Tricritical Directed Percolation Controls the Laminar-Turbulent Transition in Pipes with Body Forces

The laminar-turbulent transition in straight pipes is believed to occur through a continuous nonequilibrium phase transition in the directed percolation universality class. However, in curved pipes or in the presence of body forces it is possible to observe a discontinuous transition and other phenomenology which seem inconsistent with the emerging consensus. Here, we consider the perturbing effects of body forces and incorporate them into a minimal Landau theory of the transition. We calculate the phase diagram as a function of Reynolds number and body force strength, and show that above a threshold strength of the latter, there is a tricritical point which accounts for the observed discontinuity behavior, including spatially heterogeneous states. Our results are consistent with recent experiments in centrifugal pipes and direct numerical simulations of heated flows.