The Structure of Large- and Very Large-Scale Motions in Turbulent Pipe Flow*

Multi-point velocity measurements in turbulent pipe flow have been performed at $Re_D=1.5 \times 10^5$. Using cross-spectral and Proper Orthogonal Decomposition analysis, information is elucidated on the structure of the large- and very-large scale motions in the outer layer of wall-bounded flows. The results indicate that the large-scale motions are composed of attached eddies in the logarithmic layer but are mostly composed of detached eddies in the outer layer with a wide range of azimuthal scales. The very-large-scale motions have large radial and azimuthal scales, are concentrated around a single azimuthal mode, and make a smaller angle with the wall compared to the large-scale motions. The results support a hypothesis that only the detached large-scale motions in the outer layer align to form the very-large-scale motions.