Vortex Pairs in Transitional Poiseuille Flow

Vortices play a central role in organizing fluid motion, and their geometry and dynamics often reflect the underlying stability and energetics of the surrounding flow. In particular, streamwise vortices in generated during the transition from laminar to turbulent flow have been identified as precursors to turbulence in pipe flow, making them an important target for experimental study. Using a particle tracking velocimetry and a novel dye injection technique with a scanning volumetric imaging system, we track both the centerlines of induced streamwise vortex structures and the surrounding three-dimensional velocity field. From these measurements, we observe advection of shear away from the walls and towards the center of the pipe. Depending on the flow configuration and Reynolds number, the advected shear may undergo a Kelvin-Helmholtz type instability, leading to turbulence. This work represents a mechanistic framework for the well-documented "streak" instability that can precede transition to self-sustaining turbulence in pipe flow.