It was found that just like the case of flow past two tandem cylinders of equal diameter, a bi-stable flow situation where the flow switches intermittently between the reattached type and the co-shedding type also exists when the two tandem cylinders have different diameters. The diameter ratio of the cylinders used in the present investigation is 0.33 with the smaller cylinder upstream. It was also found that when the flow is bi-stable, the probability that the flow is of the co-shedding type Pr varies with the centre to centre spacing between the cylinders 1/d 2 from 0 to 1 over the bi-stable range and their relation is governed by a normal distribution. The range of the bi-stable flow in 1/d 2 is Reynolds number dependent. When Reynolds number increases, both the lower and upper limits of the bi-stable flow range reduce.
Problems associated with flow past a single or multiple cylinder(s) had in the past few decades attracted the interest of many fluid mechanists, probably because of their simple geometry (though complicated flow field) and frequent appearance and application in the engineering environment. Heat exchangers, over head transmission cables and offshore platforms are some of the examples that immediately come to one's mind. Most of the researchers who studied the flow associated with two cylinders chose to look at the special case of two cylinders that have the same diameter, again probably because of the simplicity of the geometry. In real engineering environment, however, cylindrical objects are not necessarily of the same diameter and problems related to flow past two or more cylinders that have different diameters are frequently encountered. Some of the research findings for the case of two identical cylinders are summarised in Zdravkovich (1977).
