Hydroelastic Response Of Flexibly-Mounted Cylinders In Harmonic Flow
Authors: Sarpkaya Turgut 
Published: October 2008

Transverse oscillations of a flexibly-mounted rigid cylinder in harmonic flow have been investigated through the use of a U-shaped vertical water tunnel. The results have shown that the largest amplitudes of oscillation for a given damping ratio, Keulegan-Carpenter number, and the Reynolds number occur when the vortex-shedding frequency is nearly in tune with the natural frequency of the cylinder. It has also been found that the amplitude of oscillations is proportional to the square of the reduced velocity defined by Um/fnD.

INTRODUCTION

The investigation of the nonlinear deterministic or statistical response of an offshore structure is of considerable interest and may be carried out through mathematical modeling, small-scale experiments, and prototype testing or through a combination of these three techniques.

Much of what is known about the wave forces and moments acting on bodies and complex structures of various shapes came from field data and experiments in wave channels. To extrapolate from these to the prediction of forces on new structures in natural waves is often very uncertain, due to difficulty or impossibility of modeling all the parameters that might have an effect. For example, there is considerable uncertainty as to how to account for the effect of currents which may be superposed on waves, for the elasticity and flow-induced oscillations of the members of the structure, for the roughness and diametral increase of the connecting members, for the effects of ambient flow with shear, etc., not to mention the two most commonly discussed dimensionless parameters, namely, the Reynolds number and the relative amplitude.

For a specific structure, it is possible to model many of the parameters in a large enough wave channel or water tunnel designed for this purpose. Nevertheless, for scientific understanding and for developing design methods, it is necessary to try to understand the various effects separately in relatively simple, idealized situations. The analogy with other branches of mechanics is obvious - the research on basic elements under controlled conditions and testing of replica models both play necessary roles.

Although some progress has been made in recent years in determining the force-transfer coefficients for sinusoidally oscillating and wavy flows about circular cylinders and piles, a complete understanding of them is still lacking at present. This is due in part to the limited availability of model and especially prototype data, and in part due to theoretical limitations. Much work still needs to be done to determine quantitatively the relative significance of the various factors enumerated above.

Studies of Reynolds number, relative amplitude, and roughness effects on the in-line and transverse force coefficients (i.e., drag, inertia, and the lift coefficients have recently been carried out by Sarpkaya. These studies were prompted by the need for systematic data and partly by the controversy concerning the influence of Reynolds number and surface roughness on the time-dependent loads on circular cylinders in harmonic flow.