Numerical investigation of flow field over four cylinders in square arrangement for different spacing ratios

Abstract

The objective of this paper is to study the effect of spacing ratio on the wake dynamics in an in-line square array of four cylinders. Two dimensional incompressible CFD simulations are done with commercial CFD software Ansys-Fluent. Diameter of cylinder is 0.01 m, and the four cylinders have identical diameter. The spacing ratios considered are 1.5, 2.0, 2.5 and 3.0, where spacing ratio is defined as ratio of distance between center of two cylinders and diameter of cylinder. Simulations are performed at a Reynolds number (Re) of 200. Water is considered as fluid medium, and velocity of water corresponding to Re of 200 is 0.0178 m/s. SIMPLE algorithm is used to solve incompressible Navier-Stokes equations, and second order accuracy is used for spatial discretization. At the chosen Re, far wake is expected to transition to turbulence, and hence κ-ω SST turbulence model is used for turbulence closure. The flow field and associated surface properties such as drag and lift force are strong function of spacing ratio. The impingement of shear layers of upstream cylinders onto to the downstream cylinders is strongly influenced by spacing ratio. The impingement and resulting flow interaction leads to asymmetry in the surface pressure distribution. This leads to generation of lift force on the cylinders. For spacing ratio of 1.5 and 2.0, the top row cylinders have positive lift coefficient, and bottom row cylinders have negative lift coefficient. For spacing ratio of 2.5 upstream cylinders have positive lift coefficient, and downstream cylinders have negative lift coefficient. All four cylinders have positive lift coefficient for spacing ratio of 3.0. Drag coefficient found to be positive for all cylinders and for all spacing ratios.