The evolution of large scale motions in turbulent pipe flow

A dual-plane snapshot POD analysis of turbulent pipe flow at a Reynolds number of 104,000 is presented. The high-speed PIV data were simultaneously acquired in two planes, a crossstream plane (2D-3C) and a streamwise plane (2D-2C) on the pipe centerline. The cross-stream plane analysis revealed large structures with a spatio-temporal extent of 1-2R, where R is the pipe radius. The temporal evolution of these large-scale structures is examined using the timeshifted correlation of the cross-stream snapshot POD coefficients, identifying the low energy intermediate modes responsible for the transition between the large-scale modes. By conditionallyaveraging based on the occurrence/intensity of a given cross-stream snapshot POD mode, a complex structure consisting of wall-attached and detached large-scale structures is shown to be associated with the most energetic modes. There is a pseudo-alignment of these large structures, that together create structures with a spatio-temporal extent of about 6R, which appears to explain the formation of the very large scale motions previously observed in pipe flow.