Inner and outer scalings in rough surface zero pressure gradient turbulent boundary layers

A new set of experiments have been performed in order to study the effects of surface roughness and Reynolds number on a zero pressure gradient turbulent boundary layer. In order to properly capture the x dependence of the single point statistics, consecutive measurements of 11 streamwise locations were performed which enabled the use of the full boundary layer equations to calculate the skin friction. This quantity was obtained within 3% and 5% accuracy for smooth and rough surfaces, respectively. For the sand grain type roughnesses used, only the Zagarola and Smits scaling, U-infinity delta(*)/delta, was able to remove the effects of roughness and Reynolds number from the velocity profiles in outer variables. However, each scaling used for the velocity deficit profiles resulted in self-similar solutions for fixed experimental conditions. When examining the Reynolds stresses in the inner region [i.e., 0 <(y+epsilon)(+)< 0.1 delta(+)], the < u(2)> component showed the largest influence of roughness, where the high peak near the wall was decreased and became nearly flat for the fully rough regime profiles. In addition, the Reynolds stresses in outer variables showed self-similarity for fixed experimental conditions. However, as the roughness parameter, k(+) increases, all Reynolds stress profiles became similar in shape indicating increased isotropy near the wall. Furthermore, the boundary layer parameters also showed a considerable increase due to roughness. (c) 2007 American Institute of Physics.