20 juillet 2021
- Numerical modeling of an in-vessel ow limiter using an immersed boundary approach / PhD defense Georis Billo
: Georis BILLO
Date de soutenance : le 20 juillet 2021 à 9h00 ; Amphi 3 Centrale Marseille
Abstract : In the framework of the development of new passive safety systems for the second and third generations of nuclear reactors, the numerical simulations, involving complex turbulent two-phase flows around thin or massive in flow obstacles are privileged tools to model, optimize and assess new design shapes. In order to match industrial demands, computational fluid dynamics tools must be the fastest, most accurate and most robust possible. The purpose of my PhD was to design and develop such a tool.
The aforementioned constaints tend to rule out a "body-fitted". Indeed, we chose a Fictitious Domain approach to deal with this problem. More precisely, the developed tool involves solving the Navier-Stokes equations using a projection scheme for a mixture fluid coupled with an Immersed Boundary (IB) approach: the penalized direct forcing method - a technique whose characteristics inherit from both penalty and immersed boundary methods - adapted to infinitely thin obstacles and to a Finite Element (FE) formulation. Various IB conditions (slip, no-slip or Neumann) for the velocity on the IB can be managed by imposing Dirichlet values in the vicinity of the thin obstacles. To deal with these imposed Dirichlet velocities, we investigated two variants: one in which we directly use the obstacle velocity and another one in which we use linear interpolation (this last variant being motivated by an increase of the space order of convergence). Several approaches were investigated (directional, mutli-directional and hybrid) for the linear interpolation of the velocity near the obstacle but, in any case, geometrical data coming from the obstacle are needed. Thus, retrieving geometrical data, generally from a Computer Assisted Design (CAD) object, is a key issue and, once again, several methods were studied and compared.
Another major issue, when dealing with numerical simulations, is validation. First, studies involving various one-phase academic test cases (such as Poiseuille, Taylor-Couette and the flow around a circular cylinder) were carried out. The results obtained were in good agreement with analytical and experimental data. Moreover, second order accuracy (in space) was numerically assessed when using linear interpolation, as expected. Then, studies involving industrial or quasi-industrial test cases were carried out to illustrates the advantages and drawbacks of this approach.
In a shortcoming second step, to face two-phase turbulent fluid simulations, some methodology modications will be considered such as adapting the projection scheme to low-compressible fluid and immersed wall-law boundary conditions (another PhD project has begun in october 2020).
o Michel Belliard, CEA Cadarache, ingénieur-chercheur, HDR, encadrant
o Pierre Sagaut, M2P2 (AMU), professeur, directeur de thèse
o Cédric Galusinski, IMATH (Université de Toulon), professeur, examinateur
o Lisl Weynans, INRIA (Université de Bordeaux), professeure assistant, HDR, rapporteure
o Stéphane Vincent, MSME (Université Gustave-Eiffel), professeur, rapporteur
o Barbara Bigot, CEA Cadarache, ingénieur-chercheur, examinatrice