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International audience; This paper is devoted to the study of convergence orders of several numericalmethods that are implemented in the TrioCFD code dedicatedto the simulation of turbulent flows and heat transfer in nuclear engineering applications. The spatialdiscretization is based on Finite Difference-Volume or Finite Element-Volumemethods. A projection method is applied to update the velocity andthe pressure. The time scheme can be either explicit or implicit, and hexahedral or tetrahedral meshes can be used for simulations. In this paper,the test cases are relative to steady Stokes problems, steady and unsteady Navier-Stokes problems, and finally the well-known lid-driven cavity flow case.The latter proposes several comparisons between our simulations and numerical data already published inthe literature, while the other cases yield the values of convergence orders by using the analytical solutions.The accuracy of the results obtained with TrioCFDdiffers according to the types of mesh used for simulations, theviscosity values or the source terms in the equations.

International audience; Two-phase turbulence has been studied using a DNS of an upward turbulent bubbly flow in a so-called plane channel. Fully deformable monodispersed bubbles are tracked by the Front-Tracking algorithm implemented in TrioCFD code on the TRUST platform. Realistic fluid properties are used to represent saturated steam and water in pressurised water reactor (PWR) conditions. The large number of bubbles creates a void fraction of 10%. The Reynolds friction number is 180. After the transitional regime, the flow is simulated until convergence of statistics is achieved. Time- and space-averaging is used to compute main variables at the average scale (e.g. void fraction, phase velocities). Budget of forces and Reynolds stresses are also computed from the local fields. They provide reference profiles to improve momentum transfer closures and turbulence modelling. The velocity profile and the flow-rate are compared to a similar single-phase flow simulation. Strong buoyancy forces create a large relative velocity. Averaged surface tension forces also play a significant role in the flow equilibrium. In the prospect of assessing a single-pressure Euler-Euler two-fluid model, the macroscopic momentum jump condition is deduced from averaging DNS fields. The resulting balance shows that the classical assumption of opposite forces acting on each phase should be revised. Indeed, neither surface tension, nor pressure difference is negligible.

International audience; The main objective of the present paper is to give an overview of the actual state of the Trio_U project. Trio_U is a CFD code developed for about 20 years in the Nuclear Energy Division of CEA. The code is designed to treat efficiently various physical problems, such as turbulent flows, fluid/solid coupling, multiphase flows or flows in porous media. The domains of application are mainly related to the nuclear industry, in particular to the interaction of turbulent flow with the solid structures of nuclear reactors, that Trio_U is able to handle successfully thanks to its massive parallelism. The Trio_U project integrates a major procedure of Verification and Validation (VetV). Numerous verification tests are performed to avoid unintentional modifications and assure a proper implementation of the numerical methods, models and options. Non-regression tests are automatically launched and provide an evaluation of the differencesbetween the development version and the latest released version. The validation is the other part of procedure. Its purpose is to carry out reference calculations on a large range of physical problems and compare their results to analytical, experimental or literature results. It leads to the definition of compromises between accuracy, robustness and restitution time and guarantees that the requirement of a correct quality of results is met. Some typical examples of recent qualification studies in nuclear domain are presented here. Most of them correspond to international experimental or industrial facilities and combine various complex physical phenomena.

International audience