[Univ of Cambridge] [Dept of Engineering]

Free Vibration of Cylindrical and Hyperboloidal Cooling-Tower Shells

by Mohey E-D El-Mously

The focus of this dissertation is the small free vibration of thin elastic hyperboloidal shells of revolution, which are often used as cooling towers in thermal power stations. The aim of the present study is to obtain a comprehensive understanding of such phenomena, in order to develop design charts that map the different regimes of structural behaviour of these shells.

The work begins with the simpler case of the cylindrical shell and encompasses tests on a range of simplified models that have been proposed in the literature, but which have not been assessed thoroughly hitherto. A first-order approximation theory for the analysis of cylindrical shells has been developed. The proposed theory aims to bridge the gap between the classical Love-Kirchhoff theory, which permits an accurate description of the shell behaviour, and the simplified version of Vlasov which only considers the effects of longitudinal stretching and circumferential bending. A direct analogy is established between the shell behaviour and the behaviour of a Timoshenko beam mounted on a Pasternak foundation. Approximate explicit formulae are derived for the fundamental frequency of the beam-foundation system, and by analogy for the shell. Criteria defining the domains of validity for the proposed theory are established.

Attention is then turned to shells with slightly curved meridians (waisted, nearly-cylindrical shells) and finally to hyperboloidal shells, both of which exhibit various complex degenerate-case effects. A simple model of waisted, nearly-cylindrical shells demonstrates cross-over phenomena when geometric parameters are varied. Further investigation shows that these phenomena are replaced by curve-veering when a more accurate shell model is used. Hyperboloidal shells predominantly demonstrate curve-veering phenomena, however there are particular geometries at which cross-over occurs. Most of the above phenomena are dominated by membrane effects. The sensitivity of these phenomena to the shell geometric parameters is examined and explained in simple terms.

An experimental investigation, using a small-scale silicone rubber model shell, confirms some main points of the analysis.

[Cambridge University | CUED | Structures Group | Geotechnical Group]

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