An Optical Tool for Measuring Vortex-Induced Vibrations of Long Flexible Cylinders

We describe the design and performance of a novel optical measurement system that can be used for measuring vortex-induced vibrations (VIV) of long flexible cylinders. We believe this is the first time this technology has been used to measure VIV in a long flexible cylinder. The emphasis is given to the measurement technique itself. It can be used either in air or in water and we believe it is a better alternative to other traditional vibration response measurement techniques used to study VIV in the laboratory. The measurement system is based on the Defocusing Digital Image Particle Velocimetry (DDPIV) concept, proposed by Willert and Gharib in [1] for measuring velocity flow fields. A set-up, which was built to conduct vortex-induced vibration measurements in a wind tunnel facility with a cross-section of 1.8 × 1.2 m, has been used in a laboratory to demonstrate the capabilities of the optical measurement system when known displacements are produced in the mappable or measurement volume. The same supporting structure designed to provide suport for the cylinder models inside the wind tunnel test section, has been used to hold a target which has been move to different known locations to measure the performance of the system. The design, description and performance of the measurement technique are discussed.