Biological & Soft Matter Seminar: Intermittent turbulence in pipe flow: excitation, self-replication and gap formation

Abstarct:

Flows in fluid dynamics often follow a familiar route to chaos upon the increase of a control parameter: starting from an instability, a sequence of bifurcations leads to chaos and eventually to turbulence – a chaotic flow involving many length and time scales. Not so in pipe flow, where turbulence appears suddenly upon the increase of the flow rate, and where it is initially interspersed with smooth, regular (laminar) flow. Explaining the transition to turbulence in pipe flow (and similar systems) has therefore been a challenge since 1883, going back to Osborne Reynolds.  I will discuss the modern understanding of this transition and the different phases of turbulent pipe flow. From a regime with localized turbulent pulses, which are similar to action potentials in a neuron, to a regime of extended turbulence with localized laminar gaps, which are mirror images of the turbulent pulses, and finally fully turbulent flow. I will describe how the transition to sustained turbulence occurs within the first regime; following an out-of-equilibrium phase transition due to the self-replication and decay of localized turbulent patches.