Numerical simulations of rod assembly vibration induced by turbulent axial flows

In this paper, various aspects relevant for nuclear applications of bare rod bundles in axial turbulent flows are studied by means of numerical simulations. The work first investigates the fluid dynamics properties of the flow field in the narrow gaps between the rigid rods and then it focuses on the study of the dynamics of the vibrations of the flexible rods. A two-rod assembly is examined first; the system consists of two identical rods in turbulent axial water flows with a small pith-to-diameter ratio and a large wall-to-diameter ratio. In the case of rigid rods, it is found that the flow field is characterized by the presence of strong axial flow pulsations in the gap between the rods with a characteristic frequency close to that observed in previous experimental works at a similar Reynolds number. Subsequently, strongly coupled numerical fluid-structure interaction (FSI) simulations are performed in order to study the flow induced vibration (FIV) of the rods. It is found that a buckling of the rods occurs because the fluid in the gaps pushes the rods apart which then undergo sustained vibrations because of the velocity fluctuations. Furthermore, due to the hydrodynamic coupling, the vibrations of the rods are not independent from each other but the system vibrates as a whole, as confirmed by the spectral analysis which shows the existence of a pair of frequencies around each of the natural frequencies of the structure in vacuo. A seven-rod assembly with the same pitch-to-diameter ratio of the two-rod case is then studied. Numerical simulations reveal the presence of strong flow pulsations in the gaps, although the frequency of the pulsations is slightly lower that than observed in the two-rod case. In the numerical FSI simulations, a very complicated rod-to-rod interaction is observed with the appearance of large vibrations and buckling deformation.