An experimental study on flow-induced vibration of three and four side-by-side long flexible cylinders

Abstract

Multiple flexible cylinder structures, such as marine risers and cables, are key equipment for the production and transportation of oil and gas in offshore engineering. Till now, little information is available on flow-induced vibration (FIV) of more than two side-by-side flexible cylinders. Though some FIV characteristics of multiple flexible cylinders can be predicted based on the knowledge of two flexible cylinders undergoing vortex shedding, there are still some other characteristics that remain unrevealed, which require further experimental investigations on more than two flexible cylinders. In this paper, the FIV of three- and four-cylinder system in a side-by-side arrangement with a distance of 6.0D between cylinder centerlines was experimentally studied in a towing tank. The flexible cylinder models with mass ratio of 1.90 and aspect ratio of 350 were mounted horizontally and towed along the tank to simulate the uniform flow. The towing velocity ranged from 0.05 to 1.0 m/s and the corresponding Reynolds number varied from 800 to 16000. The experimental results were discussed in detail, including the maximum RMS displacement amplitudes, dominant frequencies, dominant modes, time-varying displacements at the measurement points, displacement response spectra, spanwise evolutions of the RMS displacement and space-time varying displacement contour. It can be found that the multi-mode response characteristics of the three-cylinder system in a side-by-side arrangement are obviously distinct from those of the four-cylinder system due to the effect of complex wake flow behind the cylinders. These findings are very meaningful for the design of a group of flexible cylinders in engineering applications.