From transient to fully turbulent shear flows

The classical picture of the transition to turbulence in fluid flows is that of successive instabilities where starting from a stationary basic state complexity arises via a sequence of bifurcations. In contrast, shear flows undergo a sudden and direct transition from laminar to turbulent motion as the driving velocity increases. In this talk we examine the richness of this transition in pipe flow. We show that turbulence, which is transient at low Reynolds numbers, becomes sustained at a distinct critical point. The turbulent state emerging through this phase transition is a large-scale pattern consisting of localized chaotic clusters that may relaminarize, merge, nucleate new ones or annihilate each other. By further increasing the Reynolds number, we investigate the onset of fully turbulent flow. Our data show that surprisingly even at relatively high Reynolds numbers laminar islands continue to appear in an otherwise turbulent flow. The observed scaling behavior infers that a state of fully turbulent flow is only reached in the asymptotic Reynolds number limit.

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