Vortex shedding from a hemisphere in a turbulent boundary layer

Supercritical turbulent boundary layer flow over a hemisphere with a rough surface (Re = 150 000) has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen-Loeve expansion ("Proper Orthogonal Decomposition," POD). The time-dependent inflow condition is provided from a separate LES of a boundary layer developing behind a barrier fence and a set of vorticity generators. LES results using significantly different grid resolutions are compared with a corresponding wind tunnel experiment to demonstrate the reliability of the simulation The separation processes are analyzed by inspecting second-order moments, time spectra, and instantaneous velocity distributions. Applying POD, a detailed study of the spatiotemporal structure of the separation processes has been carried out. From this analysis it can be concluded that the major event in the separated flow behind the obstacle is the shedding of "von Karman"-type vortices, which can be represented by the first three energetically dominant modes.