High Reynolds number turbulent drag reduction by spanwise wall forcing

Turbulent friction drag is an inevitable source of power consumption for aircrafts, trains, pipelines, and many other industrial applications. Significant efforts are ongoing to design and study drag-reducing mechanisms, e.g. riblets, superhydrophobic surfaces and blowing/suction. In this seminar, I focus on an active controlling mechanism based on spanwise surface oscillation, leading to the generation of a streamwise travelling wave. The mechanism has been extensively studied via direct numerical simulation (DNS); it has shown the potential to reduce drag by 40%. Owing to the expensive computational cost of DNS, the prediction models for drag reduction are derived based on datasets with friction Reynolds numbers less than 2000. Motivated by Intellectual Ventures, a team of researchers have built their experimental and large-eddy simulation capabilities to study the travelling wave actuation at friction Reynolds numbers beyond 10000. Thus, providing the opportunity to study the efficacy of this mechanism at a flow regime closer to that of ground and air vehicles. As a member of this team, I present our findings. My presentation evolves around two major themes: 1) Do the high Reynolds number data agree with the past prediction models? If not, what emerging flow physics are related to the disagreement? 2) Stokes layer is an important mechanism in this problem. How does this mechanism interact with the near-wall turbulence? and how this interaction manifests in the drag reduction?