Near-surface particle image velocimetry measurements in a transitionally rough-wall atmospheric boundary layer

The Reynolds number dependence of the structure and statistics of wall-layer turbulence remains an open topic of research. This issue is considered in the present work using two-component planar particle image velocimetry (PIV) measurements acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility in western Utah. The Reynolds number (delta u(tau)/v) was of the order 10(6). The surface was flat with an equivalent sand grain roughness k(+) = 18. The domain of the measurements was 500 < yu(tau)/v < 3000 in viscous units, 0.00081 < y/delta < 0.005 in outer units, with a strearnwise extent of 6000v/u(tau). The mean velocity was fitted by a logarithmic equation with a von Karman constant of 0.41. The profile of u'v' indicated that the entire measurement domain was within a region of essentially constant stress, from which the wall shear velocity was estimated. The stochastic measurements discussed include mean and RMS profiles as well as two-point velocity correlations. Examination of the instantaneous vector maps indicated that approximately 60 % of the realizations could be characterized as having a nearly uniform velocity. The remaining 40 % of the images indicated two regions of nearly uniform momentum separated by a thin region of high shear. This shear layer was typically found to be inclined to the mean flow, with an average positive angle of 14.9 degrees.