VCTA

Virtual Cable Testing Apparatus

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.

Past Work

At the Towtank, we have been studying vortex-induced vibrations experimentally since 1992. Ram Gopalkrishnan (Sc.D., 1992) measured forces on a cylindrical section, when the cylinder was moved in beating motions (the sum of two sinusoids). He used the large tank with a special carriage carrying a two-foot long test section mounted to a linear drive and servomotor. A load cell mounted on the section recored the drag and oscillating lift forces. Using the small tank, he also investigated the effects of a heaving and pitching foil placed behind the cylinder. Jamie Anderson (Ph.D., 1995), in addition to studying fish propulsion with digital particle image velocimetry (DPIV), and REAL FISH (Giant Danio), continued work on the vortex-control problem using a foil behind a cylinder. Professor Kim Vandiver has also been involved in tests with long sections of cables and beams.

In 1993, Scott Miller and Franz Hover constructed a new apparatus for testing cable sections. Dubbed the Virtual Cable Testing Apparatus (VCTA), it differs from Gopalkrishnan's carriage in one crucial respect: the force signal is used to drive an on-line simulation of a compliant cable. This force-feedback system allows one to specify the structural dynamics of the cable in software. As a result, the system handles cable models employing nonlinearities, multiple modes, and wave phenomena.