The structure of large- and very large-scale motions in turbulent pipe flow

Multi-point velocity measurements have been performed in turbulent pipe flow at Re_D = 1.5 × 10⁵ and combined with cross-spectral and Proper Orthogonal Decomposition analysis to elucidate information on the structure of the large- and very-large scale motions in the outer layer of wall-bounded flows. The results indicate that in the outer layer the largescale motions may be composed of detached eddies with a wide range of azimuthal scales whereas in the logarithmic layer they are attached. The very-large-scale motions have large radial 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.