LES Modeling with a Multifield Approach

In nuclear power plants, flow studies with a Computational Fluid Dynamics (CFD) approach require the ability of dealing with inclusions of different sizes and shapes and turbulence effects. For this purpose, multifield methods have been developed to simulate separately the small spherical bubbles and the large deformable ones. In this article, we consider an approach in which the first range of bubbles are followed by an Eulerian dispersed method and the second structures are tracked by interface tracking methods within a two-fluid model. To deal with these large inclusions, we present and validate, in this paper, a model, called the Large Bubble Model, introduced for the simulation of large deformable interfaces between two continuous fields. The Large Bubble Model includes a surface tension model, a new drag force expression to couple the velocity of the two fields at the interface and the resolution of an interface sharpening equation to limit the numerical smearing induced by the two-fluid model. To take into account the turbulence effects, an a priori two-phase LES filtering is proposed with the two-fluid equations and the interfacial forces of the Large Bubble Model. This filtering highlights new subgrid terms compared to previous works done on the single-fluid model. Finally, DNS simulations are performed with a phase inversion test case to evaluate the order of magnitude of these terms and to compare five different turbulence models.