The effects of tunnel blockage and aspect ratio on the mean flow past a circular cylinder with Reynolds numbers between 10 4 and 10 5

Abstract

Results are presented of a series of experiments which were carried out with great care to produce accurate, comparable results in which the relatively small changes in the flow past a circular cylinder associated with varying blockage ratios can be clearly distinguished. The experiments cover a range of blockage from 2 to 16% and of aspect ratios from 4 to 10. End plates were fitted to the cylinders in all experiments.For blockage ratios less than 6%, it is shown that the effects of blockage on pressure distribution and the drag coefficient are small and that the Strouhal number is unaffected by blockage. For blockage ratios in the range 6–16%, there is considerable distortion of the flow due to blockage and the effects are complex. The pressure distribution is of a different form and the Strouhal number changes. Hoevever, conflicting influences result in a blocked drag coefficient which is not very different from that at no blockage. Reduction in aspect ratio has effects on drag coefficient and on base pressure coefficient which are similar to those associated with increase in blockage ratio.It is concluded that blockage correction procedures based on the momentum method and on the image method are unsatisfactory in their prediction of the unblocked drag coefficient but the momentum method predicts the unblocked base pressure coefficient quite well.