ABSTRACt

A symmetrical vortices model (one-vortex-pair version) to predict instantaneous in-line hydrodynamic forces on a large-diameter pipe, such as CWP of commercial size OTEC plants, was developed (Otsuka et al, 1999). An advanced symmetrical vortices model, (two-vortex-pair version) which includes effect of previously separated vortex pair, was also proposed (Otsuka et al., 2000). The purpose of this paper is to improve the two-vortex.pair version of the symmetrical vortices model to be applicable to an irregularly oscillating cylinder. In order to develop the model of vortex formations, flow visualizations of an unsteady oscillating cylinder are carried out. The results demonstrate that the vortex formations are independent of both previous and present Kc numbers, and can be approximately given by functions of only the displacement parameter of each swing. Hydrodynamic forces on an unsteady oscillating cylinder using the present method are simulated. The simulated results relatively agree with the results of force measurements.

INTRODUCTION

Since '70s, many researches on floating-type facilities for upwelling deep ocean water (DOW), mostly ocean thermal energy conversion (OTEC) plants, were carried out (e.g., Avery and Wu, 1994). In recent researches, the government of India is performing an on-site experiment of 1MW floating OTEC plant in Indian Ocean with support of Saga University OTEC research group (Ravindran, 2000). These researches suggested that the accurate dynamic analysis of the DOW upwelling pipe, so-called cold water pipe (CWP), is one of the most important technical subjects for developing commercial size floating OTEC plants. It is well known that the hydrodynamic coefficients (drag and added-mass coefficients, Cd and Ca) in Morison equation depend on Kculegan-Carpenter number (Kc) and Reynolds number (Re), which are obtained from the relation between the pipe diameter and the relative motion amplitude and the motion frequency.

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