Multiscale Multiphysics Coupling on a Finite Element Platform

In recent years numerical simulations are becoming fundamental for the design of many engineering components. For this reason many multiphysical and multiscale problems are investigated by coupling different existent software created specifically for solving each single problem. However, because of the intrinsic differences among these codes, such coupling is very challenging. In this thesis we develop a computational platform that can be used to integrate different computing tools into the common framework of the SALOME platform. Inside this platform various codes are coupled through numerical libraries with the purpose of exchanging data and melting intrinsic differences. After a description of the generic code integration procedure into the numerical platform, we introduce three classes of problems where different codes have been coupled and complex computational problems are studied. In the irst problem class, the computational platform is used to study a nuclear reactor system. We study the dynamics of a multiscale primary loop of a liquid metal reactor by coupling a mono-dimensional system code with the high resolution three-dimensional full scale core components models. Also we investigate a thermal-hydraulic-neutron multiphysics problem. The heat energy production in the reactor core, obtained by solving the neutron code DRAGON-DONJON, is coupled with the solution of the thermal-hydraulics conservative equations implemented in a in-house code. In the second problem class, we consider multiscale multiphysics Fluid Structure Interaction problems implemented in different modules of the FEMUs code. The mechanics of a three-dimensional particular component of the cardiovascular system is coupled with a mono-dimensional model that takes into account the remaining parts of a simplified circulatory system. Finally, in the last class of problems, Multiphase Fluid tructure Interaction problems are investigated by coupling the solution of a multiphase fluid interface advection VOF module with a FSI solver

On-demand Service Deployment Strategies for Fog-as-a-Service Scenarios

Service deployment at the network edge is a promising area that has been studied recently in the literature. In this work we have investigated a Fog-as-a-Service scenario, where multiple Server Fog Nodes (SFNs) can serve multiple Client Fog Nodes (CFNs) by exploiting different service deployment models, i.e., SaaS, PaaS, and IaaS, in a flexible way. The system has been modeled as a Size-Constrained Weighted Set Cover Problem aiming at maximizing the amount of satisfied CFNs exploiting a heterogeneous service deployment architecture, while minimizing the service completion time in a computation offloading scenario. In the simulation results section, we analyze the performance of different methods in terms of percentage of CFNs’ offloading requests satisfaction and offloading delay

FOG-oriented Joint Computing and Networking: the GAUChO Project Vision

This paper presents a novel architectural principle for distributed and heterogeneous systems integrating Fog Computing and Networking approaches, which has been proposed within the “Green Adaptive Fog Computing and Networking Architecture” (GAUChO) project, funded by the MIUR Progetti di Ricerca di Rilevante Interesse Nazionale (PRIN) Bando 2015 - grant 2015YPXH4W-004. In particular a modular and flexible platform has been designed and developed, supporting low-latency and energy-efficiency applications as well as security, self-adaptation, and spectrum efficiency by means of a strict collaboration among devices. Specifically, the focus here is on the design of an integrated protocol architecture supporting mobile Fog-oriented services, and the developed Fog computing testbeds

FEMuS-Platform: a numerical platform for multiscale and multiphysics code coupling

Nowadays, many open-source numerical codes are available to solve physical problems in structural mechanics, fluid flow, heat transfer, and neutron diffusion. However, even if these codes are often highly specialized in the numerical simulation of a particular type of physics, none of them allows simulating complex systems involving all the physical problems mentioned above. In this work we present a numerical framework, based on the SALOME platform, developed to perform multiscale and multiphysics simulations involving all the mentioned physical problems. In particular, the developed numerical platform includes the multigrid finite element in-house code FEMuS for heat transfer, fluid flow, turbulence and fluid-structure modeling; the open-source finite volume CFD software OpenFOAM; the multiscale neutronic code DONJON-DRAGON; and a system-scale code used for thermal-hydraulic simulations. Efficient data exchange among these codes is performed within computer memory by using the MED libraries, provided by the SALOME platform

The rapid spread of SARS-COV-2 Omicron variant in Italy reflected early through wastewater surveillance

The SARS-CoV-2 Omicron variant emerged in South Africa in November 2021, and has later been identified worldwide, raising serious concerns. A real-time RT-PCR assay was designed for the rapid screening of the Omicron variant, targeting characteristic mutations of the spike gene. The assay was used to test 737 sewage samples collected throughout Italy (19/21 Regions) between 11 November and 25 December 2021, with the aim of assessing the spread of the Omicron variant in the country. Positive samples were also tested with a real-time RT-PCR developed by the European Commission, Joint Research Centre (JRC), and through nested RT-PCR followed by Sanger sequencing. Overall, 115 samples tested positive for Omicron SARS-CoV-2 variant. The first occurrence was detected on 7 December, in Veneto, North Italy. Later on, the variant spread extremely fast in three weeks, with prevalence of positive wastewater samples rising from 1.0% (1/104 samples) in the week 5-11 December, to 17.5% (25/143 samples) in the week 12-18, to 65.9% (89/135 samples) in the week 19-25, in line with the increase in cases of infection with the Omicron variant observed during December in Italy. Similarly, the number of Regions/Autonomous Provinces in which the variant was detected increased from one in the first week, to 11 in the second, and to 17 in the last one. The presence of the Omicron variant was confirmed by the JRC real-time RT-PCR in 79.1% (91/115) of the positive samples, and by Sanger sequencing in 66% (64/97) of PCR amplicons. In conclusion, we designed an RT-qPCR assay capable to detect the Omicron variant, which can be successfully used for the purpose of wastewater-based epidemiology. We also described the history of the introduction and diffusion of the Omicron variant in the Italian population and territory, confirming the effectiveness of sewage monitoring as a powerful surveillance tool

Numerical solvers for a poromechanic problem with a moving boundary

We study a poromechanic problem in presence of a moving boundary. The poroelastic material is described by means of the Biot model while the moving boundary accounts for the effect of surface erosion of the material. We focus on the numerical approximation of the problem, in the framework of the finite element method. To avoid re-meshing along with the evolution of the boundary, we adopt the cut finite element approach. The main issue of this strategy consists of the ill-conditioning of the finite element matrices in presence of cut elements of small size. We show, by means of numerical experiments and theory, that this issue significantly decreases the performance of the numerical solver. For this reason, we propose a strategy that allows to overcome the ill-conditioned behavior of the discrete problem. The resulting solver is based on the fixed stress approach, used to iteratively decompose the Biot equations, combined with the ghost penalty stabilization and preconditioning applied to the pressure and displacement sub-problems respectively

Mathematical analysis, finite element approximation and numerical solvers for the interaction of 3D reservoirs with 1D wells

We develop a mathematical model for the interaction of a three-dimensional reservoir with the flow through wells, namely narrow cylindrical channels cutting across the reservoir. Leak off or sink effects are taken into account. To enable the simulation of complex configurations featuring multiple wells, we apply a model reduction technique that represents the wells as one-dimensional channels. The challenge in this case is to account for the interaction of the reservoir with the embedded one-dimensional wells. The resulting problem consists of coupled partial differential equations defined on manifolds with heterogeneous dimensionality. The existence and regularity of weak solutions of such problem is thoroughly addressed. Afterwards, we focus on the numerical discretization of the problem in the framework of the finite element method. We notice that the numerical scheme does not require conformity between the computational mesh of the reservoir and the one of the wells. From the standpoint of the solvers, we discuss the application of multilevel algorithms, such as the algebraic multigrid method. Finally, the reduced mathematical model and the discretization method is applied to a few configurations of reservoir with wells, with the purpose of verifying the theoretical properties and to assess the generality of the approach

A projection method for coupling two-phase VOF and fluid structure interaction simulations

The study of Multiphase Fluid Structure Interaction (MFSI) is becoming of great interest in many engineering applications. In this work we propose a new algorithm for coupling a FSI problem to a multiphase interface advection problem. An unstructured computational grid and a Cartesian mesh are used for the FSI and the VOF problem, respectively. The coupling between these two different grids is obtained by interpolating the velocity field into the Cartesian grid through a projection operator that can take into account the natural movement of the FSI domain. The piecewise color function is interpolated back on the unstructured grid with a Galerkin interpolation to obtain a point-wise function which allows the direct computation of the surface tension forces

A projection method for coupling two-phase VOF and fluid structure interaction simulations

The study of Multiphase Fluid Structure Interaction (MFSI) is becoming of great interest in many engineering applications. In this work we propose a new algorithm for coupling a FSI problem to a multiphase interface advection problem. An unstructured computational grid and a Cartesian mesh are used for the FSI and the VOF problem, respectively. The coupling between these two different grids is obtained by interpolating the velocity field into the Cartesian grid through a projection operator that can take into account the natural movement of the FSI domain. The piecewise color function is interpolated back on the unstructured grid with a Galerkin interpolation to obtain a point-wise function which allows the direct computation of the surface tension forces