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Effects of turbulence on wind induced vibrations of bluff cylinders Laneville, André

Abstract

This thesis studies the effects of turbulence, in an otherwise uniform air flow, on the static forces and vibrations of bluff cylinders. The main purpose was to investigate if the quasi-steady theory could give correct predictions when two additional variables, the scale and the intensity of turbulence, were introduced. Turbulence was produced by grids of bars of rectangular section and the measurements of its properties, with hot wires, gave results in good agreement with those of other experimenters. Then, dynamic and static models of rectangular section with H/d=0.5, 1.0, 2.0 and of D-shape, were exposed to different intensities and to different scales of turbulence. A pyramidal strain-gauge balance and an air-bearing system were used to measure the static forces and the amplitudes of oscillations respectively. The static force results indicated that an increased intensity of turbulence would turn a hard oscillator into a soft oscillator and a soft oscillator into a stable system. The results of the dynamic tests agreed, and in addition, indicated that the quasi-steady theory gave good quantitative results. The effect of the scale of turbulence did not appear important. In an attempt to understand the physical mechanism involved in the change of behaviour of the oscillators, shadowgraph and hot wire experiments on the separated shear layers were performed. The shadowgraph results indicated that an increased mixing, causing an earlier reattachment, existed inside the shear layer exposed to turbulent flow while a spectral analysis of the hot wire signals showed that transition occurred sooner in turbulent flow.

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