Large scale coherent structures in a turbulent boundary layer interacting with a cylinder wake

A turbulent boundary layer interacting with the wake of a circular cylinder, which lies perpendicular to the mean flow direction and parallel to the wall, is investigated using a 'coherent structure' type of approach. This flow configuration is of interest in terms of both heat transfer and skin friction control. In particular, the concurrent effects of increased wall heat transfer and decreased skin friction, which are brought about by the cylinder, do not satisfy the Reynolds analogy, contrary to the effects of most outer layer manipulators. In seeking to identify the physical mechanism behind the observed dissimilarity between heat and momentum transfer, information on the topology and dynamics of the large scale coherent structures is a useful complement to previous experimental results, which have been based largely on single-point measurements. Such information is obtained by applying a number of coherent structure identification techniques to multi-point hot-wire rake measurements acquired in the near and intermediate wake of the cylindrical manipulator. A conditional averaging technique yields a description of the average or dominant large scale structures, while insight into the instantaneous flow organization is provided by the use of two different turbulence filters. The relative merits of each technique are considered, followed by an analysis of the extracted coherent structures and of their contribution to the flow dynamics. It is shown that the large scale coherent structures play a significant role in the observed dissimilarity between heat and momentum transfer in the disturbed boundary layer.