Entry Date:
April 4, 1999

Virtual Cable Testing Apparatus (VCTA)

Principal Investigator Michael Triantafyllou

Co-investigators Dick Yue , J Vandiver


When fluid flows around a bluff body such as a cable, the flow is disturbed and so downstream forms a wake. The most familiar wake is perhaps the so-called Karman vortex street, characterized by strong, alternating vortices. In water, you might see it forming downstream of pilings, cables, or in front of your canoe paddle! While the wake is clearly responsible for drag exerted on the body, vortices typically exert an oscillating side force as well. One can imagine how each of the shed vortices in the Karman wake might push on the body. The behavior of these vortices and forces around a stiff body is a very interesting problem in itself.

In practice, however, very few structures are completely rigid; a cable can vibrate, a pier is only as stiff as the wood and the soil below. As a result, the oscillating forces from a vortical wake can in fact cause the structure to vibrate. This motion in turn affects the properties of the wake, and very soon we see that the dynamics of the structure and the dynamics of the wake are intimately coupled.

This is a problem of great importance in ocean engineering. For marine cables, the vibrations induced by the flow can cause large lateral deflections (typically several diameters, peak to peak). With this motion, drag forces tend to be amplified, and dynamic tensions in the cable can be quite large. The tension fluctuations may cause the cable to fail under extreme loads, or they may reduce the fatigue life of the deployment significantly.