Abstract
Abstract
Highlights
Surface roughness elements of boundary layer size and smaller are ubiquitous and have a crucial influence on flow instability and laminar–turbulent transition
The laminar–turbulent transition process in a Blasius boundary layer can take several paths depending on the level of external disturbances (Morkovin 1994), like free stream turbulence, surface roughness or controlled excitation
Vortex structures induced by an isolated static and an isolated rotating cylindrical roughness element are illustrated by means of λ2 isosurfaces (Jeong & Hussain 1995)
Summary
Surface roughness elements of boundary layer size and smaller are ubiquitous and have a crucial influence on flow instability and laminar–turbulent transition. At higher external disturbance levels, very weak streamwise vortices are able to push high-speed fluid towards the wall and the low-speed fluid to the opposite direction, i.e. the lift-up mechanism (Landahl 1980), creating elongated streamwise streaks In such circumstances, a transient algebraic increase of the perturbation, which is due to the non-normality of the controlling stability operator (Trefethen et al 1993), followed by viscous decay can be expected (Reshotko 2001). If the boundary layer flow is exposed to even higher levels of external disturbances, the non-modal growth of disturbances can evolve directly into the turbulent state, bypassing the primary instability mechanism (Morkovin 1985)
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