Vortex shedding from two surface-mounted cubes in tandem

Periodic vortex shedding from two surface-mounted cubes, of height H, in tandem arrangement placed in a thin boundary layer is investigated for a spacing 2H using phase-averaged Laser Doppler Velocimetry. Tests were conducted for a Reynolds number of 22,000, based on H and the freestream velocity, and an approximately 0.07H thick laminar boundary layer. For obstacle separations between 1.5H and 2.5H, the shedding frequency scales inversely with the obstacle spacing, S, such that the Strouhal number based on S is constant or geometrically locked. In this locked regime, periodic shedding is triggered by the interference between a vertical flow stream along the front face of the downstream obstacle and the vortex in the inter-obstacle cavity. This three-dimensional mechanism is not observed for two-dimensional geometries and helps explain why a locked regime cannot be observed for square cylinders in tandem arrangement. Furthermore, it is shown that the structure of the turbulent field in the cavity region differs significantly from that in the base region of a two-dimensional obstacle.