“Transition of transient channel flow”
Ever since the publication of the pioneering work by Osborne Reynolds in 1883 defining the concept of laminar and turbulent flows, the subject has remained a central theme of research in fluid mechanics due to its fundamental importance to the subject and its relevance to many engineering applications and the natural world. It is probably one of the most fundamental concepts that the flow in a pipe is laminar when the Reynolds number is below around 2300 and turbulent when the Re is higher. In this talk, we will present a radically new perspective of the laminar/turbulent flow and will show that the many turbulent-to-turbulent transient processes are in fact laminar-turbulent transition. We will present experimental evidence as well as DNS simulations to demonstrate that, following an increase of flow rate from an initially turbulent flow, the new flow will re-establish itself as a result of laminar-turbulent transition due to instability, rather than progressively evolving from the initial turbulent flow to a new one. The transient process is characterised by the appearance of elongated streamwise streaks followed by generation of turbulent spots which are typical of bypass transition processes. This new understanding leads to new interpretation of the statistics of transient flows and could have profound implications in the understanding and prediction of many unsteady flows encountered in practice, and lead to the development of new strategies for turbulence control, all of which are yet to be explored.

References
[1] S. He & M. Seddighi (2013) Turbulence in transient channel flow, Journal of Fluid Mechanics, 715, 60-102.
[2] S. He & M. Seddighi (2014) Transition of transient channel flow after a change in Reynolds number, Journal of Fluid Mechanics, in press.