Groups of cylinders can be found in many engineering fields such as marine and civil applications. The behavior of the group cylinders can be very complex because it undergoes the mutual effects of adjacent cylinders arranged in different positions. In this paper, the results of a study on the dynamics of a group of flexible cylinders in square arrangements along with a single (isolated) cylinder subjected to uniform cross-flow (CF) are presented. Four flexible cylinders of the same size, properties, and pretensions were tested in two configurations with different center-to-center separations. Reynolds number ranged from 1400 to 20,000 (subcritical regime).The parameter of reduced velocity reached up to 19. The aspect ratio of all the cylinders was 162 (length/diameter). Mass ratio (cylinders mass/displaced water) was 1.17. The amplitude ratio of the CF vibration of the downstream cylinders, hydrodynamic force coefficients including mean and fluctuating components of the drag and lift forces, tension variation of the downstream cylinder, and frequency responses in both CF and inline (IL) directions were analyzed. All the cylinders excited up to the second and fourth mode of vibrations for CF and IL directions, respectively. Mean drag coefficient of the upstream cylinders are almost twice those of the downstream cylinders. The mean lift coefficient is much higher for the upstream cylinders than the downstream cylinders with different positive and negative signs. The IL and CF frequencies of the downstream cylinders are lower than those of the upstream ones and the single cylinder.

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