PROPER ORTHOGONAL DECOMPOSITION ANALYSIS OF A PUFF IN A PIPE

We have performed direct numerical simulations of transitional turbulence in pipe flow for Re=2,250. The results confirm the existence of a spatio-temporal intermittency when turbulence is localized in a puff convected downstream. To analyze the turbulence, we follow a turbulent puff by a 3D moving-window centered at the location of the maximum total energy of transverse (turbulent) motion. The flow field data collected over 6,000 time instances (snapshots) have been analyzed by Proper Orthogonal Decomposition (POD) and used for identifying vortical structures. The presence of large-scale structures in a puff has been found by time-averaging of the cross-sectional turbulent velocity field and confirmed by POD analysis and by applying the Q- and λ2-criteria.