Many transitional shear flows show a regime of complex spatiotemporal dynamics that has been linked to directed percolation. Using direct numerical simulations of plane-Poiseuille flow, we identify a state of sparse oblique turbulent bands that exists in a Reynolds number range between 660 and 800, below the anticipated directed percolation transition. The localized turbulent bands will increase steadily in length in sufficiently large domains, or will sustain themselves in a cycle of band extension and breakup in periodic domains. The spreading process is not sensitive to random perturbations, and the width and tilt angle of the band are statistically invariant during the extension. The results show that at least in the case of plane-Poiseuille flow a possible transition in the directed percolation class is preceded by the formation of robust oblique turbulent bands.

• Received 23 August 2017

DOI:https://doi.org/10.1103/PhysRevFluids.3.011902