Streamwise vortical structure associated with the bursting phenomenon in a d-type rough wall boundary layer : spatial distribution of three-dimensional vortical structure and conditional averaged vorticities

Three-dimensional vortical structure associated with the bursting phenomenon in the near-wall region of a d-type rough wall boundary layer has been investigated experimentally. Velocity vector plot in several cross stream sections and conditional averaged vorticity contours were obtained from three velocity components conditionally averaged with the VITA technique. The ejection process is closely related to a pair of streamwise vortices which spanwise spacing is about 100 viscous wall units. The existence of the vortex pair suggests that the bursting phenomenon of the present rough flow can be fairly explained by means of the hairpin vortex model. While, the sweep process is concerned with large scale three-dimensional vortical structure which induces intense spanwise momentum transfer near the wall.