Linking correlations and structure - Stochastic estimation and conditional averaging

The principles and applications of stochastic estimation are reviewed and the results of various investigations as they pertain to the structure of shear flow turbulence are presented. These results support a picture in which hairpin vortices and/or vortex rings dominate in the outer layer, whereas structures elongated in the streamwise direction and possessing vorticity in the streamwise direction dominate the wall layer. They may result from vortices moving along the wall, or the vortices may result from the interaction of the streamwise streaks with the wall and with themselves.