This paper deals with non-linear vibration of a cluster of seven parallel bottom supported cantilever beams. The beams have buttons at intermediate locations, which can touch each other during vibration. Inside the subassemblies, thin circular rods are inserted tightly. The beams are fixed at the bottom. This structure represents a small portion of a typical fast reactor core. The hexagonal beams are called ‘subassemblies’. The rods represent fuel pins. The dimensions and mass distributions are scaled down such that the fundamental natural frequencies of beams are simulated. The complexity of natural frequency analysis is due to the randomness of: (1) mass distribution due to fuel pins inserted into the hexagonal tubes and (2) the noni-linear contact behaviour at the multiple button locations. The vibration analysis is carried out using ABAQUS code in time domain, by employing both 1 D (‘beam’) and 3D (‘solid’) finite element geometrical meshes. The frequencies are extracted employing Fast Fourier Transform (FFT) technique. To validate the analysis results, a benchmark experimental setup is designed with all features including dimensional tolerances. The natural vibration frequencies are determined by processing the dynamic responses under imposed uniaxial ‘sweep sine excitations’ on the structure mounted on servo-hydraulic shake table. The natural frequencies obtained through experimental and numerical simulation routes are compared and fine-tuning required in numerical simulations to predict the values close to the experimental data are identified. Based on this study, analysis guidelines, such as 1D or 3D along with associated boundary conditions, mass distribution factor, enhanced damping due to energy dissipation under multiple impacts at bottoms as well as fuel pins, were recommended for the vibration analysis of actual reactor core subassemblies.

Keywords: Vibration, Dynamics, Frequency, Fluid structure interactions, Experiment.