High-resolution simulation of flow separation from curved three-dimensional surfaces

The accurate prediction of turbulent flows separating from curved and otherwise continuous surfaces is one of the most difficult tasks for any computational framework. Among several difficulties, two major ones are the unsteady, spatially varying separation region – usually involving patches of alternately separated and attached flow and the need to resolve accurately the very thin near-wall layer that contributes greatly to the details of the separation process. The flow over a three-dimensional, hill-shaped obstacle in a duct contains most generic features encountered in almost all types of three-dimensional separation from continuous, curved surfaces. If the hill is relatively tall, as is the case herein, a closed (time-mean) recirculation bubble arises behind the hill. Moreover, the flow gives rise a variety of complex topological features, such as curved detachment and attachment lines and nodes, focal points and saddles. Large vortical structures are intermittently or periodically shed from the surfaces, resulting in complex wake structure downstream of the obstacles. Such flows are also observed to be sensitive to turbulence in the upstream flow, especially when separation is preceded by a thick turbulent boundary layer, and in the free stream above the boundary layer. All these features pose major challenges to both statistical modelling and time-resolved simulation.