conclusion. Image-based displacements analysis associated to CFD allow to perform very subtle evaluations of endograft behaviour on different temporal scales. This kind of study could be helpful both for physicians, forecasting evolution during the life span of the endograft, and for manufacturers, giving them useful indication about endograft implant and design.
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05/2020 - 22/01/2020
Artioli, E.; Beiraoda Veiga, L.; Verani, M.
An adaptive curved virtual element method for the statistical homogenization of random fibre-reinforced composites | Abstract | | In the framework of statistical asymptotic homogenization of random fibre-reinforced composites, we propose a curved virtual element procedure that allows an exact geometric representation.
We develop an approach that is able to represent exactly the involved geometry and exploits an adaptive tuning of the optimal mesh resolution through a robust and efficient residual-based a-posteriori error estimator.
Furthermore, by combining such scheme and Monte Carlo simulations, a methodology is developed to determine homogenized material moduli and representative unit cell size. A gallery of numerical tests supports the proposed approach. |
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04/2020 - 21/01/2020
Didkovskyi, O.; Azzone, G.; Menafoglio A.; Secchi P.
Social and material vulnerability in the face of seismic hazard: an analysis of the Italian case | Abstract | | The assessment of the vulnerability of a community endangerd by seismic hazard is of paramount importance for planning a precision policy aimed at the prevention and reduction of its seismic risk. We aim at measuring the vulnerability of the Italian municipalities exposed to seismic hazard, by analyzing the open data offered by the Mappa dei Rischi dei Comuni Italiani provided by ISTAT, the Italian National Institute of Statistics. Encompassing the Index of Social and Material Vulnerability already computed by ISTAT, we also consider as referents of the latent social and material vulnerability of a community, its demographic dynamics and the age of the building stock where the community resides. Fusing the analyses of different indicators, within the context of seismic risk we offer a tentative ranking of the Italian municipalities in terms of their social and material vulnerability, together with differential profiles of their dominant fragilities which constitute the basis for planning precision policies aimed at seismic risk prevention and reduction.
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