Wall-attached and wall-detached motions in wall-bounded turbulent flows*

The Townsend's attached eddies hypothesis (AEH) has received a lot of attentions in recent years. While wall-attached eddies are responsible for a number of important processes in wall-bounded turbulence, the detailed statistical behavior and many flow quantities can not be explained using only the AEH and only wall-attached eddies. A decomposition of the flow to wall-attached and wall-detached motions is useful in this context. This study presents a decomposition methodology, where inner motions are separated from outer motions. Wall-attached motions are "small-scale" outer motions, whose sizes are bounded by the local distance from the wall and the boundary layer height. The "large-scale" outer motions, on the other hand, are wall-detached motions, whose statistical behaviors are not modeled by the AEH. The decomposition methodology is used for channel, boundary layer and atmospheric surface layer flows. While there is no general theory for the outer, wall-detached motions, the statistical behaviors of wall-attached motions are very well captured by the AEH.