Low-frequency dynamics in the turbulent wake of cantilevered square and circular cylinders protruding a thin laminar boundary layer

A comparative experimental study is conducted of the flow around a cantilevered circular and square cylinder protruding a thin laminar boundary layer at a nominal Reynolds number of 10500. Spectra of the velocity and surface-pressure fluctuations indicate the existence of a low-frequency periodic instability in addition to the classical Karman-like shedding. The low-frequency fluctuations on opposing sides of the obstacles are in-phase and contribute to symmetric spatial POD modes. The instability is only observed on and in the vicinity of the obstacles. The spectral signature of this instability differs for the two geometries and evidence of interactions with the vortex shedding process is only observed for the square cylinder. Auxiliary measurements suggest that the existence of this instability depends on the boundary layer state, but is not directly related to the instability of the horseshoe vortex system. Differences in the observed dynamics and topological features between the two flows suggest that these are fundamentally distinct.Graphical abstract [GRAPHICS] .