Structure of turbulent channel flow with square bars on one wall

The organised motion in a turbulent channel flow with a succession of square bars on the bottom wall has been investigated using direct numerical simulations. Several values of the ratio w/k, where k is the bar height and w is the longitudinal separation between consecutive bars have been examined in detail. Relative to a smooth surface, the streamwise extent of the near-wall structures is decreased while their spanwise extent is increased. As w/k increases, the coherence decreases in the streamwise direction having a minimum for w/k=7. This is due to the outward motion occurring, most of all, near the leading edge of the elements. The Reynolds stress anisotropy tensor and its invariants show a closer approach to isotropy over the rough wall than over a smooth wall.