Analysis of the Coherent Structure of a d-Type Rough Wall Boundary Layer with the VITA Technique

Conditional averages of the longitudinal, normal velocities and their products have been obtained using the VITA technique in a turbulet boundary layer over a d-type rough surface at a momentum thickness Reynolds number of R_θ≒800. It is shown that the ensemble averaged patterns reflect both the well known ejection and sweep events, even in the present low Reynolds number flow. Both ejections and sweeps made a strong, intermittent contribution to the Reynolds stress and the turbulence production. Near the wall, the ejection phase plays a predominant role. The normalized averaged bursting frequency is considerably larger for the present rough wall flow than the smooth wall flow at almost the same Reynolds number. It is inferred from the increase of both the width of the intermittent region and the entrainment rate that the width of the highly irregular interface between turbulent and non-turbulent flow fields increses with a decreasing Reynolds number.