ABSTRACT

Experiments were conducted with two smooth and two sand-roughened cylinders in a harmonically oscillating flow with current to determine the drag and inertia coefficients and to examine the effect of current-induced wake biasing on the modified morison equation. The various flow parameters such as the relative current velocity, Reynolds number, and the Keulegan-Carpenter number were varied systematically and the in-line force measured simultaneously. The principal results, equally valid for smooth and rough cylinders, are as follows: the drag coefficient decreases with increasing relative current for a given Reynolds number and Keulegan-Carpenter number; the effect of wake biasing on the drag and inertia coefficients is most pronounced in the drag/inertia dominated regime; and the two-term morison equation with force coefficients obtained under no-current conditions is not applicable to the prediction of wave and current induced loads on circular cylinders.

INTRODUCTION

The analysis of the interaction of waves with pre-existing and/or wind-or wave-generated currents and the interaction of the modified wave-current combination with rigid or elastic structures and their components require different mathematical approaches, relevant observations, and experiments that are applicable to all or some of these physical circumstances.

Measurements of wave-current interaction phenomena are scarce. Among the first to perform substantial controlled experiments of this nature was Sarpkaya1 in 1955. Additional studies were conducted much later by Jonsson,2 Inman and Bowen,3 and Dalrymple and Dean4. A detailed discussion of the foregoing is given by Sarpkaya and Isaacson5.

Little information exists on the effect of the co-existing flow field (wave plus current) on hydrodynamic loading of offshore structures. The complexity of the problem stems from several facts. Firstly, an analytical solution of the problem is not yet possible even for relatively idealized situations. Secondly, Morison's equation suffers from numerous uncertainties as discussed in detail by Sarpkaya6. Thirdly, waves and currents are omni directional. Even for a simple harmonic flow-current combination the wake and the vortex shedding are biased. Finally, a field study of the problem in the practically significant range of Reynolds number, Keulegan-Carpenter number, relative current velocity, and a suitably-defined current gradient is practically impossible. In fact, any experiment addressing this question faces rather difficult problems: how should the co-existing flow field be created and what measurements should be made in order to clarify the nature of the wake biasing Measurements of the in-line and transverse forces, no matter how detailed and sophisticated, cannot lead to a unique picture concerning the nature of the particular time-dependent flow. One is forced therefore to make pressure measurements at numerous points on the cylinder (hopefully simultaneously) and to carry out extensive flow visualization studies. These, however, prove to be very difficult for a number of obvious reasons.

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