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Speed and structure of turbulent fronts in pipe flow
Published online by Cambridge University Press: 27 January 2017
Abstract
Using extensive direct numerical simulations, the dynamics of laminar–turbulent fronts in pipe flow is investigated for Reynolds numbers between $Re=2000$ and 5500. We here investigate the physical distinction between the fronts of weak and strong slugs both by analysing the turbulent kinetic energy budget and by comparing the downstream front motion to the advection speed of bulk turbulent structures. Our study shows that weak downstream fronts travel slower than turbulent structures in the bulk and correspond to decaying turbulence at the front. At $Re\simeq 2900$ the downstream front speed becomes faster than the advection speed, marking the onset of strong fronts. In contrast to weak fronts, turbulent eddies are generated at strong fronts by feeding on the downstream laminar flow. Our study also suggests that temporal fluctuations of production and dissipation at the downstream laminar–turbulent front drive the dynamical switches between the two types of front observed up to $Re\simeq 3200$.
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- © 2017 Cambridge University Press
References
Song et al. supplementary movie
Advection of vortices near a weak downstream front at Re = 2600. The flow is from left to right, therefore, the bulk of the slug is on the left and the laminar flow downstream is on the right. Vortices, visualised by using the ?_2 criterion, are shown in the frame of reference co-moving with the front.
Song et al. supplementary movie
Advection of vortices near a strong downstream front at Re = 5000. The flow is from left to right, therefore, the bulk of the slug is on the left and the laminar flow downstream is on the right. Vortices, visualised by using the ?_2 criterion, are shown in the frame of reference co-moving with the front.
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