Hydrodynamic coefficients of two stationary tandem cylinders at varying spacing ratios and Reynolds numbers

This experimental study investigates the hydrodynamic characteristics of flow past two stationary tandem cylinders in a large-scale towing tank. The experiments cover spacing ratios from G/D=1.5 to 4.0 and Reynolds numbers (Re) ranging from the subcritical to the early critical regime (2.65×104–5.29×105). Separate force measurements on the upstream and downstream cylinders are combined with system-level quantities, including total drag, differential drag, and torsional moment, to clarify how G/D and Re jointly affect the force responses. This dataset addresses the limited availability of force measurements for tandem cylinders at high Reynolds numbers, where spacing ratio and Reynolds number are varied together while individual-cylinder and system-level loads are reported in the same experiment. The results show that the upstream cylinder exhibits a hydrodynamic trend with respect to Re similar to that of an isolated cylinder. In the transition in boundary layer 1 (TrBL1) regime, steady lift forces develop on both cylinders with opposite directions, and the mean lift forces on the downstream cylinder vanish when G/D<2.0. The torsional moment weakens in the critical regime. The onset of the TrBL0 regime for the tandem configuration is delayed relative to that of an isolated cylinder, and this delay becomes more pronounced as G/D decreases. Within the tested spacings, an abrupt transition between G/D = 3.0 and 4.0 is observed in the subcritical regime through changes in hydrodynamic coefficients and shedding-frequency characteristics, whereas this spacing sensitivity weakens as the early critical regime is approached.