This paper presents a simple theory to predict hydrodynamic forces acting on arbitrarily oscillating bodies. The theory is based on the assumption that the Morison formula can be applied with constant drag and mass coefficients provided that wake velocities produced by the body motions in all past history are properly corrected for. The wake velocity is calculated by "the unsteady turbulent wake theory". The theory is verified by comparing with experimental results on a single circular cylinder as well as two interacting cylinders. It is demonstrated that the theory accurately predicts hydrodynamic forces on the body especially In the range of relatively large KC numbers.
In order to design many types of offshore structures the prediction of hydrodynamic forces acting on them is very Important. These hydrodynamic forces can be due to waves, currents, oscillatory motions of the structure itself or various combinations of these effects. In practical design of offshore structures hydrodynamic forces acting on a well submerged. oscillating body are In general calculated by the following well known Morison formula by using drag and mass coefficient. Co and CM which mainly depend on the geometry of the body, the Reynolds number(Rn) and the Keulegan- Carpenter number(KC). Because of the difficulties and complexities of the theoretical treatment of the ambient flow field around the oscillating bodies, the hydrodynamic force coefficients. Co and CM, have so far been determined from experiments such as U-tube measurements. A number of experimental data of these hydrodynamic force coefficients have been published (for instance see Sarpkaya and Isaacson, 1981) Such experiments, however, have been performed for rather simple bodies such as two-dimensional cylinders as function of Rn, KC, surface roughness. etc. In recent years considerable effort has been spent on developing theoretical models for calculating the flow field behind an oscillating body.