Skin friction drag reduction over staggered three dimensional cavities

The effect of three-dimensional staggered circular cavities on a zero-pressure gradient incompressible turbulent boundary layer was studied. Two key parameters were varied, being the ratio of the diameter, d, to the depth, h, of the cavity, d/hand the Reynolds number based on the diameter of the cavity,Rd. Velocity profile measurements showed that for the cases of d/h >1an increase in skin friction drag was experienced with respect to a smooth surface, but for d/h ≤1the drag increment was almost negligible and in some cases it was lower than that of a smooth surface by up to 10%. Measurements along the spanwise plane showed the presence of organised transverse velocity components which bear some resemblance with the flow over riblets. The skin friction drag appears to be a strong function of Rd, where for Rd>5500a drag increment is experienced which could potentially be due to shear layer breakdown and more production of turbulence.