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 1268 products
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45/2021 - 07/02/2021
Diquigiovanni, J.; Fontana, M.; Vantini, S.
Distribution-Free Prediction Bands for Multivariate Functional Time Series: an Application to the Italian Gas Market | Abstract | | Uncertainty quantification in forecasting represents a topic of great importance in statistics, especially when dealing with complex data characterized by non-trivial dependence structure. Pushed by novel works concerning distribution-free prediction, we propose a scalable procedure that outputs closed-form simultaneous prediction bands for multivariate functional response variables in a time series setting, which is able to guarantee performance bounds in terms of unconditional coverage and asymptotic exactness, both under some conditions. After evaluating its performance on synthetic data, the method is used to build multivariate prediction bands for daily demand and offer curves in the Italian gas market. The prediction framework thus obtained allows traders to directly evaluate the impact of their own offers/bids on the market, providing an intriguing tool for the business practice. |
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44/2021 - 07/01/2021
Gentili, G.G.; Khosronejad, M.; Bernasconi, G.; Perotto, S.; Micheletti, S.
Efficient Modeling of Multimode Guided Acoustic Wave Propagation in Deformed Pipelines by Hierarchical Model Reduction | Abstract | | The fi nite element based hierarchical model (HiMod) reduction technique is here
applied, for the fi rst time, to model guided acoustic wave propagation in deformed
pipelines in a linear regime. This method turns out to be extremely efficient to
discretize the linearized Helmholtz equation for acoustic waves. The selection of
a suitable modal transverse basis for the acoustic fi eld allows us to speed up the
computation by orders of magnitude with respect to a standard 3D finite element
discretization. |
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43/2021 - 06/24/2021
Salvador, M.; Fedele, M.; Africa, P.C.; Sung, E.; Dede', L.; Prakosa, A.; Chrispin, J.; Trayanova, N.; Quarteroni, A.
Electromechanical modeling of human ventricles with ischemic cardiomyopathy: numerical simulations in sinus rhythm and under arrhythmia | Abstract | | We developed a novel patient-specific computational model for the numerical simulation of ventricular electromechanics in patients with ischemic cardiomyopathy (ICM). This model reproduces the activity both in sinus rhythm (SR) and in ventricular tachycardia (VT). The presence of scars, grey zones and non-remodeled regions of the myocardium is accounted for by the introduction of a spatially heterogeneous coefficient in the 3D electromechanics model. This 3D electromechanics model is firstly coupled with a 2-element Windkessel afterload model to fit the pressure-volume (PV) loop of a patient-specific left ventricle (LV) with ICM in SR. Then, we employ the coupling with a 0D closed-loop circulation model to analyze a VT circuit over multiple heartbeats on the same LV. We highlight similarities and differences on the solutions obtained by the electrophysiology model and those of the electromechanics model, while considering different scenarios for the circulatory system. We observe that very different parametrizations of the circulation model induce the same hemodynamical considerations for the patient at hand. Specifically, we classify this VT as unstable. We conclude by stressing the importance of combining electrophysiological, mechanical and hemodynamical models to provide relevant clinical indicators in how arrhythmias evolve and can potentially lead to sudden cardiac death. |
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42/2021 - 06/23/2021
Calissano, A.; Fontana, M.; Zeni, G.; Vantini, S.
Conformal Prediction Sets for Populations of Graphs | Abstract | | In the latest years, scholars started focusing on how to develop statistical tool for the analysis of population of complex data, such as sets of labelled or unlabelled graphs graphs. The present works adds to this literature by focusing on a strangely overlooked area, namely the formulation of prediction sets.
By exploiting cutting edge techniques in the realm of machine learning, we propose a forecasting method for populations of both labelled and unlabelled graphs based on Conformal Prediction, able to identify prediction regions. Our method is model-free, achieves finite-sample validity, is computationally efficient and it identifies interpretable prediction sets, in the shape of a parallelotope. To explore the features of this novel forecasting technique, a simulation study and and a real-world example are presented. |
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41/2021 - 06/21/2021
Costa, G., Cavinato, L., Maschi, C., Fiz, F., Sollini, M., Politi, L. S., Chiti, A., Balzarini, L., Aghemo, A., di Tommaso, L., Ieva, F., Torzilli, G., Viganò, L.
Virtual Biopsy for Diagnosis of Chemotherapy-Associated Liver Injuries and Steatohepatitis: A Combined Radiomic and Clinical Model in Patients with Colorectal Liver Metastases | Abstract | | Non-invasive diagnosis of chemotherapy-associated liver injuries (CALI) is still an unmet need. The present study aims to elucidate the contribution of radiomics to the diagnosis of sinusoidal dilatation (SinDil), nodular regenerative hyperplasia (NRH), and non-alcoholic steatohepatitis (NASH). Patients undergoing hepatectomy for colorectal metastases after chemotherapy (January 2018-February 2020) were retrospectively analyzed. Radiomic features were extracted from a standardized volume of non-tumoral liver parenchyma outlined in the portal phase of preoper- ative post-chemotherapy computed tomography. Seventy-eight patients were analyzed: 25 had grade 2–3 SinDil, 27 NRH, and 14 NASH. Three radiomic fingerprints independently predicted SinDil: GLRLM_f3 (OR = 12.25), NGLDM_f1 (OR = 7.77), and GLZLM_f2 (OR = 0.53). Combining clinical, laboratory, and radiomic data, the predictive model had accuracy = 82%, sensitivity = 64%, and specificity = 91% (AUC = 0.87 vs. AUC = 0.77 of the model without radiomics). Three radiomic parameters predicted NRH: conventional_HUQ2 (OR = 0.76), GLZLM_f2 (OR = 0.05), and GLZLM_f3 (OR = 7.97). The combined clinical/laboratory/radiomic model had accuracy = 85%, sensitivity = 81%, and specificity = 86% (AUC = 0.91 vs. AUC = 0.85 without radiomics). NASH was predicted by conventional_HUQ2 (OR = 0.79) with accuracy = 91%, sensitivity = 86%, and specificity = 92% (AUC = 0.93 vs. AUC = 0.83 without radiomics). In the validation set, accuracy was 72%, 71%, and 91% for SinDil, NRH, and NASH. Radiomic analysis of liver parenchyma may provide a signature that, in combination with clinical and laboratory data, improves the diagnosis of CALI. |
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39/2021 - 06/15/2021
Barnafi, N.; Di Gregorio, S.; Dede', L.; Zunino, P.; Vergara, C.; Quarteroni, A.
A multiscale poromechanics model integratingmyocardial perfusion and systemic circulation | Abstract | | The importance of myocardial perfusion at the outset of cardiac disease remains largely understudied. To address this topic we present a mathematical model that considers the systemic circulation, the coronary vessels, the myocardium, and the interactions among these components. The core of the whole model is the description of the myocardium as a multi-compartment poromechanics system. A novel decomposition of the poroelastic Helmholtz potential involved in the poromechanics model allows for a quasi-incompressible model that adequately describes the physical interaction among all components in the porous medium. We further provide a rigorous mathematical analysis that gives guidelines for the choice of the Helmholtz potential.
To reduce the computational cost of our integrated model we propose decoupling the deformation of the tissue and systemic circulation from the porous flow in the myocardium and coronary vessels, which allows us to apply the model also in combination with pre-computed cardiac displacements, obtained form other models or medical imaging data.
We test the methodology through the simulation of a heartbeat in healthy conditions that replicates the systolic impediment phenomenon,
which is particularly challenging to capture as it arises from the interaction of several parts of the model. |
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40/2021 - 06/15/2021
Martinolli, M.; Cornat, F.; Vergara, C.
Computational Fluid-Structure Interaction Study of a new Wave Membrane Blood Pump | Abstract | | Purpose: Wave Membrane Blood Pumps (WMBP) are novel pump designs in which blood is propelled by means of wave propagation by an undulating membrane. In this paper, we computationally studied the performance of a new WMBP design (J-shaped) for different working conditions, in view of potential applications in human patients.
Methods: Fluid-Structure Interaction (FSI) simulations were conducted in 3D pump geometries and numerically discretized by means of the Extended Finite Element Method (XFEM). A contact model was introduced to capture membrane-wall collisions in the pump head. Mean flow rate and membrane envelope were determined to evaluate hydraulic performance. A preliminary hemocompatibility analysis was performed via calculation of fluid shear stress.
Results: Numerical results, validated against in-vitro experimental data, showed that the hydraulic output increases when either the frequency or the amplitude of membrane oscillations were higher, with limited increase in the fluid stresses, suggesting good hemocompatility properties.
Also, we showed better performance in terms of hydraulic power with respect to a previous design of the pump. We finally studied an operating point which achieves physiologic flow rate target at diastolic head pressure of $80$ mmHg.
Conclusions: A new design of WMBP was computationally studied.
The proposed FSI model with contact was employed to predict the new pump hydraulic performance and it could help to properly select an operating point for the upcoming first-in-human trials. |
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37/2021 - 06/11/2021
Dassi, F.; Fumagalli, A.; Mazzieri, I.; Scotti, A.; Vacca, G.
A Virtual Element Method for the wave equation on curved edges in two dimensions | Abstract | | In this work we present an extension of the Virtual Element Method with curved edges for the numerical approximation of the second order wave equation in a bidimensional setting. Curved elements are used to describe the domain boundary, as well as internal interfaces corresponding to the change of some mechanical parameters. As opposite to the classic and isoparametric Finite Element approaches, where the geometry of the domain is approximated respectively by piecewise straight lines and by higher order polynomial maps, in the proposed method the geometry is exactly represented, thus ensuring a highly accurate numerical solution. Indeed, if in the former approach the geometrical error might deteriorate the quality of the numerical solution, in the latter approach the curved interfaces/boundaries are approximated exactly guaranteeing the expected order of convergence for the numerical scheme. Theoretical results and numerical findings confirm the validity of the proposed approach. |
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