Ëxperiments for High Reynolds Numbers VIV on Risers

™ In this paper the results of high Reynolds VIV experiments with a new test set-up in MARIN's high speed towing tank are presented. A circular cylinder of 206 mm in diameter and 3.84 m in length was towed at constant speed through the basin at Reynolds numbers up to 5.5 x 10 s. Tests with a stationary cylinder and tests with a freely vibrating cylinder were carried out. Our VIV experiments at high Reynolds numbers are as far as we know new. When comparing the new high Reynolds results with the available VIV data for lower Reynolds number, significant differences are observed. The measured in-line drag coefficient of the freely vibrating cylinder in the early critical Reynolds regime is for instance significantly lower than the known values for lower Reynolds numbers. Also the measured lift and added mass in our high Reynolds experiments differ significantly from the known values. It is assumed that the differences are related to the subtle behavior of the boundary layer. For a smooth stationary cylinders it is well known that the boundary layer changes significantly in the early critical regime, leading for instance in a significant reduction of the in-line drag coefficient. Due to the turbulent transition in the boundary layer, the wake becomes narrower and less organized, resulting in smaller hydrodynamical loads. k The new high Reynolds VIV data can have an important impact on *he design of deep water riser and mooring systems. Additional research is therefore needed on the VIV behavior of slender cylinders in high Reynolds flow.