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.
