Wake dynamics of a cantilevered circular cylinder of aspect ratio 4

Highlights: • POD analysis carried out on velocity field data obtained in a cantilevered circular cylinder. • A new low frequency instability is identified for cantilevered cylinder flows of moderate aspect ratio. • The low frequency instability comprises a significant portion of the fluctuating energy in the wake. • The low frequency instability persists for laminar and turbulent vortex shedding conditions. • The instability is not observed if the incoming boundary layer is tripped, i.e, turbulent. - Abstract: Near-wake characteristics of a low aspect ratio ($h/d=4$) cantilevered circular cylinder protruding a thin laminar boundary layer were investigated both numerically ($Re=300$) and experimentally ($Re=10,400$). Despite the substantial differences in the investigated Re, the wake dynamics show striking similarities and appear governed by similar instability mechanisms: (i) a Kármán-like vortex shedding instability, and (ii) a low-frequency instability related to the flow over the free end and near the cylinder-wall junction. Attention is drawn to the low-frequency instability, which comprises a significant portion of the kinetic energy content in the wake, and has not been reported in previous experimental or numerical investigations. It appears to be characteristic of intermediate aspect ratio cantilevered circular geometries and the boundary layer state, since the phenomenon is not observed for turbulent boundary layers of similar thickness.