Vortex Shedding and Flow-Induced Vibration of Two Cylinders in Tandem

This paper presents a study of the interaction between a fluid flow and two cylinders in a tandem arrangement. The cylinders are identical in size and either rigidly mounted, or elastically mounted but restricted to one degree of freedom in the cross-flow direction. The Reynolds number is Re = UD/v = 200 where U is the freestream velocity, D is the cylinder diameter and v is the kinematic viscosity. The sharp interface immersed boundary method is used to conduct two-dimensional simulations of the interaction between fluid and the two structures as a function of elasticity level quantified by the reduced velocity (U* = U / f(N) . D) where f(N) is the natural structural frequency of each cylinder in vacuo, and pitch p which is the streamwise distance between the centres of the two cylinders. In the first stage, aerodynamic forces, frequency spectrum and amplitude of oscillation have been measured as a function of p for rigid cylinder system. The results showed that in the rigid two-cylinder system, there are four distinct regimes. In the second stage, a similar study with varying pitch but also with varying reduced velocity, U*, has been conducted for elastically mounted cylinders. It is found that for systems with very small p, the behaviour is highly nonlinear and the oscillation of both bodies exceeds that of a single isolated cylinder. Over a critical value of p the oscillation of the front cylinder is very similar to a single cylinder system and is therefore essentially independent of pitch. However, the rear cylinder behaviour is strongly dependent on the pitch. The rear cylinder can oscillate with an amplitude which is higher or lower than the amplitude of oscillation for a single cylinder depending on the U* value.