Analysis of fluctuating force acting on two cylinders in different arrangements through Lattice Boltzmann Method

The force components on two circular cylinders in tandem, side-by-side and staggered arrangements and in a uniform incoming flow are investigated systematically with Reynolds number being taken Re=200. The numerical simulations are undertaken using the lattice Boltzmann method and the boundary condition is imposed though the immersed boundary method. At an intermediate centre-to-centre spacing which is based on the ratio of distance between cylinder centres and the cylinder diameter, the fluid force is periodic and dominated virtually by a single frequency component in the tandem arrangement and is random with a continuous frequency spectrum in the side-by-side arrangement. Here random means that the components of the force are not in the form of discrete frequencies, but are in the form of continuous spectrum. Detailed analysis has been undertaken to show this evolution process with the alignment angle α, which is defined as the angle between the incoming flow direction and the line linking two cylinders. As α increases, the frequency component of the force on each individual cylinder changes from a single value to multiple ones, then to a large number of discrete ones and eventually to a broadband continuous spectrum. The results have important implications to vortex induced vibration and fatigue analysis of the cylinder, which has important applications, such as risers in offshore engineering.