Abstract

The statistics of orientation and location of passive vectors in an unstable periodic flow of Couette system are investigated numerically with special attention to the correlation with the coherent structures in the flow. The flow domain is divided into many small cubic regions and the distribution of directions of many passive vectors in each cube is calculated at several temporal phases of the unstable periodic flow. It is found that in most of cubes the passive vectors align either in a single direction (linear region) or on a plane (planar region). The unstable periodic flow exhibits the characteristic features of Couette turbulence having the streamwise vortices and the low-speed streaks. The linear regions are observed in streamwise vortices and low-speed streaks (or the ejection region near the wall), the planar regions in the periphery of vortices and in the sweep region near the wall. The dispersion in direction of passive vectors depends on their near-past history over the characteristic times of the unstable periodic flow.