An evaluation of vortex shedding over slender structures using large-eddy simulation

Abstract

Turbulent flows around slender structures are common in nature and occur in many applications, including flows around tall buildings and long-span bridges. Understanding and predicting the properties of these flows are necessary for a safe, effective and economical engineering design. Experimental techniques are expensive and often provide data that is not sufficiently detailed for the structural engineer. With increasing computational power it has become possible to investigate these flows using numerical techniques. A number of numerical approaches have been proposed over the last half century. Of these, one of the most effective techniques, that is currently on the verge of being viable for common use in industry, is that of Large-Eddy Simulation (LES). This thesis illustrates the appealing aspects of LES for the use in wind engineering.

Measurement of forces on a bluff body placed in a flow is of considerable importance to the wind engineer. It is well established in the literature that the peak pressures on the body are due to large coherent structures in the flow, denoted vortices. The atmospheric boundary layer, in which all civil engineering structures are situated, is almost always turbulent, and it has a strong influence on vortex formation. In the present work, to generate in flow turbulence for LES, a synthetic in flow condition was utilised to analyse the effects of freestream turbulence over the CAARC standard tall model building. With the use of pressure statistics and conditional sampling, the sensitivity of the peak loading to the freestream turbulence effects were analysed, showing a marked increase for both turbulence intensity and length scale of the flow. The formation of vortices over a long-span bridges can incur severe aeroelastic instabilities. A fluid-structure coupling method was proposed for the study of freely-vibrating cylinder (simulating a bridge section) undergoing Vortex-Induced Vibration (VIV) and torsional flutter. The Motion Induced Vortex was identified as the fundamental cause of both cases. Spanwise correlations and pressure statistics were used to determine the effects of the motion and freestream turbulence on both phenomena. It was found that small-scale turbulence (length scales size of the cylinder) had a diminishing effect on the cylinder's response (compared to smooth flow). When the length scales were increased to those found in atmospheric wind, the cylinder's response was increased for the torsional motion.

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

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