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How turbulence arises in simple shear flows, such as pipes and channels has been an open question for over a century. In these flows turbulence is found despite the linear stability of the laminar flow and transition is caused by finite amplitude perturbations. Despite numerous experimental and theoretical studies it has not been possible to determine a well defined critical point nor to clarify the nature of the transition. It will be shown for the examples of pipe and Couette flow that the onset of sustained turbulence is a nonequilibrium phase transition. The critical point is determined by resolving the extremely long time scales of the underlying growth and decay processes. By detailed numerical simulations and experimental measurements close to the transition point we also determine the critical exponents and show that this transition falls into the directed percolation universality class.

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James D. Bjorken, Stanford University
HS III
Contact: not specified