ABSTRACT:

The measurements of the water wave diffraction by a large surface piercing circular cylinder due to the action of both regular and random waves were carried out in a wave flume at Ocean Engineering Centre, Indian Institute of Technology, Madras, India. The measured diffracted wave fields at selected locations around the vicinity of the cylinder are validated with the results of MacCamy and Fuchs Linear Diffraction theory (L.O. theory). The agreement between measurements and theory was quite satisfactory for scattering parameter up to about 0.9, beyond which discrepancies are noticed, especially for steeper waves. The effect of side walls of the wave flume on the measurement of diffracted wave field are also discussed. With respect to the action of random waves on the cylinder, it is found that the L.D. theory predicts the diffracted wave elevations reasonably well for a narrow band incident wave spectrum.

1. INTRODUCTION

A knowledge of the scattered wave field around the vicinity of Ocean structures is important to predict the dynamic characteristics of Offshore supply vessels and crafts approaching the structure, since the motions of these vessels very much depend on the diffracted waves. MacCamy and Fuchs (1954) derived a closed form solution to determine the wave Induced pressures, forces, moments on a fixed vertical cylinder and the diffracted wave field In its vicinity due to the action of monochromatic waves in a constant water depth. Several researchers, notably, Jen (1967), Lebreton and Cormault (1969), Watanabe and Horikawa (1974) correlated the measured wave forces on cylinders With the results of L.D. theory and commented Its suitability for practical usage. Validation of L.D. theory with the measurements of dynamic pressures exerted on vertical cylinders have been reported by Chakrabarti and Tam (1975), Nakamura (1976) and Neelamani et al (1989).

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