Fluid-structure interaction in a cluster of cylinders exposed to axial flow: from low-order models to fully coupled CFD-CSM methods

Rod bundles in nuclear reactors are continuously optimized in order to provide excellent heat transfer, while ensuring a low pressure drop and safe operation. As the resulting structure can be quite flexible, vibrations due to the interaction with the fluid flow might occur. These vibrations are typically caused by turbulence in the flow and the resulting amplitudes are relatively small. However, these small vibrations might lead to fretting wear and hence to long-term damage. In order to test for wear damage, the coupled dynamics of the structure and the fluid flow have to be known. The presence of an external fluid flow has a number of effects on the dynamics of a submersed structure: e.g., a solitary cylinder exposed to an external flow experiences added mass and damping due to the presence of the surrounding fluid. At high axial flow velocities relative to the stiffness of the cylinder, previous work showed that coupled instabilities such as flutter and divergence occur. In addition to these phenomena, a cluster of cylinders experiences inter-cylinder coupling: pressure perturbations in the fluid due to the movement or acceleration of one cylinder forces another cylinder to move. Consequently, the different cylinders move in organized patterns. In this contribution, these patterns will be predicted and analyzed. In a first step, the inter-cylinder coupling is computed by using simplified (low-order) models, which are based on 2D-potential flow analysis. Those results serve as a precursor step for fully coupled, transient and 3D CFD-CSM simulations of those vibration patterns.