Laborhalle 3D - Foyer

Prof. Jorge Peixinho, University of Le Havre, F

Wed, Nov. 19th 2014, 3:30 pm

Laminar-turbulent transition in the flow through a gradual expansion in a circular pipe

Abstract

We present the results of a combined experimental and numerical investigation of the transition to turbulence in a slowly diverging pipe, i.e. a gradual expansion in a circular pipe. The science of sudden expansion flow has been studied and indicate a very different transition scenario from circular pipe flow. Initially, the inlet velocity profile is parabolic along with the formation of axisymmetric recirculation region in the divergent section. There is a critical velocity for the existence of the recirculation. When the flow increases further, a subcritical transition for localised turbulence arises. The experiments describe the growth of the turbulent patch. These turbulent patches are reminiscent of turbulent puffs in uniform pipe flow suggesting that the observed patches may contain solutions similar to those in pipe flow. In order to quantify further the localised turbulence regime, relaminarisation experiments were performed. The turbulent patches are generated, then the flow velocity is reduced and the decay to laminar flow is monitored. We observe a linear increase of the relaminarisation time. This will be discussed and reanalyzed. Moreover, a direct numerical simulation is carried out to study the onset of localised turbulence and the amplitude of perturbation required to trigger them.