Numerical simulations of vortex-induced vibrations in a 7-rod bundle compared to experimental data

Being able to quantify mechanical vibrations is of key importance for the safety of nuclear power plants, as they are able to induce damage. In this work, numerical simulations are used to simulate water flow through a densely packed bundle of 7 rods, mimicking an experimental setup used at Delft University of Technology. This flow configuration is chosen to resemble the coolant flow through a nuclear reactor core. Because of the wall proximity, a considerable velocity difference between the narrow gaps and the subchannels exists, with an inflection point in the velocity profile. This yields an unstable situation, and large vortices are continuously created through a mechanism similar to the Kelvin–Helmholtz instability. The vortex streets in between the rods are associated with a fluctuating pressure field, causing so-called vortex-induced vibrations of the rods. These vibrations are well-known to cause damage to structures in cross-flow. In this case they are of smaller amplitude, but could still cause fretting or fatigue damage. The experimental setup contains 7 steel cylinders, encased in a hexagonal duct. The central rod contains a section where the steel is replaced by a water-filled silicone tube, clamped at both extremes to the steel rod, and the vibrations of this section are examined. The numerical approach consists of coupled fluid-structure interaction (FSI) simulations, with the flow being modelled using computational fluid dynamics (CFD) and the structure using computational solid mechanics (CSM). For the CFD-part, the software package ANSYS Fluent is used, implementing the finite volume method. A deformable grid is applied with the Arbitrary Eulerian–Lagrangian (ALE) formulation. The k-ω SST model is employed for the turbulence. Streamwise periodic boundary conditions are used. As the flow is then inherently developed, this allows for a great reduction in domain length, keeping the simulation computationally manageable. Structural simulations are performed by the Abaqus software, implementing the finite element method. Quadratic elements are used for the silicone part. An in-house code manages the coupling, using the quasi-Newton algorithm IQN-ILS. The available experimental data consist of Laser Doppler Anemometry (LDA) measurements and high-speed camera footage of the wall movement of the silicone rod. Equivalent data is collected from the numerical simulations. The simulations are repeated for different flow rates. The frequency spectrum of the coherent structures, and the frequency and amplitude of the wall movement are compared for each operating point, as well as their trend as a function of the flow rate. The dominant frequencies found in the simulation results were similar to the experimental results, although slightly higher. They also showed a linear trend, just like the experiments. A larger mismatch was present for the structural response, the frequencies found using the FSI model being more than twice as high.