Author(s): Kyoungsik Chang; George Constantinescu; Seung-O Park
Linked Author(s): George Constantinescu
Keywords: Coherent Structures; Stratified flows; Large Eddy Simulation; Cavity flows; Mass Exchange
Abstract: The flow past a two-dimensional (2D) cavity situated in a channel is studied using Large Eddy Simulation (LES). The flow just upstream the cavity corresponds to a developing laminar boundary layer profile. First, 3D LES is used to investigate the main instabilities and coherent structures present in this flow. Then, the unsteady purging mechanism corresponding to ejection of a neutrally buoyant and of a dense miscible contaminant (two non-zero Richardson numbers are considered) introduced in the cavity at a certain time is studied using 2D LES. It is found that the mechanism of removal of the contaminant is very different between the buoyant and non-buoyant cases. In the non-buoyant case the contaminant is removed from the cavity mostly due to the interaction between the vortices that are shed at a regular (fundamental) frequency (St~0. 5) in the detached shear layer on top of the cavity with the main recirculation eddies inside the cavity. In the buoyant cases, after the initial stages of the mass exchange, the interfacial instabilities are playing a major role in the entrainment process. The main phenomenon is the presence of a strong internal wave which interacts with a recirculation region inside the cavity situated near the trailing edge in between the shear layer and the internal wave. The density variation across this internal wave region is strong. Through this interaction the denser fluid is extracted from the region beneath the internal wave where it is concentrated.