An extensive program of forced and self excited (vortex-induced) vibration tests on a single circular cylinder moving in uniform current has been performed. For both self excited and forced vibration tests two cases were investigated. one in which the cylinder was restrained in the in-line direction and the other in which it was supported on suitable springs. The vibrational response and the hydrodynamic forces on the cylinder both in the in-line and the transverse directions were measured. This paper reports the hydrodynamic force components with frequencies higher then the cylinder motion frequency (m) in the in-line and the transverse direction. When the in-line direction is spring-supported the test results reveal large higher harmonics in the in-line drag force and the lift force. Apart from a constant drag the in-line force, has 2m. 4m and 6m components. On the other hand, the lift force in addition to the component of m consists of components with frequencies equal to 3m and 5m." For given true reduced velocity and transverse reduced amplitude. it seems that the amplitudes of these higher harmonics depend mainly on the motion amplitude in the in-line direction and the phase angle between the motions in the two directions. If the cylinder is fixed in the in-line direction or it vibrates in a "arc" type pattern the components of 4m and 600 in the in-line force and of 300 and 5ω in the lift force are vanishing. A hydrodynamic force model based on the relative flow velocity may qualitatively describe the phenomenon.
Many experiments with flexible supported sectional stiff cylinder segments have been performed to study the interaction between flow and the cylinder motion. When the hydrodynamic force acting on the cylinder is concerned test results are scarce and show a relatively large spread.
