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ROUGHNESS INDUCED TRANSITION IN WALL BOUNDED FLOW: A VORTICITY POINT OF VIEW

Saikishan Suryanarayanan
Department of Aerospace Engineering and Engineering Mechanics The University of Texas at Austin Austin, Texas 78712

David B. Goldstein
Department of Aerospace Engineering and Engineering Mechanics The University of Texas at Austin Austin, TX 78712

Garry L. Brown
Department of Mechanical and Aerospace Engineering Princeton University Princeton, NJ 08544

Sinopsis

We have recently demonstrated, using experiments and DNS, how boundary layer transition caused by a single discrete roughness element (DRE) can be inhibited by appropriate addition of a discrete roughness element (Sharma et al, 2014; Suryanarayanan et al, 2017a) or distributed roughness (Kuester et al, 2014; Suryanarayanan et al, 2017b). In this paper we explore in greater detail the roughness induced transition (RIT) control via application of a single additional discrete roughness element, and utilize the unique capabilities of our simulation setup to examine the underlying mechanics. Notably we show that for the parameter regimes in which we are interested (Reδ* ~ 1480, k+~15; δ*-boundary layer displacement thickness, k - height of the DRE), we obtain nearly identical results for the boundary layer and for Couette flow, suggesting that the flow evolution is dominated by the near-wall dynamics in case of both the RIT and RIT cancellation cases. Analysis of the evolution of different vorticity components and the associated analysis of the production and dissipation terms provide insights into the transition process and the roles of different components of vorticity. We thus explain why the control works from a vorticity point of view, complementing the related experimental/CFD work (Berger et al, 2017) that examines some of these results from an instability point of view. Using specially engineered simulations, we also address fundamental questions about the receptivity, the deterministic amplification of the steady vortical perturbations, the apparent 'modal' amplification of unsteady vortical perturbations, the subsequent evolution to a chaotic state, and how viscosity and inlet unsteadiness affect the evolution. The present results show that the investigation of RIT leads to novel control techniques and offers the potential to provide further understanding of the basic transition mechanisms in wallbounded flow.