Characterizing differences between surface-mounted square and circular cylinder wakes protruding thin laminar boundary layers.

A comparative experimental study is conducted for the flow around a cantilevered circular and a square cylinder protruding thin laminar boundary layers at a Reynolds number of 10500. Comparison to cases with thin turbulent boundary layers shows differences in the turbulent wake structure and dynamics. Unlike for turbulent boundary layers, spectra of velocity and surface pressure fluctuations show the existence of a low-frequency instability in addition to the periodicity associated with Karman-like shedding. The low-frequency fluctuations on opposing obstacle sides are in-phase and contribute to symmetric POD modes. The instability is only observed on and near the obstacles. The low-frequency spectral signature differ for the two geometries. Interactions between the low-frequency instability and shedding process only occur for the square cylinder. Auxiliary measurements suggest that the instability’s existence depends on the boundary layer state, but is not directly related to the instability of the horseshoe vortex system. In addition to the dynamics, the topologies of the velocity field and surface flow patterns also indicate that the circular and square geometries give rise to fundamentally distinct wakes.