ABSTRACT:

In this paper, the viscous forces on and the flow field around oscillating circular cylinders with various roughness height are experimentally investigated. In-line and lift forces on oscillating roughened cylinders have been measured in a small water tank. The results show that the drag, inertia and lift coefficients depend on relative roughness height (K/D), which is the ratio of the roughness height (K) to the cylinder diameter (D). The dependency of the drag coefficient on the relative roughness height drastically changes at K/D of about 0.025, where the roughness height is almost the same as the boundary layer thickness of a smooth cylinder. Lift force frequency shifts to higher values at K/D larger than 0.025. Flow velocity near the cylinder surface has also been measured by a hot-wire anemometer in air. The instantaneous fluid velocity profiles obtained near the cylinder surface have been observed to also be affected by the relative roughness height.

INTRODUCTION

The roughness effect associated with marine growth is one of the most important factors to be considered in the estimation of wave loads on offshore structures. It has been pointed out that the roughness effects on hydrodynamic forces on an oscillating cylinder depend on several parameters: the roughness height, the roughness plugging density, the shape of the roughness elements, and so forth. Most researchers on the roughness effects have focused their attention on the relationship between the forces and the relative roughness height (K/D), which is defined as the ratio of the roughness height (K) to the cylinder diameter (D). Sarpkaya et al. (1977) measured hydrodynamic forces on sand-roughened cylinders in plane oscillatory flow, and reported that the drag coefficient increases with increasing relative roughness height at high Reynolds numbers due to the activation of transition in the flow.

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