Roughness induced transition: A vorticity point of view

This paper explores the mechanisms underlying roughness induced transition (RIT) caused by discrete roughness elements (DREs) using immersed boundary direct numerical simulations. We show via favorable comparison between RIT in Blasius boundary layers and equivalent Couette flows that linear instability of the boundary layer profile does not play a significant role for the DREs considered (k < 0.6 delta*, where k is the height of the DRE) and that k(+) = u(tau)k/nu is the dominant parameter (for a given shape of the DRE) which strongly affects the transition location. For a suitable range of k(+), the flow evolution can be separated into four distinct stages: (i) generation of vortical disturbances at the roughness, (ii) a steady and spatial amplification of a three dimensional disturbance, (iii) the emergence and amplification of unsteady disturbances, and (iv) the emergence of chaotic behavior leading to a "turbulent wedge" (with a relatively high mean wall shear stress). Each of these stages is studied in detail. A mechanistic understanding of RIT is suggested which includes a new and fundamental understanding of the final stage. Novel results include the description of a mutual stretching mechanism leading to the near wall amplification of streamwise vorticity at the onset of stage IV, complementary interpretations of the lift up and the "modal instability" using a control volume formulation for different components of the enstrophy, and a demonstration of a passive RIT mitigation strategy using an "anti-roughness" element (i.e., a second downstream roughness element), which exploits this understanding of RIT mechanisms from the vorticity-based analysis. Published under license by AIP Publishing.