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

Abstract 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 Karman 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 .