Vortex-induced vibration (VIV) in flexible cylinders typically exhibits the coexistence of several natural modes. In this work, we conduct a fundamental investigation of coupled VIV responses in a simplified scenario where rigid-body and flexible modes coexist. The experimental setup comprises a flexible cylinder of 0.7 m in length, with external diameter, aspect ratio, and mass ratio of \(D=25\) mm, \(L/D=28\) , and \(m^*\approx 4.5\) , respectively, which is vertically cantilevered from a one-degree-of-freedom leaf spring support, with natural frequency controlled by changing its spring stiffness. Through this assembly, it is possible to investigate the modal coexistence due to VIV when the natural frequency of the rigid body is properly calibrated to values close to the first two natural frequencies of the flexible cylinder. These conditions were investigated for a wide range of 20 reduced velocities. For measurement purposes, a motion capture system recorded rigid-body dynamics, while four inertial measurement units (IMUs) placed inside the flexible cylinder were used to record acceleration and rotational rate data during testing. A signal processing procedure has been developed and applied to transform accelerations into displacements along the flexible cylinder, which is a complete reference framework for the analysis of simultaneously excited VIV responses. In this sense, frequencies and nondimensional amplitudes are evaluated during the tests, obtained based on the application of Fourier and Hilbert–Huang transforms. These findings enrich VIV research by providing information that can improve the theoretical models currently used to predict coupled VIV responses, particularly regarding the interdependence between natural modes in relation to the reduced velocity that has excited them. The contribution is intentionally fundamental, offering clean benchmarks that stimulate an informed discussion of the topic.
