It is a well-known fact that the force coefficients used in design of subsea cylindrical structures do not come from free VIV tests, but from forced oscillation experiments. It is much easier and more reliable to quantify the forces using forced vibration due to the constancy of the desired amplitude and frequency. Hence, it is of fundamental importance to quantify the hydrodynamic coefficients based on forced vibration experiments for accuracy in design.
This paper presents the results of the experimental measurement of hydrodynamic forces on a circular hollow cylinder undergoing prescribed harmonic motions in a steady uniform flow. This study aims at quantifying the hydrodynamic coefficients in in-line and cross flow directions under different combinations of frequency and amplitude of oscillation. All experiments are performed at Reynolds number of 12,000 with non-dimensional frequency ranging from 0.4 < fosc/f0< 2.2 and non-dimensional amplitudes of 0.2 ≤ A/D ≤ 1.0. The experiments were conducted using a purpose-built forced vibration rig mounted onto a 40 m towing tank facility. A cylinder of diameter 0.06m and length 0.6m was attached vertically to the rig and towed at a constant speed while its motion was prescribed at a fixed amplitude and frequency in the transverse direction.
Fluid force analysis and its breakdown into components in-phase with velocity and acceleration are presented to investigate the general response characteristics of varying amplitude and frequency of motion. Three-dimensional plots of the response as a function of frequency and amplitude are plotted in order to offer a better representation of the fluid force coefficients.