Speaker
Description
Most flows of industrial and technical relevance are turbulent. Turbulent flows typically exhibit larger viscous drag compared to an equivalent laminar flow. This fact prompted numerous efforts in reducing such excess drag – the so-called turbulent drag – during the last decades, enticed by potentially huge economic and environmental benefits. The challenges range from discovering efficient means of reducing turbulent drag, through finding practical ways of implementing them, to bridging the gap between the simple fundamental flows necessarily considered in research and the complex flows occurring in real applications.
Particularly this last point has been hindering the development of turbulent drag reduction techniques. For instance, there is active debate on whether the estimates for drag reduction obtained experimentally or numerically at typically relatively low Reynolds number can be extrapolated to the larger Reynolds numbers of most applications. Similarly, most research investigations so far considered incompressible flows, and flows in which friction is the only source of drag, thereby neglecting compressibility effects, or the interaction with other possible sources of drag.
In this contribution, I address some of the open questions mentioned above, finding the answer to some, and highlighting where further efforts are still required. The active drag reduction strategy of spanwise wall forcing, as well as the passive drag-reducing surface corrugations known as ribelts are taken as example turbulent drag reduction techniques to analyse the behaviour of turbulent drag reduction.
