Investigation of the cylinder separated shear-layer physics by large-eddy simulation

The transition process to turbulence occurring within the separated shear layers of a circular cylinder is investigated by the large-eddy simulation methodology. The Reynolds number (Re=8000) is sub-critical, meaning that upstream separation is laminar. In this study, we desire to improve our understanding of the shear-layer properties as well as assess the capability of the solution methodology to accurately resolve the fundamental characteristics. In the computation, the dynamic eddy-viscosity model is implemented to handle the turbulent scales cut off by the grid-filter. However, the grid-scale level within the shear layer resolves the majority of turbulent scales of interest. The governing equations are re-cast into a curvilinear coordinate framework to accommodate a non-orthogonal grid comprised of line clustering near the cylinder periphery and within the shear-layer region. Two fundamental frequencies persist throughout the entire transition process; one identifying von Kármán shedding and the other denoting the Bloor “transition wave”. Only two other mixed modes are clearly discernible. Transition begins approximately 1/4 diameters from separation and concludes about one diameter further downstream. All the characteristic trends of the shear layer, in terms of their growth rate and dependence on Re, that were established by M.F. Unal, D. Rockwell [J. Fluid Mech. 190 (1988) 491] have been verified by the present simulation to the higher Re.