Discontinuous transition to shear flow turbulence

The transition to turbulence in wall bounded shear flows is characterized by the co-existence of turbulent and laminar regions. As discussed in a variety of recent studies this transition type corresponds to a continuous phase transition and falls into the universality class of directed percolation. We will demonstrate here that the nature of this transition fundamentally changes in the presence of body forces. We present experimental and numerical studies of more complex situations such as vertically heated pipes, curved pipes and MHD channel flow. In all these situations, with increasing amplitude of the body force, spatio-temporal intermittency is suppressed, and eventually the familiar flow structures such as turbulent puffs, slugs or stripes, are fully suppressed. Instead of continuous the phase transition becomes discontinuous and hence the flow transitions directly from laminar to fully turbulent as the Reynolds number is increased. As shown the mechanism responsible for this drastic change is the suppression of spatial coupling and the elimination of energy transfer between laminar and turbulent regions.