Reduction of fluid forces acting on a single circular cylinder and two circular cylinders by using tripping rods

The effects of tripping rods on flow characteristics and fluid forces acting on a single cylinder and on two cylinders in side-by-side and tandem arrangements were investigated in a smooth cross stream at a Reynolds number of 5.5 x 10(4). At first, tripping rods of 4, 5 and 6 mm in diameter were used on a single cylinder of diameter 49 mm. The angular position of the tripping rods was varied from 20degrees to 60degrees. The optimum angular position of tripping rods for reducing fluid forces was found to be 30degrees. Two tripping rods, each of 5 mm in diameter, positioned symmetrically at this angle reduced steady drag, fluctuating drag and fluctuating lift forces acting on the cylinder by 67%, 61% and 87%, respectively. A bistable nature of the flow on the single cylinder appears when the tripping rods were positioned at 41degrees-44degrees. Next, fluid forces acting on two circular cylinders in a side-by-side or tandem arrangement were reduced by using tripping rods on both cylinders. A significant reduction in both steady and fluctuating fluid forces acting on the side-by-side cylinders was observed when the gap spacing between the cylinders was greater than the diameter of a cylinder. In the case of tandem cylinders with tripping rods, fluid forces acting on the upstream cylinder were almost independent on the downstream cylinder at any spacing of the cylinders. The critical spacing at which a bistable flow appears was found at spacing ratios of 3.0 and 2.6 for the plain cylinders and the cylinders with tripping rods, respectively. At the critical spacing, however, there was no considerable jump in fluid forces acting on the upstream cylinder when tripping rods were used on the cylinder. Wavelet analysis was used to study the bistable flow for the single cylinder with tripping rods and for the tandem cylinders. (C) 2003 Elsevier Ltd. All rights reserved.