This paper presents some experimental measurements of wave forces on a transversely oscillating slender cylinder which represents a slender offshore member in waves. An integrated Program based on Labview graphical language is described in detail which allows controlling the cylindrical model to undergo a desired orbital motion. The phase shift between the arrival of the crest of the incident wave and the crest of the cylinder trajectory, ratio of the cylinder oscillation frequency to the wave frequency (f o/f w) and ratio of the oscillation amplitude to diameter (A / D) are effectively controlled so that all tests are repetitive. Results indicate that the phase shift may affect the wave force time history due to wake biasing effect; both the amplitude ratio and frequency ratio may have a significant influence on hydrodynamic coefficients M C and D C.
Vortex-Induced-Vibration (VIV) is an important topic in ocean engineering which involves fluid-structure interactions of small structural members. In uniform flow cases, if the vortex shedding frequency approaches the natural frequency of a lightly damped cylinder, the vibration of the cylinder may become stronger, and this strong vibration can drive the eddies to be shed at a frequency ranging between the natural frequency (N f) of the cylinder and the Strouhal frequency (f / S t U /D). This phenomenon is usually called "lock-in" between the frequency of vortex shedding and the frequency of the vibrating cylinder. Under lock-in conditions, strong resonant vibration occurs and the lift forces are amplified by the increase of vortex strength. Reviews on this topic have been given by Blevins (1990) and Sarpkaya (2004). A similar phenomenon may also occur under certain conditions for an elastically mounted cylinder in waves.
