Analysis of the small-scale structure of turbulence on smooth and rough walls

Energy spectra and structure functions of the streamwise and wall-normal turbulent velocity components in an open channel flow are experimentally investigated as a function of the distance from the wall. Both smooth- and rough-wall conditions are considered, with special attention to the zone close to the wall. In the core region, the small-scale turbulent flow field is always characterized by a high level of isotropy, while a strong anisotropy is found at small scales in the near-wall region. In the smooth-wall case, the extent of the scaling range increases as the wall is approached, but with exponents which are different from the classical ones. In the rough-wall case, the roughness strongly interacts with turbulence, destroying the scaling regions at small scales through the imposition of its characteristic scales. A lower level of intermittency and anisotropy is also observed at the small scales for rough-wall conditions. Energy spectra and structure functions suggest a connection of these behaviors with the turbulent energy directly injected into the flow by the roughness elements. The integral structure functions show that roughness effects exceed the sole modification of local shear and reveal the direct impact of wakes and vortices generated by the roughness in the near-wall region.