Vortex-induced vibration of a cylinder with two degrees of freedom

In this work, we study the response of an elastically mounted cylinder, which is free to move in two degrees of freedom in a fluid flow, and which has low mass and damping. There has been a great deal of work concerned with bodies restrained to move in the direction transverse to the free stream, but very few studies which comprise motion in both the transverse (Y) and in-line (X) directions. In such cases, it has generally been assumed that in-line response would dramatically change the character of the wake vortex dynamics as well as the transverse body response. We find in the present work that, surprisingly, the freedom to move in two directions has very little effect on the transverse response, the modes of vibration, or the vortex wake dynamics (for a body of similar low mass ratio (relative density) in the range m* = 5-25). For low values of normalised velocity (U* similar to 2-4) below the classical synchronisation regime for transverse response, we find two in-line vibration modes, which are associated with symmetric and antisymmetric vortex wake modes. corresponding well with the modes discovered by Wooton et al. and by King for a flexible cantilever. Coupled with a parallel effort by D.O. Rockwell's group at Lehigh, these experiments form the first such studies in which both the oscillating mass and the natural frequency are precisely the same in the X and Y directions. A principal conclusion from this investigation is that it demonstrates the validity, for bodies in two degrees of freedom, of employing the existing comprehensive results for bodies restrained to vibrate only in the transverse Y-direction, even down to low mass ratios of m* = 5. (C) 2003 Elsevier Ltd. All rights reserved.