The moving mesh methods are specific tools implemented and emphasized in the last years in the fluid dynamics simulation code STAR-CCM+ as a natural and interesting extension of the classical CFD applications, allowing for the simulation of the effective motion of objects which interact with the fluid around. The methods we focus on involve rigid motions coupled with grid deformation motions and, recently, overlapping grids motions. In the first case, it is necessary to recreate the deformed mesh. In this sense, we develop and implement techniques for the automation and the optimization of the re-meshing process and for the mesh-independence of the solution. In the case of the overset mesh method, the re-meshing is not necessary, the data exchange between the moving object and the fluid background takes place across a volumetric interface. The challenges we face consist in applying this method to complex geometries-parts of experimental facilities, with narrow flow paths and moving bodies with accurate design details.
In the framework of the MAXSIMA project, whose objective is the safety assessment in the nuclear facility MYRRHA, these methods are used in order to reproduce and test the passive activation of the control rods system in the case of an extreme event such as the emergency shutdown of the reactor. The application aims in the first place to prove the feasibility of such innovative simulations, to validate the simulations against experimental tests, which are ongoing in Belgium during this project and finally to provide inputs for the safety analyses. Concretely, the simulation provides information concerning the control rod insertion time (estimated at less than 1 second), the correct passive operation based on the forces acting on the moving assembly-drag, buoyancy, gravity, the pressure drops and the mass flows supplied by the pumps and by the natural circulation of the liquid metal in the primary loop of MYRRHA.
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