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Robert J. Martinuzzi
Department of Mechanical and Manufacturing Engineering Schulich School of Engineering, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

Brian Havel
Advanced Fluid Mechanics Research Group Department of Mechanical and Materials Engineering Faculty of Engineering, University of Western Ontario London, Ontario, N6A 5B9, Canada


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 22000, 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 linearly with the obstacle spacing. In this lock-in 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 gap. The mechanism is different from that observed for two-dimensional geometries and helps explain why lock-in cannot be observed for square cylinders in tandem arrangement. The shedding event is accompanied by an abrupt increase in the random contribution to the turbulence field.