From single obstacles to wall roughness: some fundamental investigations based on DNS results for turbulent channel flow

Turbulent flow through a plane channel with only one smooth wall is analyzed based on DNS results for three Reynolds numbers. The opposite wall has 2D bars of size k x k attached with the distance to each other as the crucial parameter. When they are close together they act as a wall roughness whereas they are single obstacles in character when they are far apart. These two extreme cases show very different coherent structures in the vicinity of the wall attached bars. The categories single obstacles and wall roughness are introduced as an alternative to the often used categorization in terms of k- and d-type roughness. Visualization of the coherent structures is achieved by introducing constant local entropy generation as a parameter. Finally it is discussed whether results gained in a channel with one rough wall can be transferred to the more realistic case when both walls are rough.