  Turbulent flow profiles are known to change between low- (LDR) and
high-extent drag reduction (HDR) regimes. It is however not until recently that
the LDR-HDR transition is recognized as a fundamental change between two DR
mechanisms. Although the onset of DR, which initiates the LDR stage, is
explainable by a general argument of polymers suppressing vortices, the
occurrence of HDR where flow statistics are qualitatively different and DR
effects are observed across a much broader range of wall regions remains
unexplained. Recent development of the vortex axis tracking by iterative
propagation algorithm allows the detection and extraction of vortex axis-lines
with various orientations and curvatures. This new tool is used in this study
to analyze the vortex conformation and dynamics across the LDR-HDR transition.
Polymer effects are shown to concentrate on vortices that are partially or
completely attached to the wall. At LDR, this effect is an across-the-board
weakening of vortices which lowers their intensity without shifting their
distribution patterns. At HDR, polymers start to suppress the lift-up of
streamwise vortices in the buffer layer and prevent their downstream heads from
rising into the log-law layer and forming hairpins and other curved vortices.
This interrupts the turbulent momentum transfer between the buffer and log-law
layers, which offers a clear pathway for explaining the distinct mean flow
profiles at HDR. The study depicts the first clear physical picture regarding
the changing vortex dynamics between LDR and HDR, which is based on direct
evidence from objective statistical analysis of vortex conformation and
distribution.
