Evolution of Vortex Lines in Transitional Pipe Flow

Concentrated vortex lines spontaneously arise in turbulent flows, acting as organizing features of the flow. In order to study the interactions between vortices and background flows in a controlled environment, we have built an experimental setup to generate concentrated streamwise vortices in pipe flow at for Reynolds numbers below 2000. We image the resulting flows in 3D using a combination of fluorescently dyed vortex cores and randomly distributed tracer particles. This allows for simultaneous reconstruction of the velocity field and a high fidelity tracking of the vortex shape. These experiments demonstrate a sharp transition in the vortex line behavior at Re ~ 1000, well below the full turbulent transition. The goal of this project is to shed new light on how vortices interact with walls and background flows, quantifying these interactions in terms of energy, helicity, and enstrophy.