Interaction of turbulent spots in pipe flow
The process of transition from laminar to turbulent regime in shear driven flows is still an unresolved issue. Localized turbulent regions or spots occur in pipe flow for Reynolds numbers around 2000. Typically in this regime an intermittent change between laminar and turbulent flow is observed (Wygnanski). Indeed, even if a large section of the laminar flow is uniformly perturbed localized turbulent spots emerge rather than an extended region of turbulence. A good understanding of this localization process is crucial for the comprehension of the transition to turbulence. We investigate the interaction of such turbulent spots in pipe flow for Reynolds numbers from 1900 to 2500. Turbulence is created locally by injecting a jet of water through a small hole in the pipe wall. For small perturbation frequencies the spacing of the turbulent spots downstream is inversely proportional to the frequency. It is observed that for distances less than approximately 20 pipe diameters turbulent spots start to interact and annihilate each other. The interaction distance is measured as a function of Reynolds number. We are also studying the effect of amplitude of the perturbations on the mutual interaction of the puffs. This investigation is closely related to spatially turbulent laminar periodic patterns which were earlier observed in other shear driven flows like Taylor-Couette or plane Couette (Prigent et al), (D. Barkley and L. Tuckerman).