Wake interference effects for two surface-mounted cubes in tandem

Interference effects on the mean flow field, aerodynamic forces, and vortex shedding in the wakes of two cubes in tandem have been experimentally investigated, considering different boundary layer thicknesses of delta/D=0.8 and 1.3. The cubes had center-to-center longitudinal spacing ratios of L/D=1.5 to 5 and were exposed to an incoming flow with a Reynolds number of Re =7.5x10(4)- 9x10(4) based on the cubes' width D and the freestream velocity. For the thin boundary layer, the flow showed an intermittent regime with two Strouhal numbers for L/D<2.25, a co-shedding regime with synchronized shedding for L/D>2.25, and evidence of a cavity-locked regime around L/D=2.75. The intermittent regime featured flow reattachment but was not bistable, corresponding instead to unsynchronized shedding from both the upstream and downstream cubes. The mean drag force, normal force, and bending moment coefficients were examined across the regimes as functions of L/D and delta/D, showing greater variation for the downstream cube. Particle image velocimetry measurements revealed that the probability of reattachment on the downstream cube decreased, a second horseshoe vortex formed in the gap, and flow separated from the downstream cube's leading edges for high L/D. When increasing delta/D, vortex shedding from the upstream cube was delayed, leading to a high Strouhal number single-wake regime for L/D<2.5. The shorter formation region caused fluid to enter the gap, giving higher drag force coefficients for the downstream cube for small L/D, while the decreased probability of reattachment on the top of the cube led to a small increase in its normal force coefficient.