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 1349 products
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20/2012 - 04/24/2012
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 - 04/15/2012
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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18/2012 - 04/14/2012
Formaggia, L.; Vergara, C.
Prescription of general defective boundary conditions in fluid-dynamics | Abstract | | This work reviews and extends to a more general setting some strategies
to impose defective boundary conditions to fluid-dynamic problems investigated
by the authors in the last years. We focus here to the steady Stokes
problem as a paradigm for the unsteady and non-linear cases. We show
the well posedness of the proposed approaches and discuss their relative
benefits. |
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17/2012 - 03/20/2012
Manzoni, A.; Quarteroni, A.; Gianluigi Rozza, G.
Computational reduction for parametrized PDEs: strategies and applications | Abstract | | In this paper we present a compact review on the mostly used techniques for computational reduction in numerical approximation of partial differential equations. We highlight the common features of these techniques and provide a detailed presentation of the reduced basis method, focusing on greedy algorithms for the construction of the reduced spaces. An alternative family of reduction techniques based on surrogate response surface models is briefly recalled too. Then, a simple example dealing with inviscid flows is presented, showing the reliability of the reduced basis method and a comparison between this technique and some surrogate models. |
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16/2012 - 03/14/2012
Cutri', E.; Zunino, P.; Morlacchi, S.; Chiastra, C.; Migliavacca, F.
Drug delivery patterns for different stenting techniques in coronary bifurcations: a comparative computational study | Abstract | | The treatment of coronary bifurcation lesions represents a challenge for the interventional cardiologists due to the lower rate of procedural success and the higher risk of restenosis. The advent of drug eluting stents (DES) has dramatically reduced restenosis and consequently the request for re-intervention. The aim of the present work is to provide further insight about the effectiveness of DES by means of a computational study that combines virtual stent implantation, fluid dynamics and drug release for
different stenting protocols currently used in the treatment of a coronary artery bifurcation. An explicit dynamic finite element model is developed in order to obtain realistic configurations of the implanted devices used to perform fluid dynamics analysis by means of a previously developed finite element method coupling the blood flow and the intramural plasma filtration in rigid arteries. To efficiently
model the drug release, a multiscale strategy is adopted, ranging from lumped parameter model
accounting for drug release, to fully 3-D models for drug transport to the artery. Differences in drug delivery to the artery are evaluated with respect to local drug dosage. This model allowed to compare alternative stenting configurations (namely, the Provisional Side Branch, the Culotte and the Inverted Culotte techniques), thus suggesting guidelines in the treatment of coronary bifurcations lesions and addressing clinical issues such as the effectiveness of drug delivery to lesions in the side branch, as well as the influence of incomplete strut apposition and overlapping stents. |
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15/2012 - 03/11/2012
Mengaldo, G.; Tricerri, P.; Crosetto,P.; Deparis, S.; Nobile, F.; Formaggia, L.
A comparative study of different nonlinear hyperelastic isotropic arterial wall models in patient-specific vascular flow simulations in the aortic arch | Abstract | | Blood flow in major arteries gives rise to a complex fluid-structure interaction (FSI) problem. The mechanical behaviour of the tissues composing the vessel wall is highly nonlinear. However, when the wall deformation is small it can be argued that the effect of the non-linearities to the flow field is small and indeed several
authors employ a linear constitutive law. In the aortic arch, however, the deformations experienced by the vessel wall during the heart beat are substantial.
In this work we have implemented different non-linear constitutive relations for the vessel wall. We compare the flow field and related quantities such as wall shear stress obtained on an anatomically realistic geometry of aortic arch
reconstructed from clinical images and using physiological data. Particular attention is also devoted to the efficiency of the algorithms employed. The fluid-structure interaction problem is solved with a fully coupled approach and using
the exact Jacobians for each different structural model to guarantee a second order convergence of the Newton method. |
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14/2012 - 03/05/2012
Fumagalli, A.;Scotti, A.
An unfitted method for two-phase flow in fractured porous media. | Abstract | | We propose an efficient computational method to simulate two-phase flow in fractured porous media.
Instead of refining the grid to capture the flow along the faults or fractures, we represent them as immersed interfaces with
reduced model for the flow and suitable coupling conditions. We allow for non matching grids between the porous matrix and the fracture to increase the flexibility of the method in realistic cases. We employ the extended finite element method for the Darcy problem and a finite volume method for the saturation equation, with a numerical flux that yields the correct entropy solution in the case of discontinuous flux function at the interface between the fracture and the porous matrix. |
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13/2012 - 02/14/2012
Formaggia, L.; Guadagnini, A.; Imperiali, I.; Lever, V.; Porta, G.; Riva, M.; Scotti, A.; Tamellini, L.
Global Sensitivity Analysis through Polynomial Chaos Expansion of a basin-scale geochemical compaction model | Abstract | | We present a model-driven uncertainty quantification methodology based on the use of sparse grids sampling techniques in the context
of a generalized Polynomial Chaos Expansion (GPCE) approximation of a basin-scale geochemical evolution scenario. The approach is illustrated through a one-dimensional example involving the process of quartz cementation in sandstones and the resulting effects on the dynamics of the vertical distribution of porosity, pressure and
temperature. The proposed theoretical framework and computational tools allow performing an efficient and accurate Global Sensitivity Analysis (GSA) of the system states (i.e., porosity, temperature, pressure and fluxes) in the presence of uncertain key mechanical and geochemical model parameters as well as boundary conditions. GSA is grounded on the use of the variance-based Sobol indices. These allow
discriminating the relative weights of uncertain quantities on the global model variance and can be computed through the GPCE of the model response surface. Evaluation of the GPCE of the random model response is performed through the implementation of a sparse grid interpolation technique in the space of the selected uncertain
quantities. As opposed to a standard Monte Carlo sampling, the use of sparse grids polynomial interpolants renders computationally affordable and reliable evaluations of the required indices. GPCE can then be employed as a surrogate model of the system states to quantify uncertainty propagation through the model in terms of the
probability distribution (and its statistical moments) of target system states. |
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