AIR FLOW OVER ROUGHNESS DISCONTINUITY.

Measurements of mean velocity, mean-square turbulent velocity, turbulent shear stress, one-dimensional spectrum, and mass concentration distributions following a step increase in surface roughness of a wind-tunnel boundary-layer flow are presented. The mean velocity distributions agree well with Nickerson's numerical calculations for a small roughness change. The mixing-length distribution in the 'transitory' region is not experimentally consistent with that established for fully-developed turbulent boundary layer. Turbulent intensity and shear stress are generated progressively towards the upper layer as one moves downstream from the roughness discontinuity. The high frequency end of the spectra in the 'transitory' region can be exactly represented by the high frequency shape of the undisturbed turbulent boundary layer. Self-preserving mass concentration profiles are in general possible for both the vertical and horizontal distributions. The adjustment of the mean motion to the roughness change is more rapid than that of the turbulence. (Author)