PIV-based characterization of puffs in transitional pipe flow

High frame rate stereo-PIV was conducted on circular cross sections of pipe flow to examine velocity variations inside turbulent puffs. A registration method was developed to recognize strong ejections of fluid from the wall near a puff’s trailing edge, and the method was employed to determine ensemble averages of multiple puff occurrences. Both individual puff reconstructions and the ensemble average revealed that these ejections were accompanied consistently by a hairpin vortex that was, in turn, frequently part of a streamwise-aligned hairpin sequence. This sequence is associated with a region that starts near the wall one diameter upstream of the puff’s trailing edge and spreads to the center of the pipe at the trailing edge before shrinking back towards the wall over the next three diameters downstream. The sequence is accompanied by additional hairpins and disturbances that extend over the pipe cross section already by the trailing-edge location. The puff data were used also to search for azimuthal modal patterns within streamwise velocity variations. These modal patterns were shown to be a robust feature of experimental puffs. Upstream patterns were typically disrupted by the trailing edge ejection and hairpin, resulting in downstream patterns of different mode and character. On average, modal patterns existed over longer distances than those identified previously in numerical simulations.