Direct numerical simulations of turbulent boundary layers over the cube-roughened walls were performed with
varying spanwise extent. Recent work by Lee et al. (2011) showed that the inner layer statistics are influenced by the
streamwise pitch over the 3-D square-edged roughness and these have the maximum at px/k=4 with pz/k=2. However,
the present simulations showed that the surface drag, including form drag and roughness function, etc., is also affected
by spanwise spacing and the maximum contribution occurs at pz/k=3 with px/k=3 which has the roughness density, 0.111.
This result might indicate that the main parameter for the wall friction is roughness density rather than roughness
configurations. In addition, it was shown that outer peaks of the Reynolds stresses are increased with increasing
spanwise length, implying stronger inner and outer layer interactions at larger pz/k, which is consistent with those of
streamwise spacing. This signifies streamwise and spanwise spacing as the principal variable in the outer layer.