Judy Maksoud
International Editor
Offshore
Researchers are making strides in experimentation targeting a better understanding of vortex induced vibration (VIV), but there is still a long road ahead. Dr. Peter Bearman of the Imperial College, London, told participants at the 24th annual Offshore Mechanics and Arctic Engineering conference in Helkidiki, Greece, that although research has moved the understanding of VIV in the right direction, much more research needs to be done.
Experimentation results using computational fluid dynamics (CFD) are improving our understanding of VIV, Bearman said, and prediction reliability is getting better. However, there is a real need to improve VIV predictability. The industry also needs new proven techniques for VIV suppression, he said.
Opening the VIV discussion, Bearman quoted an 1497 observation by Leonardo da Vinci: "Moving water strives to maintain the course pursuant to the power which occasions it." Finding an obstacle in its path, water continues its circular motion, da Vinci wrote.
This raises a significant question, Bearman said. "If vortices were known in the 15th century, why haven't we solved the VIV problem?" In fact, the reasons for that are many, and researchers are working to identify the obstacles that lay in the path of better understanding.
Most VIV research is single-mode analysis that attempts to determine what the fluid is doing, Bearman said. Researchers have concluded that changes in mass ratio have little influence on VIV and that decreasing the mass ratio reduces oscillation frequency in still water, he said. To reduce VIV response, researchers need very large increases in damping, he continued, but it is unlikely that sufficient damping can be produced to achieve this.
According to Bearman, typical experimentation has been a rigid cylinder on a platform.
"What we see from these types of single-mode experiments is that as the current starts increasing, there is a large increase in amplitude," he said. Cylinder response over a wide range of flow speeds is due to a fluid/structure interaction that synchronizes the cylinder/fluid, Bearman explained. The big question that comes out of the current mode of experimentaiton is, "Can single-mode experiments apply to a multi-mode condition?"
Bearman pointed to current 2D CFD experimentation, simplified modeling, and modeling based on 2D measurements. He noted that each method has strengths and weaknesses, but none of them is able to accurately show drag coefficient. "All of them fall short in some way," he said.
Though experimentation and research are moving forward the industry's understanding of VIV, much more work remains to be done, Bearman said.
06-13-05
