Direct Numerical Simulations of flows in channels are presented to demonstrate that the wall-normal Reynolds stress is a fundamental quantity to assess when the flow past smooth and rough surfaces becomes turbulent. In this paper it is demonstrated that only when (u) over tilde'(+)(2)vertical bar(w) (+ indicates wall units and w the value at the plane of the crest of the roughness elements) is greater than the threshold value of 0.6, there is a transition from a laminar to a turbulent regime. This statement has been checked by changing the shape, the density and the distribution of solid obstacles. The threshold value for (u) over tilde (2)'vertical bar(+)(w) is reached only if the height k(+) of the protuberance is greater than 15. In addition, the simulations for distributed elements have been used to understand the deformation of the Tollmien-Schlichting (T-S) waves by the roughness. Animations show the formation of elongated streaks near the smooth wall, and indicate difference in the shape of structures near the rough surface.
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