  Depending on the type of flow the transition to turbulence can take one of
two forms, either turbulence arises from a sequence of instabilities, or from
the spatial proliferation of transiently chaotic domains, a process analogous
to directed percolation. Both scenarios are inherently continuous and hence the
transformation from ordered laminar to fully turbulent fluid motion is only
accomplished gradually with flow speed. Here we show that these established
transition types do not account for the more general setting of shear flows
subject to body forces. By attenuating spatial coupling and energy transfer,
spatio-temporal intermittency is suppressed and with forcing amplitude the
transition becomes increasingly sharp and eventually discontinuous. We argue
that the suppression of the continuous range and the approach towards a first
order, discontinuous scenario applies to a wide range of situations where in
addition to shear, flows are subject to e.g. gravitational, centrifugal or
electromagnetic forces.
