Turbulent flow around two interfering surface-mounted cubic obstacles in tandem arrangement

The flow around tow in-line surface-mounted cubes in a thin laminar boundary layer was experimentally investigated as a function of obstacle spacing for a Reynolds number of 22,000 based on approach velocity and cube height. Mean velocity measurements with Laser Doppler Velocimetry and surface flow pattern, obtained with an oil film technique, show that three distinct mean flow field structures exist based on obstacle spacing. Frequency spectra of velocity and surface pressure fluctuations reveal that these structures are related to three regimes of wake flow periodicity. For small spacing, the shear layer separating from the first cube reattaches on the sides of the second obstacle and wake periodically can only be detected in the wake of the downstream cube. For a critical spacing range, the fluctuations in the gap and wake lock-in. For larger spacings, a second horseshoe vortex appears at the windward base of the second cube. Observations using dye-injection and smoke-wire techniques are consistent these results.