Scalar flashes as memory carrier and turbulent spots composed of reverse and forward hairpins in pipe flow

Feasibility and accuracy of direct simulation of spatially-developing pipe transition from weakly perturbed laminar inflow was demonstrated in Wu et al (PNAS, 112, 7920-7924, 2015). Here, we study scalar flash, turbulent spot and transition statistics in a 500 radii-long pipe configuration at Reynolds number 6500.  During the late stage of transition, second-order statistics such as the rate of dissipation of turbulent kinetic energy exhibit substantial overshoot, which is accounted for by the observed stronger mid-to-high frequency content in the spectra of the rate of dissipation. Transitional turbulent spots are found to have a dual-type structural composition. The near-wall region consists of primarily reverse hairpin vortices with their head element directing towards the upstream direction and towards the wall. The core region of the spots on the other hand is populated by normal hairpin vortices. Passive scalar injected at the center of the inlet plane develops during transition into what we call the Type-1 and Type-2 flashes.  At several hundred radii downstream of transition where the flow is fully-developed and turbulent, the Type-2 scalar flashes serve as carriers of persistent memory of the far upstream transition.