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 compute water flow and vibration in a densely packed bundle of 7 rods, mimicking an experimental setup. 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 vibrations of the rods. 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). 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.

No abstract

No abstract

Flexible cylinders surrounded by a fluid flow that is dominantly aligned with the axis of the cylinders can be found in several applications. Examples with a flow confined to a narrow region around the cylinder(s) can be found in tube bundles of heat exchangers and reactor cores and also in air-jet weaving machines. This research analyses the flow-induced vibration of these two different cases with flexible cylinders in confined axial flow using numerical fluid-structure interaction (FSI) simulations. The FSI simulations of both cases use a partitioned framework, meaning that a computational fluid dynamics (CFD) solver is coupled with a finite element analysis (FEA) structural solver. The dynamic and kinematic equilibrium conditions at the contact surface between the fluid and the structure are satisfied by performing coupling iterations between both solvers in each time step. Convergence of these iterations is accelerated using quasi-Newton coupling techniques. For the case of the tube bundle, the modal characteristics have been identified for a tube bundle when they are submerged in an axial fluid flow. Furthermore, different types of flow-induced vibration have been studied. The flow speed has been increased in an FSI simulation of a single cylinder surrounded by an annular fluid domain, resulting first in static buckling and then in flutter at higher flow speeds. For the case of the air-jet weaving machines, the cylinder represents a smooth yarn which is accelerated by an air jet in the main nozzle of the machine, consisting of a long tube with small diameter. FSI simulations of a yarn clamped at the upstream end have been performed using the arbitrary Lagrangian-Eulerian formulation with deforming grids in the fluid domain. This work demonstrates the feasibility of analyzing and predicting flow-induced vibration of cylinders in confined axial flow by performing FSI simulations. The results of simulations are compared with those of experiments for tubes in axial flow and for a yarn in a nozzle of an air-jet weaving machine.

No abstract