  Turbulent-laminar intermittency, typically in the form of bands and spots, is
a ubiquitous feature of the route to turbulence in wall-bounded shear flows.
Here we study the idealised shear between stress-free boundaries driven by a
sinusoidal body force and demonstrate quantitative agreement between turbulence
in this flow and that found in the interior of plane Couette flow -- the region
excluding the boundary layers. Exploiting the absence of boundary layers, we
construct a model flow that uses only four Fourier modes in the shear direction
and yet robustly captures the range of spatiotemporal phenomena observed in
transition, from spot growth to turbulent bands and uniform turbulence. The
model substantially reduces the cost of simulating intermittent turbulent
structures while maintaining the essential physics and a direct connection to
the Navier-Stokes equations.
  We demonstrate the generic nature of this process by introducing stress-free
equivalent flows for plane Poiseuille and pipe flows which again capture the
turbulent-laminar structures seen in transition.
