Computer simulations of the flow around a pair of two-dimensional, tandem circular cylinders in a flow, for Reynolds numbers in the range 1–40, are described. Cylinder surface-to-surface separations in the range $0.1 (D = cylinder diameter) were considered. The computed wake of a single cylinder at these low to moderate Reynolds numbers was in surprisingly good agreement with the laminar wake approximation, and a simple theory is presented to explain this agreement. With tandem cylinders, the drag on the downstream cylinder is a monotonic function of the cylinder separation. The laminar wake approximation can be used to explain reasonably well the variation in drag. The drag on the upstream cylinder is also a monotonic function of separation distance provided that the Reynolds number is less than about 10. For Reynolds numbers between 10 and 40, the upstream cylinder drag first decreases as separation increases up to a few diameters and then increases monotonically with separation distance.

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