The Onset of Metastable Turbulence in Pipe Flow

The onset of turbulence in pipe flow has been a fundamental challenge in physics, applied mathematics, and engineering for over 140 years. To date, the precursor of this laminar-turbulent transition is recognized as transient turbulent spots or puffs, but their defining characteristics - longevity, abrupt relaminarization, and super-exponential lifetime scaling - have been lack of first-principles explanations. By combining extensive computer simulations, theory, and verifications with experimental data, we identify distinct puff relaminarizations separated by a critical Reynolds number, which are defined by a noisy saddle-node bifurcation derived from the Navier-Stokes equations. Below the critical number, the mean lifetime of puff follows a square-root scaling law, representing an intrinsically deterministic decay dominated by the critical slowing down. Above the critical value, the bifurcation's node branch creates a potential well stabilizing the turbulence, while the saddle branch mediates stochastic barrier-crossing events that drive memoryless decay - a hallmark of metastable states. Accordingly, the mean lifetimes are solved theoretically and can be fitted super-exponentially. By quantifying the deterministic and stochastic components in the kinetic energy equation, the lifetime statistics of puff are analyzed in a unified framework across low-to-moderate Reynolds number regimes, uncovering the mechanisms governing the transition to metastable turbulence in pipe flows.