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Title: When is the onset of shear-flow turbulence discontinuous?
Plain: Adding forces such as buoyancy, pipe curvature or magnetic fields to a pipe or channel flow can turn the gradual onset of turbulence into an abrupt jump with hysteresis, like boiling. No rule predicts which flows behave which way.
Precise: In wall-bounded shear flows with body forces (curved, heated or magnetohydrodynamic pipes and channels) or with suppressed large-scale flow, the turbulent fraction can jump discontinuously to zero, with metastable turbulence (Yang et al., Nature Physics 2026; Gome et al., PRL 2024, plane Couette flow). Find a criterion, in measurable quantities such as the kinetic-energy advection across laminar-turbulent interfaces or the fluctuation intensity in a one-field stochastic front model, that predicts whether the transition is continuous (directed percolation) or first order, and locate the tricritical point if one exists.
Settled by: A parametric study in one flow family, such as a pipe with tunable body force, mapping the order of the transition against the proposed control parameter, including the tricritical point.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: open
Status note: The 2024 and 2026 works propose different controlling quantities (fluctuation intensity versus interfacial energy flux).
Posed since: 2024