Experimental investigation on multi-mode flow-induced vibrations of two long flexible cylinders in a tandem arrangement

Laboratory tests of flow-induced vibrations (FIV) of two tandem long flexible cylinders were conducted in a towing tank. The two identical cylinder models with an aspect ratio of 350 and a mass ratio of 1.90 were towed along the tank by a carriage to generate a uniform flow. The towing velocity was in the range of 0.05-1.0 m/s, and the corresponding Reynolds number approximately ranged from 800 to 16,000. Strain gauges were used to measure the time-varying strains in both cross-flow (CF) and in-line (IL) directions. A modal analysis approach was adopted to reconstruct the displacement response based on the strain signals in measurement positions. Five center-to-center separation distances (T/D = 4.0, 6.0, 8.0, 10.0 and 16.0) were tested in the experiments. The influence of separation distance on the multi-mode FIV behaviors of two tandem flexible cylinders was investigated from various aspects, e.g. strain and displacement responses, dominant frequencies and modes, mean drag force coefficients. Experimental results show that the multi-mode FIV behaviors of the upstream cylinder are similar to an isolated single cylinder within the separation distance range tested. The response of the downstream cylinder obviously differs from the upstream one due to the wake shielding effects generated by the upstream cylinder. In addition, there are no clear trends associated with the separation distance regarding the response amplitude of the downstream cylinder. (C) 2017 Elsevier Ltd. All rights reserved.