Out in the open ocean, metal cables secure monitoring stations, which are exposed to constantly changing conditions above and below the surface, to the sea floor. When the cables fail, more is lost than the equipment. For example, the Tropical Atmosphere Ocean array, a series of nearly 70 regularly spaced moorings spanning the Pacific, supplies real-time data to weather prediction centers around the globe. At the same time, it serves as the cornerstone for oceanographic research into the workings of the El Niño phenomenon. These cables are replaced annually, before they fail.
One reason for cable failure is fatigue resulting from violent vibrations. Professor George Em Karniadakis (Center for Fluid Mechanics, Turbulence and Computation at Brown University) is using the massively parallel IBM RS/6000 POWERparallel System (SP) at the Cornell Theory Center to examine the conditions that may cause the phenomenon--lock-in--behind this violent behavior.
Karniadakis and his Ph.D. student David Newman are building upon what we know about lock-in in two dimensions and are creating 3D computer simulations of cable vibration. The picture is much more complex in 3D. The group plans to develop computer models that can explain the relationship between wake behavior and cable vibration. Such models can be tuned in response to input from experiment and ultimately customized and simplified for specific engineering applications.
Karniadakis' models track the vibrating cable and the flowing water, concentrating the calculations at or near the cable in order to see the fine scale interactions that generate the vortices. The move to CTC's SP will allow a ten-fold increase in the size and complexity of the model, which will now simulate a cable with a length equivalent to 100 diameters. The added length provides for more realistic modeling of the interactions of different vibration modes. This work will be compared with companion experiments at Massachussetts Institute of Technology conducted by Prof. M. Triantafyllou.
Vortex-induced vibrations cause trouble above water as well as below. The same phenomena that wear out mooring cables also affect the performance of structural cables, for example those that hold up suspension bridges. Karniadakis' work is a major contribution to the study of complex systems in computational fluid dynamics. And it will also have many practical applications.
Lock-in
Cables at Sea
Turbulent Wakes