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 1249 products
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06/2017 - 01/27/2017
Ekin, T.; Ieva, F.; Ruggeri, F.; Soyer, R.
On the Use of the Concentration Function in Medical Fraud Assessment | Abstract | | We propose a simple, but effective, tool to detect possible anomalies in the services prescribed by a health care provider (HP) compared to his/her colleagues in the same field and environment. Our method is based on the concentration function which is an extension of the Lorenz curve widely used in describing uneven distribution of wealth in a population. The proposed tool provides a graphical illustration of a possible anomalous behavior of the HPs and it can be used as a pre-screening device for further investigations of potential medical fraud. |
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05/2017 - 01/20/2017
Menafoglio, A.; Hron, K.; Filzmoser, P.
Logratio approach to distributional modeling | Abstract | | Symbolic data analysis (SDA) provides a unified approach to analyze distributional data, resulting from capturing intrinsic variability of groups of individuals as input observations. In parallel to the SDA approach, a concise methodology has been developed since the early 1980s to deal with compositional data — i.e., data carrying only relative information — through the logratios of their parts. Most methods in compositional data analysis aims to treat multivariate observations which can be identified with probability functions of discrete distributions. Nevertheless, a methodology to capture the specific features of continuous distributions (densities) has been recently introduced. The aim of this work is to describe a general setting that includes both the discrete and the continuous setting, and to provide specific details to both frameworks focusing on the implications on SDA. The theoretical developments are illustrated with real-world case studies. |
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04/2017 - 01/19/2017
Dede', L; Garcke, H.; Lam K.F.
A Hele-Shaw-Cahn-Hilliard model for incompressible two-phase flows with different densities | Abstract | | Topology changes in multi-phase fluid flows are difficult to model within a traditional sharp interface theory. Diffuse interface models turn out to be an attractive alternative to model two-phase flows. Based on a Cahn-Hilliard-Navier-Stokes model introduced by Abels, Garcke and Grun (Math. Models Methods Appl. Sci. 2012), which uses a volume averaged velocity, we derive a diffuse interface model in a Hele-Shaw geometry, which in the case of non-matched densities, simplifies an earlier model of Lee, Lowengrub and Goodman (Phys. Fluids 2002). We recover the classical Hele-Shaw model as a sharp interface limit of the diffuse interface model. Furthermore, we show the existence of weak solutions and present several numerical computations including situations with rising bubbles and fingering instabilities. |
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03/2017 - 01/13/2017
Fumagalli, I.; Parolini, N.; Verani, M.
On a free-surface problem with moving contact line: from variational principles to stable numerical approximations | Abstract | | We analyze a free-surface problem described by time-dependent Navier-Stokes equations. Surface tension, capillary effects and wall friction are taken into account in the evolution of the system, influencing the motion of the contact line - where the free surface hits the wall - and of the dynamics of the contact angle. The differential equations governing the phenomenon are first derived from the variational principle of minimum reduced dissipation, and then discretized by means of the ALE approach. The numerical properties of the resulting scheme are investigated, drawing a parallel with the physical properties holding at the continuous level. Some instability issues are addressed in detail, in the case of an explicit treatment of the geometry, and novel additional terms are introduced in the discrete formulation in order to damp the instabilities. Numerical tests assess the suitability of the approach, the influence of the parameters, and the effectiveness of the new stabilizing terms. |
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02/2017 - 01/13/2017
Arena, M.; Calissano, A.; Vantini, S.
Monitoring Rare Categories in Sentiment and Opinion Analysis - Expo Milano 2015 on Twitter Platform. | Abstract | | This paper proposes a new aggregated classification scheme aimed
to support the implementation of text analysis methods in contexts
characterised by the presence of rare text categories. The proposed
approach starts from the aggregate supervised text classifier developed
by Hopkins and King and moves forward relying on rare event sampling
methods. In details, it enables the analyst to enlarge the number
of text categories whose proportions can be estimated preserving the
estimation accuracy of standard aggregate supervised algorithms and
reducing the working time w.r.t. to unconditionally increase the size
of the random training set. The approach is applied to study the daily
evolution of the web reputation of Expo Milano 2015, before, during
and after the event. The data set is constituted by about 900,000
tweets in Italian and 260,000 tweets in English, posted about the event
between March 2015 and December 2015. The analysis provides an
interesting portray of the evolution of Expo stakeholders’ opinions over
time and allow to identify the main drivers of Expo reputation. The
algorithm will be implemented as a running option of the next release
of R package ReadMe. |
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01/2017 - 01/04/2017
Riccobelli, D.; Ciarletta, P.
Rayleigh-Taylor instability in soft elastic layers | Abstract | | This work investigates the morphological stability of a soft body composed of two heavy elastic layers, attached to a rigid surface and subjected only to the bulk gravity force. Using theoretical and computational tools, we characterize the selection of different patterns as well as their nonlinear evolution, unveiling the interplay between elastic and geometric effects for their formation.
Unlike similar gravity-induced shape transitions in fluids, as the Rayleigh-Taylor instability, we prove that the nonlinear elastic effects saturate the dynamic instability of the bifurcated solutions, displaying a rich morphological diagram where both digitations and stable wrinkling can emerge. The results of this work provide important guidelines for the design of novel soft systems with tunable shapes, with several applications in engineering sciences. |
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58/2016 - 12/27/2016
Antonietti, P. F.; Bruggi, M. ; Scacchi, S.; Verani, M.
On the Virtual Element Method for Topology Optimization on polygonal meshes: a numerical study | Abstract | | It is well known that the solution of topology optimization problems may be affected both by the geometric properties of the computational mesh, which can steer the minimization process towards local (and non-physical) minima, and by the accuracy of the method employed to discretize the underlying differential problem, which may not be able to correctly capture the physics of the problem. In light of the above remarks, in this paper we consider polygonal meshes and employ the virtual element method (VEM) to solve two classes of paradigmatic topology optimization problems, one governed by nearly-incompressible and compressible linear elasticity and the other by Stokes equations. Several numerical results show the virtues of our polygonal VEM based approach with respect to more standard methods. |
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57/2016 - 12/24/2016
Bassi, C.; Abbà, A.; Bonaventura, L.; Valdettaro, L.
Large Eddy Simulation of gravity currents with a high order DG method | Abstract | | This work deals with Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of turbulent gravity currents, performed by means of a Discontinuous Galerkin (DG) Finite Element method.
In particular, a DG-LES approach in which the filter operation is built in the numerical discretization has been employed, similarly to VMS approaches.
Numerical simulations of non-Boussinesq lock-exchange benchmark problems show that, in the DNS case, the proposed method allows to correctly reproduce relevant features of variable density flows with gravity. Moreover LES results highlight the excessively high dissipation of the Smagorinsky model with respect to the Germano dynamic procedure, providing a first indication of the superiority of dynamic models in the context of gravity currents. |
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