Pipe flow clogged with turbulence
Résumé
The flow inside a circular pipe is known to support localised patches of turbulence travelling downstream. When the diameter of the pipe is slowly increasing, seemingly analogous but motionless patches can congest the flow and lead to disastrous energetic performance. Until now, no theory has predicted quantitatively the flow rates at which such a steady turbulent puff could be expected. The recent large-scale numerical study by Selvam etal. (J. Fluid Mech., vol. 771, 2015, R2) offers the first realistic simulation of this phenomenon, with a detailed analysis of the bifurcation sequence leading from the laminar flow to the localised turbulence regime. Similarities and differences between straight and divergent cases are also discussed.