Thermal-Hydraulic Simulations of Turbulent Flows using the Immersed Boundary Method for Innovative Nuclear Reactor Safety Devices

Abstract

In the framework of safety-system developments for new generations of nuclear power plants, we model turbulent flows inside industrial passive-safety devices, based on the fluidic diode principle, for design optimization. Considering a statistical processing or an expert advice, this geometrical optimization would require a specific meshing of each evaluated shape. Facing a large number of calculations and to avoid the tedious re-meshing task, we use the fictitious domain approach allowing us to dissociate the computational domain from the geometrical one. Using in-house industrial software, we are not able to modify the equations governing the physical problem in the code. Hence, we choose the Penalized Direct Forcing (PDF) method. It consists of a penalized forcing term, written in a direct forcing formulation, added to the governing equations to take account immersed boundaries. Considering industrial turbulent flow problems, calculated by the RANS approach and unresolved boundary layers, we need an appropriate treatment of immersed walls. An immersed wall model, based on the power law, is chosen because of its ability to model the velocity profile within the entire boundary layer, to deal with non-uniform distributions of distances to the immersed wall and to be explicit. Consequently, we develop a finite element version of the power law approach and its variants in our PDF method, validate this approach on analytical test cases and compute industrial cases. In particular, we highlight the need of turbulent viscosity control near the immersed wall and the impact of the power law variants on the mechanical constraint estimation on the immersed wall.