Hydrodynamic forces acting on rough circular cylinders in a harmonically oscillating flow at high Reynolds numbers were experimentally investigated. Especially the effects of the roughness parameters, that is roughness height and roughness density, were investigated. Two kinds of roughness height (5mm, 10mm) and three kinds of roughness density (20%, 60%, 80%) were used in the experiments.
Experiments were carried out using a large circular cylinder 1.2m in diameter and 1.5m in length at TSMB (Tsu Ship Model Basin, NKK). End plates were set at the upper and lower ends of the large cylinder to eliminate the three dimensional effects. In-line and lift forces acting on cylinders were measured at Keulegan-Carpenter numbers from 6 up to 50 and at two Reynolds numbers (0.5X106, 1.0X106). Experiments show that the roughness has great effects on the hydrodynamic forces even at high Reynolds numbers. The three dimensional effects on hydrodynamic coefficients were investigated by using small cylinders.
Recently ocean development is spreading to the deep sea and arctic zones. Therefore, the high level technology is needed to design the offshore structures to withstand the severe conditions. Hydrodynamic forces are a principal condition of design and it is very important to estimate the hydrodynamic forces acting on offshore structures for estimating the stability and the workability of them.
Marine foulings such as barnacles, attach to the surface of offshore structures, but few offshore structures dock in order to have removed the foulings. Therefore, researching the effects of marine fouling is important to predict the hydrodynamic forces acting on offshore structures Offshore structures consist mainly of cylinders such as circular cylinders, of which the Reynolds numbers are very high (105 -107). It seems that the roughness effect especially at high Reynolds numbers should be investigated (Ref.l).
The roughness effect on a circular cylinder in a uniform flow, was investigated first by Fage (Ref.2) and later by Achenbach (Ref.3, 4). According to their results, the critical Reynolds number decreases with increasing roughness, and the transition of boundary layers occur at lower Reynolds numbers. The drag coefficients of rough cylinders in the supercritical range of Reynolds numbers, is always larger than that of smooth cylinders.
In harmonically oscillating flows, the roughness effects on the hydrodynamic forces were investigated experimentally by Sarpkaya (Ref.5, 6). He carried out a series of experiments with sandroughened cylinders, of which the relative roughness is 0.001–0.02. He concluded that the drag, lift and inertia coefficients of rough cylinders become constant for the sufficiently large values of "roughness Reynolds numbers" and depend only on Keulegan-Carpenter numbers and the relative roughness. Matten used a more realistic roughness, that is, rigid marine growth (Ref.7). His results showed that roughness made no significant difference to the lift force. But few experiments were carried out at high Reynolds numbers.
The roughness effect becomes greater as the marine foulings grow and occupy a large part of surface, but the effect of the increase in the areas covered with roughness, has not been investigated very much.
