Flow pulsations and vibrations simulation

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SESAME Newsletter #1 (March 2016) 

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Vortex-induced vibration is a term which almost exclusively refers to cross-flow induced vibrations. However, that does not imply that there cannot be any vortical motion and resulting vibration in purely axial flow conditions. If a dense cluster of cylinders is mounted in axial flow, a flow instability similar to a Kelvin-Helmholtz instability has been observed.This instability was discovered with the advent of nuclear reactors as the amount of mixing and heat transfer between different subchannels was too high to be explained by turbulent diffusion.

At first, it was believed that the enhanced mixing originated from secondary flows driven by the anisotropy of turbulence. Although such secondary flows occur in these bundles, their effect turned out to be small compared to another flow pattern: periodic large-scale vortices. These vortices arise from the interaction of the high-speed flow in the subchannels and the low-speed flow in the gap between cylinders.

In WP1, we aim at predicting vibrations occurring in tightly packed rod bundles with axial flow in a close collaboration between one experimental and two numerical groups. In this way, experimental and cross-computational validation can be performed. The preliminary mock-up design under consideration in this study is a 7-rod bundle with a pitch (distance between the centres of the cylinder) over diameter ratio of 1.1

The first computational results (performed by Ghent University) show that unsteady Reynolds-Averaged Navier-Stokes simulations (URANS) are able to predict the wavy character of the flow field. This wavy character is shown in the figure. Note that one cylinder is missing for visualisation purposes. The performed simulations have allowed to identify the main frequency of the instability.

In a subsequent series of calculations, a part of the central cylinder is considered to be flexible. The flexibility was chosen in such a way that the combined flow-structural system is still inherently stable (i.e. no fluid-elastic instabilities are occurring). In a series of fluid-structure interaction simulations, the actual vibration of the flexible part was computed. The resulting amplitude of oscillation is fairly small (up to 1% of the gap width). However, even small amplitude motion can lead to long-term damage.

More results are currently on their way and we are looking forward to the different cross-comparisons.