UNIFORM MOMENTUM ZONES IN A TURBULENT BOUNDARY LAYER OVER PERFORATED WALLS

The modification of the boundary layer grazing over staggered arrays of cavities is investigated by means of Particle Image Velocimetry. The skin friction drag is reduced, confirming the recent investigations conducted with hot wire anemometry on similar configurations. The skin friction drag reduction is more pronounced when the diameter of the cavities is larger. Changes in the Uniform Momentum Zones (UMZs) of the boundary layer suggest that the coherent structures are modified by the cavities. In fact, the average number of UMZs decreases and the average size of the first zone (which corresponds to the logarithmic layer), as well as the average overall size, increases. The trend of these two variations with the diameter is not in line with the skin friction reduction. The reduction of the mean modal velocity of the UMZs follows the same trend of the skin friction with respect to the diameter of the cavities. For this reason it is believed that this parameter is better suited to emphasize the link between the modifications of the UMZs and the skin friction.