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The first long-lived turbulent structures observable in planar shear flowstake the form of localized stripes, inclined with respect to the mean flowdirection. The dynamics of these stripes are central to transition, and recentstudies proposed an analogy to directed percolation where the stripes'proliferation is ultimately responsible for turbulence to become sustained. Inthe present study we focus on the internal stripe dynamics as well as on theeventual stripe expansion, and we compare the underlying mechanisms in pressureand shear driven planar flows, respectively plane-Poiseuille and plane-Couetteflow. Despite the similarities of the overall laminar-turbulence patterns, thestripe proliferation processes in the two cases are fundamentally different.Starting from the growth and sustenance of individual stripes, we find that inplane-Couette flow new streaks are created stochastically throughout the stripewhereas in plane-Poiseuille flow streak creation is deterministic and occurslocally at the downstream tip. Because of the up/downstream symmetry, Couettestripes, in contrast to Poiseuille stripes, have two weak and two stronglaminar turbulent interfaces. These differences in symmetry as well as ininternal growth give rise to two fundamentally different stripe splittingmechanisms. In plane-Poiseuille flow splitting is connected to the elongationalgrowth of the original stripe, and it results from a break-off / shedding ofthe stripe's tail. In plane-Couette flow splitting follows from a broadening ofthe original stripe and a division along the stripe into two slimmer stripes.

Dynamics and proliferation of turbulent stripes in plane-Poiseuille and plane-Couette flows