Directed percolation transition to turbulence in pipe flow

Directed percolation (DP) has recently emerged as the potential explanation of the transition to turbulence in shear flows, yet direct evidence in terms of the corresponding universal critical exponents is scarce and has so far only been obtained for wall-driven (Couette-type) flows. Conversely, for common pressure driven flows such as pipe flow the excessive time scales at play prohibit direct observations. At the same time, the overall relevance of DP remains debated given that in previous cases it only appears to apply to a narrow parameter regime and falls short of explaining the complete transition process to fully turbulent flow. We here show that in weakly curved pipes the time scales relevant to transition dramatically reduce, finally enabling the measurement of the critical exponents for an open, pressure driven flow. Surprisingly, due to a fundamental change in turbulence interactions, the observed scaling range is not limited to a narrow initial stage but directed percolation describes the entire transition, from laminar to fully turbulent motion.