The present work investigates the structure associated with flow past two tandem circular cylinders with a diameter ratio of 0·33. The smaller cylinder is always upstream. The two parameters varied in the present work are the spacing between the cylinders denoted by l/d2where l is the centre to centre spacing and d2 is the diameter of the downstream cylinder, and the Reynolds number Re based on d2 Just like the case of flow past two equal size cylinders, a critical spacing was found to exist. For spacing less than the critical value, shear layers that separate from the upstream cylinder reattach onto the downstream cylinder (reattached flow regime) whereas for spacing larger than the critical value both cylinders shed vortices (co-shedding flow regime). For diameter ratio dl/d2 of 0·33 and at Re ≈ 6·27 × 104, the critical spacing is at l/d2 ≈ 1·8 to 2·2. In 1·8 ≤ l/d2 ≤ 2·2, bi-stable flow situation where the flow structure changes intermittently between the reattached and the co-shedding types was observed and the probability that the flow is co-shedding within the bi-stable regime is found to follow a normal distribution.It was also found that the reattached flow regime can be sub-divided into two sub-regimes. In 0·66 ≤ l/d2 ≤ 1·2 (sub-regime 1), the vortex formation length, lf, of vortices shed by the downstream cylinder appears to be shorter, resulting in larger drag force on the downstream cylinder and larger r.m.s. pressure around it. In 1·4 ≤ l/d2 ≤ 1·18 (subregime 2), lf appears to be larger which results in a smaller drag force and smaller r.m.s. pressure distribution when compared with sub-regime 1. Only the downstream cylinder has a Strouhal number S2 associated with it in the reattached flow regime. The general trend is that of a slight reduction in S2 with increasing l/d2 which is consistent with the relation of larger lf at larger l/d2 in the reattached flow regime. In the coshedding regime, the Strouhal numbers of both cylinders are similar.

The Reynolds number was varied in the range 3·15 × 104 ≤ Re ≤ 8·81 × 104. It was observed that both the mean drag and the r.m.s. lift and drag forces reduce in magnitude when Re was increased. From the spectra of the lift force, reduction in C'L was found to be due to a reduction in the regularity of the vortex shedding process. From the pressure distribution of the downstream cylinder at constant l/d2, it was found that the reduction in mean drag with increasing Re is caused by an increase in base pressure which in turn is likely to be the consequence of an increase in vortex formation length. The upper and lower limits of the range in the lldi at which bistable flow was detected were found to decrease with increasing Re. At l/d2 = 2·0, the flow was of the reattached type at Re = 3·15 × 104 and 4·75 × 104 but became bi-stable at Re ≥ 6·27 × 104.