MOX Reports
The preprint collection of the Laboratory for Modeling and Scientific Computation MOX. It mainly contains works on numerical
analysis and mathematical modeling applied to engineering problems. MOX web site is mox.polimi.it
Found 1253 products
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03/2011 - 01/13/2011
Garegnani, G.; Rosatti, G.; Bonaventura, L.
Mathematical and Numerical Modelling of Fully Coupled Mobile Bed Free Surface Flows | Abstract | | We analyze the most widely used formulations of sediment transport modelling in the framework of one dimensional channel flow, thereby assessing the impact of typical simplifying assumptions, such as low sediment concentration and decoupling of hydrodynamics and bed evolution. As a result, the importance of using a quasi two-phase formulation is highlighted. Starting from the
a quasi two-phase model equations, under the hypothesis of quasi-steady free surface and mixture flows, we derive a simplified equation for the bed evolution, that is also valid in the large sediment concentration regime. The solution of such a simplified equation provides a useful benchmark for numerical methods aimed at computing approximate solutions of the quasi two-phase system.
Finally,we propose and evaluate a highly efficient and accurate semi-implicit and
semi-Lagrangian numerical method for quasi two-phase mobile bed system. |
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02/2011 - 01/10/2011
Lassila, T.; Quarteroni, A.; Rozza, G.
A reduced basis model with parametric coupling for fluid-structure interaction problems | Abstract | | We present a new model reduction technique for steady fluid-structure interaction problems. When the fluid domain deformation is suitably
parametrized, the coupling conditions between the
fluid and structure can be formulated in the low-dimensional space of geometric parameters.
Moreover, we apply the reduced basis method to reduce the cost of repeated fluid solutions necessary to achieve convergence of fluid-structure iterations. In this way a reduced order model with reliable a posteriori error bounds is obtained. The proposed method is validated with an example of steady Stokes flow in an axisymmetric channel, where the structure is described by a simple 1-d generalized string model. We demonstrate rapid convergence of the reduced solution of the parametrically coupled problem as the number of geometric parameters is increased. |
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01/2011 - 01/05/2011
Dalla Rosa, M.; Sangalli, L. M.; Vantini, S.
Dimensional Reduction of Functional Data by means of Principal Differential Analysis | Abstract | | We explore the use of principal differential analysis (PDA) as a tool for performing dimensional reduction of functional data sets. In particular, we compare the results provided by PDA and by functional principal component
analysis (FPCA) in the dimensional reduction of three synthetic data sets, and of a real data set concerning 65 vascular geometries (i.e., the AneuRisk data set).
The analyses of the synthetic data sets show that PDA can provide an alternative and effective representation of functional data that is always
easily interpretable in terms of constant, exponential, sinusoidal, or dampedsinusoidal
functions and not affected by the presence of clusters or strong correlations among the original components. Moreover, in the analysis of
the AneuRisk data set, PDA is able to detect important features of the data that FPCA is not able to detect. |
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43/2010 - 12/10/2010
Pennati, G.; Dubini, G.; Migliavacca, F.; Corsini, C.; Formaggia, L.; Quarteroni,A.; Veneziani, A.
Multiscale Modelling with Application to Paediatric Cardiac Surgery | Abstract | | The study of the haemodynamics of 3D models
of vascular districts with complex anatomy with the aid of numerical models requires to prescribe correct conditions at the boundary of the district of interest. The cardiovascular system is a highly integrated circuit and global systemic
effects cannot be neglected even when the interest lays on a specific local area.
To this aim, a novel approach, called geometrical multiscale, has been devised where models of
different level of detail (and computational cost) are coupled together. In particular, we focus in this work on an application where the 3D Navier–Stokes equations are coupled with a non-linear system of ordinary differential
equations governing the systemic circulation. The application is the simulation of different procedures for paediatric cardiac surgery. |
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42/2010 - 12/03/2010
Baraldo, S.; Ieva, F.; Paganoni, A. M.; Vitelli, V.
Generalized functional linear models for recurrent events: an application to re-admission processes in heart failure patients | Abstract | | An effective methodology for dealing with data extracted from clinical heart failure databases and the Public Health Database is proposed. A model for recurrent events is used for modeling the occurrence of hospital readmissions in time, thus deriving a suitable way to compute individual cumulative hazard functions. Estimated cumulative hazard trajectories are then treated as functional data, and their relation to clinical relevant responses is studied in the framework of generalized functional linear models. |
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41/2010 - 11/24/2010
Ambrosi, D.; Arioli, G.; Nobile, F.; Quarteroni, A.
Electromechanical coupling in cardiac dynamics: the active strain approach | Abstract | | The coupling between cardiac mechanics and electric signaling is addressed in a non–standard framework in which the electrical potential dictates the active strain (not stress) of the muscle. The physiological and mathematical motivations leading us to this choice are illustrated. The propagation of the electric signal is assumed to be governed by the FitzHugh–
Nagumo equations, rewritten in material coordinates with a deforming substrate; the solution is compared with the rigid case and differences in celerity and width of a pulse are discussed. The role of visco-elasticity is pointed
out. We show that the stretching of coordinates is insufficient to originate electromechanical feedback; nevertheless, it can increase the energy of a perturbation enough to produce a traveling pulse: an energy estimate and numerical evidence are reported. To support these conclusions, numerical simulations in two dimensions show the interplay between electric propagation and mechanical strain. |
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40/2010 - 11/21/2010
D'Angelo, C.; Scotti, A.
A Mixed Finite Element Method for Darcy Flow in Fractured Porous Media with non-matching Grids | Abstract | | We consider an incompressible flow problem in a N-dimensional fractured porous domain(Darcy’s problem). The fracture is represented by a (N − 1)-dimensional interface, exchanging fluid with the surrounding media. In this paper we consider the lowest-order(RT0, P0) Raviart-Thomas mixed finite element method for the approximation of the coupled Darcy’s flows in the porous media and within the fracture, with independent meshes for the respective domains. This is achieved thanks to an enrichment with discontinuous basis functions on triangles crossed by the fracture and a weak imposition of interface conditions.
First, we study the stability and convergence properties of the resulting numerical scheme in the uncoupled case, when the known solution of the fracture problem provides an immersed boundary condition. We detail the implementation issues and discuss the algebraic properties of the associated linear system. Next, we focus on the coupled problem and propose an iterative porous domain / fracture domain iterative method to solve for fluid flow in both the porous media and the fracture and compare the results with those of a traditional monolithic approach.
Numerical results are provided confirming convergence rates and algebraic properties predicted by the theory. In particular, we discuss preconditioning and equilibration techniques to make the condition number of the discrete problem independent of the position of the immersed interface. Finally, two and three dimensional simulations of Darcy’s flow in different configurations (highly and poorly permeable fracture) are analyzed and discussed. |
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39/2010 - 11/16/2010
D'Angelo, C.
Finite Element Approximation of Elliptic Problems with Dirac Measure Terms in Weighted Spaces. Applications to 1D-3D Coupled Problems | Abstract | | In this work we study the stability and the convergence rates of the finite element approximation of elliptic problems involving Dirac measures, using weighted Sobolev spaces and weighted discrete norms. Our approach handles
both the cases where the measure is simply a right hand side or it represents an additional term, i.e. solution-dependent, in the formulation of the problem.
The main motivation of this study is to provide a methodological tool to treat elliptic problems in fractured domains, where the coupling terms are seen as Dirac measures concentrated on the fractures. We first establish a decomposition lemma, which is our fundamental tool for the analysis of the considered problems in the non-standard setting of weighted spaces. Then, we consider the stability of the Galerkin approxima-
tion with finite elements in weighted norms, with uniform and graded meshes.
We introduce a discrete decomposition lemma that extends the continuous one and allows to derive discrete inf-sup conditions in weighted norms. Then, we focus on the convergence of the finite element method. Due to the lack of regularity, the convergence rates are suboptimal for uniform meshes; we show that using graded meshes optimal rates are recovered. Our theoretical results are supported by several numerical experiments. Finally, we show how our theoretical results apply to certain coupled problems involving
fluid flow in porous three-dimensional media with one-dimensional fractures, that are found in the analysis of microvascular flows.
Keywords: elliptic problems, measure, Dirac measure, weighted spaces, nite element method, graded
mesh, error estimates, reduced models, multiscale models, microcirculation. |
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