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Transition to turbulence – a new perspective on transient turbulent flow

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If you have a question about this talk, please contact Amélie Lamarquette.

We will present a new perspective of transient turbulent flow and show that in such flows, turbulence does not progressively evolve from one state to another. Instead, the flow is characterised by the development of a laminar boundary layer followed by transition to turbulence. The talk will begin with a review of DNS results, followed by discussing recent laboratory experiments of a flow accelerating from an initially turbulent state following the opening of a valve, together with LES of the experiments and extended Stokes first problem solutions for the early stages of the flow. As predicted by DNS simulations of the near-step flow acceleration, the flow in the experiment has also been found to respond to the acceleration in a manner that is closely analogous to a laminar flow accelerating from rest. In both cases, the primary consequence of the acceleration is the temporal growth of a boundary layer from the wall, gradually leading to a strong instability causing transition. In this interpretation, the initial turbulent fluctuations can be regarded as noise in an otherwise well-defined flow behaviour. We observe the spontaneous appearance of turbulent spots and discontinuities in the velocity signals in time and space, revealing rich detail of the transition process, including a striking contrast between streamwise and wall-normal fluctuating velocities. The work has recently been extended to three-dimensional transient flow.

This talk is part of the Fluids Group Seminar (CUED) series.

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