Dynamic response of three long flexible cylinders subjected to flow-induced vibration (FIV) in an equilateral-triangular configuration

A three-cylinder system with an equilateral-triangular configuration represents one of the most common, basic units of multi-cylinder systems in many engineering fields. In this paper, an experimental investigation of the flow-induced vibration (FIV) of a three-cylinder system in an equilateral-triangular arrangement was conducted in a towing tank to study their response characteristics. Three identical flexible cylinder models with a centre-to-centre distance of 6.0 times the cylinder diameter were towed along the tank to generate a uniform flow. The Reynolds number ranged from approximately 1600 to 16000 depending on the towing velocity, which was in the range of 0.10-1.00 m/s. The time-varying bending strains of the flexible cylinders in the cross-flow (CF) and in line (IL) directions were measured using strain gauges at different measurement positions. The displacement responses were reconstructed by a modal analysis method based on the acquired strain signals. The experimental results (e.g., displacement amplitudes, response frequencies, dominant modes and motion trajectories of the flexible cylinders) were presented to reveal the FIV response features. Three typical cases of the equilateral-triangular configuration (A, B and C) corresponding to flow incidence angles of 0 degrees, 30 degrees, and 60 degrees were selected and discussed. (The incidence angle is the angle between the flow direction and the line joining the centre of one cylinder to the centre of the equilateral-triangular configuration.) In Cases A and B, the upstream cylinder is insignificantly affected by the downstream cylinders and behaves like an isolated flexible cylinder; meanwhile, the upstream cylinders in Case C vibrate similarly to two side-by-side flexible cylinders with weak mutual interference. Striking departures exist in the IL oscillations of the downstream cylinders due to the notable effects of the upstream cylinders. The phenomenon of mixed modes appears in the IL vibrations of the downstream cylinders. Furthermore, the IL response spectra always contain a robust lower frequency component equal to the dominant frequency in the CF direction. Moreover, eight-, half-moon- and C-shaped vibration figures are comparatively uncommon for the downstream cylinders.