Vortex-Induced Loading on Offshore Structures: A Selective Review of Experimental Work
Authors: Sheppard D.M. , Omar A.F. 
Published: October 2008

This paper is a review of experimental research on vortex-induced transverse forces on smooth, rigid, fixed and elastically mounted circular cylinders, subjected to a variety of steady and periodic flows. The salient flow and geometric parameters influencing the vortex shedding process and thus the transverse force are outlined and discussed. Using data from a number of investigators', attempts are made to illustrate both accepted findings and behavioral trends in the available data. Experimental work by the authors presently underway with oscillatory, nonplanar flows is described and some preliminary results discussed. Areas where information is sparse or nonexistent or where there is significant scatter in existing data-are outlined as topics for future work.

INTRODUCTION:

When a viscous fluid such as water or air flows past a blunt body with sufficient velocity, flow separation occurs and a wake region is formed. Over a wide range of flow and structure parameters of interest, vortices are observed to form near the points of flow separation. For symmetric structure shapes, void of sharp edges, such as right circular cylinders, vortices are formed on both sides of the body. Under certain conditions these vortices remain attaclled to the body while under other conditions they are shed from the body in or out of phase with each other. The net effect of this phenomenon is a fluctuation in the points of flow separation, which in turn causes a time varying distribution of normal and tangential stresses over the body. This results in time dependent inline and transverse loads on the structure, even when the flow is steady and planar (uniform). The processes associated with flow separation are complex and difficult to predict. Yet minor changes in the separation point can result in relatively large changes in both the inline and the transverse forces on the structure. This flow instability problem is sensitive to perturbations such as those introduced by surface roughness, motion of the body, free stream turbulence, flow orientation relative to the structure, flow around the ends of the structure, etc. In an attempt to understand and model this phenomenon, researchers have isolated various aspects of the problem starting with the (seemingly) simplest case and moving toward the more complex flow and structure situations. The processes are of course nonlinear and thus their individual effects cannot be simply superimposed to obtain the combined effect. However, much can be learned about the mechanisms involved and some guidance for the design engineer can be achieved by such a process.

Vortex induced loads are of interest in a number of engineering disciplines, and of particular importance in the design of offshore structures. Structural elements are constantly subjected to loading due to wind and/or ocean currents and waves. Most flow situations encountered in nature are turbulent, nonplanar (nonuniform), and unsteady. To further complicate matters the structural element of interest is often in close proximity to other members, compliant, and perhaps partially covered with biofouling.