Aerodynamics of smooth and turbulent flows past stationary and oscillating square cylinders

Abstract

A cylindrical structure with a square section is a simple geometry, which is widely used in engineering industry, such as offshore platform risers, high-rise buildings, long-spanned bridge and pantograph of high speed train. Except for a very low inflow velocity, flow past a square cylinder forms a pair of vortices, which shed asymmetrically in the wake. The asymmetric vortex shedding causes a change of lift of the cylinder and leads to the oscillation of structure. The aim of this thesis is to analyse the mechanism of flow past a square cylinder between stationary and oscillating settings, in particular the similarities and differences of flow features. Additionally, the turbulent inflow past a square cylinder can substantially change the flow behaviours compared to those with smooth inflow. In this work, the freestream turbulence effects on the aerodynamics of a square cylinder are investigated for different integral length scales and turbulence intensities.

In this thesis, I have developed a flow solver based on the finite volume method to simulate flow past a square cylinder. An efficient immersed boundary method is implemented to model the stationary and oscillating square cylinders at Reynolds number 22,000. In the study of similarities and differences between stationary and oscillating square cylinders, it is found that at the resonant reduced velocity, the oscillation triggers an earlier flow reattachment over the side surfaces of the cylinder compared to the stationary square cylinder. This earlier reattachment in the oscillating square cylinder causes a more random and chaotic flow field downstream the reattachment point. In the study of freestream turbulence effects, an efficient turbulence generation technique is used, and turbulence intensities and integral length scales of freestream turbulence are investigated. From this study, it is found that the wake is more sensitive to the different turbulent inflow conditions than the shear layer over the side surfaces. The freestream turbulence with high turbulence intensity can affect the flow field within the shear layer over the side surfaces. Moreover, with a forced oscillation at resonance reduced velocity, it is found that the vortex shedding frequency is independent of the turbulent inflow conditions, whereas the length scales of flow structures and temporal response of surface force are affected by freestream turbulence.

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Identifiers

ORCID for Zhengtong Xie: orcid.org/0000-0002-8119-7532

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