An experimental investigation of two-degrees-of-freedom VIV trajectories of a cylinder at different scales and natural frequency ratios

This paper presents the results of two-DOF VIV experiments on horizontal cylinders with lengths of 120 cmand diameters of 5 cm, 11 cm and 20 cm under conditions of different in-line and transverse natural frequencies. By comparative analysis of vibration trajectories, it has been found that the oscillation frequencies of cylinders in in-line and cross-flow directions with different scales all appeared to have more than one dominant frequency at some stages, That is, the in-line dominant frequency was equal to one to three times the cross-flow one under conditions of certain scales, natural frequencies and reduced velocities. Additionally, more than one dominant frequency was also observed in transverse oscillation, so the trajectories of cylinders were significantly different from the traditional “figure 8” shape. Particularly for small cylinders, the change in the vibration trajectory was the most obvious. Studying the evolution of the cylinder VIV trajectories at various scales and different conditions revealed that the appearance of various forms of cylinder vortex vibration trajectories was mainly due to the scale of the cylinder, the transverse and in-line natural frequencies, the frequency ratio and the mass ratio.