Structure identification in pipe flow using proper orthogonal decomposition.

The energetic motions in direct numerical simulations of turbulent pipe flow at Re=685 are investigated using proper orthogonal decomposition. The procedure is extended such that a pressure component is identified in addition to the three-component velocity field for each mode. The pressure component of the modes is shown to align with the streamwise velocity component associated with the large-scale motions, where positive pressure coincides with positive streamwise velocity, and vice versa. The streamwise evolution of structures is then visualized using a conditional mode, which exhibit a strong similarity to the large-scale, low-momentum motions. A low-pressure region is present in the downstream section of the structure, and a high-pressure region is present in the upstream section.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.