An experimental study is carried out to investigate the unsteady aerodynamic loading on cylinders in tandem configuration in the sub-critical Reynolds number range. Experiments are performed using highly instrumented cylinders, with several static pressure taps and dynamic pressure transducers at different spanwise and peripheral locations. The effects of cylinders gap distance (L/D) on the static and dynamic surface pressure, coherence and turbulence length-scale have been investigated. The surface pressure results have shown a critical cylinders gap distances between L/D = 3 − 3.7, in which the flow patterns undergo an abrupt change in the reattachment and the co-vortex shedding regimes. For both cylinders, it is observed that the fundamental vortex shedding Strouhal number decreases with increasing L/D from 1.2 to 3, while for L/D > 3, the Strouhal number increases and approaches to that of the single cylinder model (St ≈ 0.2). The spanwise coherence results have also shown that the vortex shedding structures have a high energy level and a long spanwise length scale, while the three-dimensional flow structures within the boundary layer have a much lower energy level and shorter correlation length. The existence of the broadband content of the spanwise coherence in the wake and especially close to the base of the cylinder is prominent at smallest cylinders gap distance (L/D = 1.2). Furthermore, the spanwise coherence at the peripheral angles close to the gap region is found to be purely tonal in the case of the downstream cylinder.
