Identifying the Power-In Region for Vortex-Induced Vibrations of Long Flexible Cylinders

The primary objective of this research is to locate the source of the vortex-induced vibrations (VIV) for long flexible cylinders at high-mode number and to help determine the source region for future predictions. The two Gulf Stream tests were conducted to collect data on a scale-model pipe that was excited at high-mode numbers. The high density of the sensors on the pipe allowed for analysis that had not previously been done. Two methodologies are presented to locate the area of the region that is the source of the vibration. In VIV, the current which causes the vibrations is important, because the speed of the current will determine the frequency of the vibration. Therefore, one important question is which section of the pipe will be the source of the vibrations for a known current profile. This source region is known as the power-in region. Regions on the pipe that are not a source of power instead damp the structural vibrations. Once the region where the vibration originated has been found, the different phenomena that effect the location of the power-in region that were discovered are shown. Four different factors are presented that effect the locations of the power-in region: the angle of the pipe with respect to the vertical, the gradient of the current direction, the current profile, and the end effects at high mode number. A dimensionless parameter is presented which help in the prediction of VIV given a current profile. The power-in factor predicts the region where the source of the vibration occurs using a combination of the current velocity and the source region length.