Slug genesis in cylindrical pipe flow

Abstract : Transition to uniform turbulence in cylindrical pipe flow occurs experimentally via the spatial expansion of isolated coherent structures called slugs, triggered by localized finite-amplitude disturbances. We study this process numerically by examining the preferred route in phase space through which a critical disturbance initiates a slug. This entails first identifying the relative attractor - edge state - on the laminar-turbulent boundary in a long pipe and then studying the dynamics along its low-dimensional unstable manifold, leading to the turbulent state. Even though the fully turbulent state delocalizes at Re ˜ 2300, the edge state is found to be localized over the range Re = 2000-6000, and progressively reduces in both energy and spatial extent as Re is increased. A key process in the genesis of a slug is found to be vortex shedding via a Kelvin-Helmholtz mechanism from wall-attached shear layers quickly formed at the edge state's upstream boundary. Whether these shedded vortices travel on average faster or slower downstream than the developing turbulence determines whether a puff or a slug (respectively) is formed. This observation suggests that slugs are out-of-equilibrium puffs which therefore do not co-exist with stable puffs. © 2010 Cambridge University Press.
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Y. Duguet, A.P. Willis, R.R. Kerswell. Slug genesis in cylindrical pipe flow. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2010, 663 (novembre), pp.180-208. ⟨10.1017/s0022112010003435⟩. ⟨hal-01021118⟩

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