POD-based analysis of the onset of turbulence in pipe flow

The Openpipeflow Navier-Stokes solver (openpipeflow.org) was used to perform direct numerical simulations of a laminar-turbulent transitional flow in a pipe for Re=1,920. The onset of turbulence is manifested by a localized turbulent zone called "puff". Using the proper orthogonal decomposition (POD), we modify the various components of the velocity fluctuations to disrupt the self-sustaining mechanism of turbulence. Velocity components have been expanded into POD modes and several modes have been removed. The simulation resumes with the field reconstructed from the truncated POD modes. We have found that the turbulence was maintained even by the first two POD azimuth modes, which accounted for 17% of the overall azimuth energy. The complete elimination of the radial component did not prevent the onset of turbulence. We found that the removal of all longitudinal fluctuations, that is, the elimination of inflection points in the longitudinal velocity profile, but retaining only the first two azimuthal POD modes, also did not prevent the onset of turbulence. We believe that POD-based manipulation of velocity components can be valuable for understanding and managing laminar-turbulent transitional flow.