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

The investigation is concentrated on two important quantities - the Strouhal number and the mean base suction coefficient, both measured at the mid-span position. Reynolds numbers from about 50 to 4 x 10(4) were investigated. Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30). It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime. However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder. At higher Re, the results at different aspect ratios were compared with those of a 'quasi-infinite cylinder' and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, 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 smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and 'irregular flow' was found at intermediate Reynolds number ranges in the subcritical regime (Re approximate to 2 x 10(3)).