Abstract
The study of forced and free vibration of a cylinder has long been isolated. The internal relationship between free vibration and forced vibration has rarely been investigated. In this paper, the relationship between the forced and free vibration of a cylinder was established. A series of numerical simulations of a cylinder undergoing forced oscillations at a wide range of vibration amplitudes and frequencies were carried out, with the flow solver viv-FOAM-SJTU developed based on the open-source platform OpenFOAM. Complex demodulation analysis was conducted to quantify the spatial-temporal phase relationship between the forces and the displacement of the cylinder. It was found that, at some particular oscillating amplitudes and frequencies, the phase angle switched between positive and negative values, which corresponds to a vortex mode transferring from the 2P mode to the 2 P O mode. This distinct new mode “ 2 P O ” was closely related to the intermittent jumping between lower and upper branches of the amplitude responses of VIV. A prediction model was developed to obtain the VIV amplitude responses based on the numerical results of forced oscillation. The prediction results of three points located separately in the initial, upper, and lower branches of VIV agreed well with experimental measurements of an elastically mounted cylinder. This prediction model was thus expected to be suitable for predicting the response of VIV.
Highlights
When a flow passes a cylinder, the oscillatory shedding of vortices into the wake would cause fluctuating lift and drag forces on the cylinder
We found that the wake could switch intermittently between the 2P and 2POVERLAP mode (2PO) modes, even if the cylinder was vibrating with constant amplitude and frequency
Numerical Fluid Model. e fluid motion is represented by the incompressible unsteady Reynolds-Averaged Navier–Stokes (URANS) equations
Summary
When a flow passes a cylinder, the oscillatory shedding of vortices into the wake would cause fluctuating lift and drag forces on the cylinder. E motions of the cylinder would in turn change the vortex wake. If the cylinder was elastically mounted, the fluctuating forces could excite the vibrations of the cylinder. Such a problem of fluid-structure interaction was called vortex-induced vibrations (VIV). VIV was a highly specialized subject that incorporates fluid mechanics, structural mechanics, vibrations, computational fluid dynamics (CFD), etc. Studies for VIV could be divided into two categories: free vibration of a cylinder and forced vibration of a cylinder
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