The hydrodynamic forces on a smooth horizontal circular cylinder exposed to oscillating flow have been experimentally investigated at Reynolds numbers (Re) in the range 20,000 - 260,000 (subcritical and transcritical regimes) and Keulegan-Carpenter numbers (Kc) in the interval from 5 to 40. In the tests, the Re number and Kc number were varied systematically. The drag force coefficient Co and inertia force coefficient CM in Morison equation have been determined through the use of Least Square Method. Both total in-line force coefficient CF and transverse force (lift) coefficient CL have been analysed in terms of their maximum and root mean square values. All the in-line and lift force coefficients were given as a function of Re and Kc number, and also their deviations with the average value have been shown. The principal results are as follows: for the Re > 80,000, all the hydrodynamic force coefficients, including CD" CM" CF and CL, are at best very weak functions of Reynolds number, and each of them tends towards a certain constance with increasing Kc number; for the Re < 80,000, the drag force coefficient CD decreases with increasing Re number, and inertia force coefficient CM increases with increasing Re number. The tendencies of drag and inertia coefficients versus Ke number for the Re > 105 are very similar to the others, which are very close to the Rodenbusch and Cutierrez's (1983) but are somewhat larger than the Sarpkaya's (1976 and 1986) and Bearman et al's (1985).
The Morison equation has been used to determine the wave forces on small membered offshore structures for many years. Since no satisfactory theoretical means are available to obtain the hydrodynamic coefficients, their values are determined experimentally. Similarly, tests have been carried out in uniform flow, with harmonically oscillating now.