AN EXPERIMENTAL INVESTIGATION OF THE FLOW AROUND A CIRCULAR-CYLINDER - INFLUENCE OF ASPECT RATIO

The investigation is concentrated on two important quantities - the St rouhal number and the mean base suction coefficient, both measured at the mid-span position. Reynolds numbers from about 50 to 4 x 10(4) wer e investigated. Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plat e diameter ratios between 10 and 30). It was not possible, by using th ese end plates in uniform flow and at very large aspect ratios, to pro duce parallel shedding all over the laminar shedding regime. However, parallel shedding at around mid-span was observed throughout this regi me in cases when there was a slight but symmetrical increase in the fr ee-stream velocity towards both ends of the cylinder. At higher Re, th e results at different aspect ratios were compared with those of a 'qu asi-infinite cylinder' and the required aspect ratio to reach conditio ns independent of this parameter, within the experimental uncertaintie s, are given. For instance, aspect ratios as large as L/D = 60-70 were needed in the range Re approximate to 4 x 10(3)-10(4). With the small est relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and 'irregu lar flow' was found at intermediate Reynolds number ranges in the subc ritical regime (Re approximate to 2 x 10(3)).