Large-scale flow dynamics around two surface-mounted cubes in tandem

The wake of two surface-mounted cubes in tandem features complex flow regimes, which include intermittent reattachment at small spacing ratios L/D, changing to a cavity-locked regime and synchronized shedding at higher L/D. While the mean flow field has been widely studied, the dynamic processes responsible for these regimes require further investigation, motivating the present study. The dynamic flow field around two surface-mounted cubes in tandem with L/D=2, 2.5, and 4 was investigated based on large-eddy simulations of the flow for Re=1×104 and a turbulent ground plane boundary layer of thickness δ/D=0.8. A finite-impulse response filter-based spectral proper orthogonal decomposition was performed on the three-dimensional flow field, revealing an unsynchronized shedding regime with two co-existing modes for L/D=2. Mode A was characterized by vortex shedding from the downstream cube at a higher frequency, while mode B featured stronger but low-frequency shear layer oscillation from the upstream cube. A transition state was observed for L/D=2.5 and a fully synchronized shedding regime was observed for L/D=4. A low-frequency drift mode was identified for all cases, with two modes for L/D=4. The drift mode was found to trigger mode A for L/D=2, and it showed a correlation with the reverse flow region, downwash intensity, and fluctuating drag force on the cubes for all L/D. While the downstream cube wake did not change significantly with L/D, an alternate shedding of base-like vortices was found, distinguishing it from the wake of an isolated cube.