| The flow around two interfering surface-mounted cubes of height, H, in a thin boundary layer was experimentally and numerically investigated as a function of the obstacle spacing, S, at a Reynolds number of Re H = 22,000 in a thin boundary layer. The mutual aerodynamic interference between the two surface-mounted cubes is described in terms of changes in vortex shedding frequency and in terms of changes in the mean and turbulent field structures. Mean, instantaneous and phase-averaged velocity data from Laser Doppler Velocimetry (LDV) and surface pressure data were analysed to characterize three flow regimes. The studies were complemented by surface pattern and laser-sheet visualizations. The results were compared to the two-dimensional counterpart of this flow. Based on vortex shedding behaviour, it had been established that four distinct flow regimes exist: "one-body", bi-stable, lock-in, and quasi-isolated. For short spacings S/H<1.5, the shear layer separating at the leading edge of the upstream obstacle reattaches on the sides of the second obstacle giving rise to two intermittent modes of oscillations. For this mechanism to occur it is suggested that sufficient circulation must freely enter the gap, which is not the case for the 2D geometry. To broaden the understanding of the lock-in regime, 1.5< S/H<2.3, detailed LES simulations were conducted. The LES predicted the Strouhal number correctly, as well as the macroscopic and turbulent features of the flow field. It had been shown using phase-averaging that the vortex shedding is not like the traditional Karman type of mechanism. Instead the top shear layer from the first cube interferes with the vortex formation and triggers shedding. The result is a streamwise shed vortex that can be traced into the wake of the second obstacle on its opposite side. Using the LDV measurements and LES simulations it has been shown that an arch-shaped vortex dominates the inter-obstacle cavity.;Keywords. Vortex shedding, two cubes, Laser Doppler Velocimetry, LDV, lock-in, horseshoe vortex, triple-decomposition, turbulence, aerodynamics, streamwise vortex, phase-averaging, flow visualization, flow separation, tandem cylinders. |
