Hydrodynamic Lift and Drag on Rough Circular Cylinders
Authors: Sarpkaya T. 
Published: October 2008

The purpose Of this paper is to compare the existing hydrodynamic force data On the effect Of rigid roughness On cicular ylinders and to discuss critically the fluid-mechanical and/or facility-dependent causes of the differences.

INTRODUCTION

The research On the effect Of roughness On fluid resistance is unguided by theory: Experiments determine the force-transfer coefficients. However, the acquisition of reliable data is fairly difficult, it being more so in the ocean environment than in the laboratory. The non-repeatability of the natural occurrences and the difficulty of quantifying the prevailing conditions lead to large scatter, particularly when the equipment is not well suited to the task under consideration. Even in the absence of macroscopic fouling, the frictional resistance of submerged surfaces increases with time due to microscopic fouling - the formation Of a film. In mid 70's, in the absence Of any data appropriate to periodic or wavy flows, it has been assumed that the drag coefficients obtained from tests in steady flow over artificially- or marine-roughened cylinders are applicable to wave force calculations, at least when the loading is predominantly drag' The first warning against such a simplistic assumption was given by Sarpkaya1–2 in l976. Since then numerous experiments have been conducted and it has been generally shown that roughness (a) increases tube diameter, projected area, displaced volume and hence the hydrodynamic loading; (b) increases both drag and lift coefficients, leading to larger fluid forces (the decrease in the inertia coefficient (Cm = 1+ Ca) is more than offset by volume increase}; (c) increases mass, reduces natural frequency, and leads to a larger dynamic amplification factor; (d) increases structural weight, both in water and above the mean water level; and (e) increases turbulence intensities, Reynolds stresses, correlation length, vortex strength, and the lift coefficient. Equally important is the fact that roughness accumulates primarily in the upper most regions of an offshore structure where the wave- and current-induced velocities are largest. All these lead to the conclusion that the effect of roughness must be carefully and periodically assessed. Here, only some of the more ambitious and properly documented experiments which have a particular bearing on the understanding of fluid-mechanical and experimental factors governing the effects of rigid roughness on cylinders will be compared with the hope that the findings will provide guidance for future experiments and further clarify the effects of roughness. The purpose of the paper is not to compare the relative magnitudes of the drag loading obtained from oscillatory flow experiments with those obtained in real sea conditions. Such comparisons have been rnade3 a number of times in the past and it has been generally concluded that the sea data yield lower drag coefficients for apparently similar levels of roughness because of random kinematics and reduced coherence.

MATOR FACILITIES AND EXPERIMENTS

Some of the most frequently used test facilities are wave basins, wave flumes, U-shaped water tunnels, and water basins equipped with oscillating carriages. Even though, the resulting motions are theoretically the same, experimentally it does matter what is oscillated: the fluid or the cylinder. The variability of the kinematics of the ocean environment is not conducive to the reduction of time-invariant force-transfer coefficients for the assessment of Morison's equation.