A framework for understanding the flow around groups of cylinders

The interaction between fluid flow and an array of cylinders in a tandem arrangement with Re = UD/ν = 200 (where U is the free stream velocity, D is the diameter of the cylinder and ν is the kinematic viscosity) is studied numerically in this paper. All the cylinders are rigidly mounted and equal in size and the pitch (p = distance between centre of cylinders) is consistent between any two consecutive cylinders. The sharp interface immersed boundary method is used to simulate the fluid and cylindrical structures in two dimensions. In the first step of this study, the two-cylinder system with varying pitch, 1.1 ≤ p ≤ 10 is studied. There are four distinct regimes as a function of p, delineated by the maximum lift coefficient and mean drag coefficient data. In the second step of this study, one representative pitch, p = 6.0, is selected and the fluid-structure interaction problem is extended systematically by adding further cylinders in the wake. The contours of streamwise mean flow velocity for two to five cylinders show that there is a region in which the flow is convectively unstable. This region starts after interaction of fluid with the second cylinder and its length is dependent on the number of cylinders. The length of the convective region increases when the third cylinder is added to the system but it follows by a decrease when adding further cylinders in the wake.