Statistical model and universality class for interacting puffs in transitional turbulence

To understand the universality class of the laminar-turbulent transition in pipe flow, we measure how turbulent regions known as puffs proliferate and interact. Because of the huge length and time scales required to reach the critical regime, we model the dynamics from the effective forces we measure in experiments. Away from the transition, we find a crystal-like pattern with spatio-temporal intermittency. Renormalization group analysis and numerical simulations of both discrete and continuous models of puff interactions show that interactions are irrelevant at the 1+1-dimensional directed percolation fixed point but several finite-size artifacts mask the critical scaling regime in practice.