Date of Award

2009

Degree Type

Thesis

Department

Mechanical Engineering

First Advisor

Rashidi, Majid

Subject Headings

Wind-pressure -- Measurement, Structural dynamics

Abstract

Structural dynamic analyses of elongated masts subjected to various wind speeds are presented in this work. The masts are modeled as vertically supported cantilever beams,with one end fixed to the ground, and the other end free. The external excitation forces acting of the masts are the results of vortex shedding represented by a sinusoidal time dependent functions. The frequencies of these sinusoidal functions are dictated by the Strouhal numbers associated of the flow regimes crossing over the masts. To enhance the vibratory behavior of a typical mast, under the influence of flow induced vibrations,three different mass distributions along the length of the mast were considered. The different mass distributions were achieved by tapering the mast along its length,allocating more of the mass at its fixed end, and gradually decreasing it toward its free end. Three different tapering angles were considered for these studies. All results were compared with the results obtained for a straight circular cross-section cylindrical mast having the same overall length and mass. For a mast length of 25 meters and a total mass of 1782.74 kg, three tapered angles of 0.229, 0.458 , and 0.596 degrees, were considered. These analyses show that the first natural frequencies of the tapered masts increases from that of the straight mast. The first natural frequency of the straight mass was determined to be 0.2845 Hrz. The corresponding first natural frequencies for the masts with tapered angle of 0.229, 0.458, and 0.596 became 0.417, 0.5911, and 0.7435Hrz. respectively. In addition to the natural frequency analyses, dynamic responses analyses of these masts were determined under the influence of the harmonic excitations resulting from the vortex shedding cause by the wind flow. For the tapered angles chosen in these studies the maximum displacement of the free ends of these masts were determined. For a wind speed of 10 m/s the free-end displacements of the tapered masts were determined to be (500, 1000, 1300)* 10⁻⁶ meters

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