DNS of transitional rough channels

Abstract

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 (+ 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 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.

Keywords:

• direct numerical simulation
• transition, turbulent boundary layers

Acknowledgements

The support of MIUR (60%) and PRIN grants are acknowledged. Computational time was provided by CASPUR; some of the simulations were performed in the Puertorico clusters and in the DIMA cluster bought by PRIN resources. A particular thank to S. Leonardi and G.F. Carnevale for their comments on the first draft. The final draft was improved with the help of S. Pirozzoli. F. Fabiani was of help to produce the animations.