Subcritical transition to turbulence in diverging pipe flows

Transition from laminar to turbulent regime is accompanied by a large change in flow related processes such as mixing, heat transfer and drag friction that increase dramatically. This work tries to addresses the fundamental question of the transition instability in diverging pipe flows of an ultimate active control strategy focusing on the fundamental understanding of the flow physics.

From a theoretical point of view, the classical linear stability theory predicts large critical speeds for transition, whereas experiments indicate the occurrence of transition at lower flow rates. The current work intends to clarify this scientific ambiguity and to shade more lights into the transition mechanisms and their consequences on critical flows.

Therefore, the proposed work uses direct numerical simulations (DNS) approach and compare the results with on-going well-designed experiments to better understand the effects of flow and geometrical parameters such as inlet imperfection and diverging angle on the transition to turbulence for such simple yet important geometries. In its extreme limit and in accordance with the experimental observation, we found a new transition mechanism as well as its dependency to wall imperfection.