Transition without puffs and slugs in curved pipes

Spatio-temporal intermittency, i.e. the coexistence of laminar and turbulent domains, is one of the central characteristics of the transition to turbulence in pipe and other shear flows. As shown in recent studies this transition is continuous, i.e. with decreasing Reynolds number the fraction of the flow that is turbulent decreases gradually and reaches zero at a critical point. In this talk we present experiments in curved pipes where for certain parameter regimes spatio-temporal intermittency is shown to be marginalized. That is, with decreasing Reynolds number the flow transitions from fully turbulent to laminar and the transition appears to become discontinuous. As we show localized turbulent structures such as puffs and slugs are unstable and below critical they shrink in size and the flow relaminarizes, whereas above critical they continue to expand until the flow is turbulent throughout.