A numerical study of flow past arrays of cylinders at low Reynolds number

Abstract

Coastal erosion along shorelines has increasingly been prevalent. A popular method for alleviating this problem is to use breakwaters, made of bamboo, known as “bamboo fencing”. The thesis numerically investigates steady, low-velocity flow past several geometrical configurations of bamboo fencings, modelled as array of cylinders using the ANSYS®-Fluent software. The Reynolds number based on incoming flow velocity (U∞), cylinder diameter (D), and fluid kinematic viscosity is Re = 100. Validation is done for mesh convergence and time-step size for the two-dimensional flow past a circular cylinder. For a pair of side-by-side arranged cylinders, it is found that the effect of traverse gap (T) between the cylinder becomes negligible at about T/D = 15. For the staggered arrangement of array of cylinders with 3 to 13 columns, cylinders located at any position experience similar value of drag coefficient except for those located at the last column whose drag coefficient is much smaller. For the aligned arrangement with 3 to 5 columns, cylinders at the first column experience largest drag while the drag forces of cylinders for the rest are of similar, lower value. The pressure drop across the fence of cylinders is found to be directly proportional to the combined drag, as suggested by the theory. The summation of drag force and pressure drop increase linearly as more column is added but the increasing rate is larger for the staggered configuration. The investigation also considers other types of arrangement, found in the real world. It is found that the zigzag arrangement has drag summation and pressure reduction that are only slightly larger than the 5-column staggered arrangement, while consists of a larger number of cylinders. For the triangular and diamond modules, cylinders at the far left and far right in the lateral direction experience largest drag. The summation of drag force and pressure drop of the diamond module are approximately twice as much as those of the triangle. For the T-shape and line-shape, the drag summation and pressure drop are significantly smaller relative to the zigzag, the triangular, and the diamond modules. Lastly, the tail of the T-shape does not significantly alter the summation of drag force and pressure drop.