Experimental study on the vortex-induced vibration of towed pipes

Abstract

We experimentally attempted to understand the vibration characteristics of a flexible pipe excited by vortex shedding. This has been extensively studied in the previous decades (for example, see Sarpkava 1979 Journal of Applied Mechanics 46, 241-258; Price et al. 1989 Eighth International Conference on Offshore Mechanics and Arctic Engineering. The Hague-March 19-23. 447-454. Yoerger et al. 1991 Journal of Offshore Mechanics and Arctic Engineering, Transaction of Engineers 113, 117-127 Grosenbaugh et al. 1991 Journal of Offshore Mechanics and Arctic, Engineering, Transaction of Engineers 113, 199-204; Brika and Laneville 1992 Journal of Fluid Mechanics 250, 481-508, Chakrabarti et al. 1993 Ocean Engineering 20, 135-162, Jong 1983 Ph.D. Dissertation, Department of Ocean Engineering, M.I.T.; Kim et al. 1986 Journal of Energy Resources Technology, Transactions of American Society of Mechanical Engineers 108, 77-83). However, there are still areas that need more study. One of them is the relation between spatial characteristics of a flow-induced vibrating pipe, such as its length, the distribution of wave number, and frequency responses. A non-linear mechanism between the responses of in-line and cross-flow directions is also an area of interest, if the pipe is relatively long so that structural modal density is reasonably high. In order to investigate such areas, two kinds of instrumented pipe were designed. The instrumented pipes, of which the lengths are equally 6 m, are wound with rubber and silicon tape in different ways, having different vortex-shedding conditions. One has uniform cross-section of diameter of 26.7 mm, and the other has equally spaced four sub-sections, which are composed of different diameters of 75.9, 61.1, 45.6 and 26.7 mm. Both pipes are towed in a water tank (200 In x 16 In x 7 m) so that they experienced different vortex-shedding excitations. Various measures were obtained from the towing experiment, including frequency responses, the time-domain tracing of in-line and cross-flow responses, and Wigner-Ville distributions. The experimental results analyzed by using these measures exhibit several valuable features. One of them is that the natural frequencies and their corresponding strain mode shapes dominate the strain response of the uniform pipe. However for those of non-uniform pipe, the responses are more likely local and many modes participate in it. (C) 2002 Academic Press.