In experimental modelling of flow over very rough, urban-like surfacesHowever, this type of profile is only true in the inertial sublayer of a boundary layer that has adjusted to the underlying roughness. When the wind encounters a new surface roughness, the extent of the disturbed boundary layer is about 1/10 of the fetch, and only the bottom 10% of this internal boundary layer is in equilibrium with the new surface. Since many experimental studies are done with a fetch of building obstacles of the order of 100H or less, where H is the average building height, the extentof the inertial sublayer should only be about 1H. This is less than the depth of the typical roughness sublayer, in which the local perturbations due to individual obstacles disrupt the flow. Thus, in practice there should be no distinguishable layer inwhich the conditions for a semi-logarithmic profile are satisfied. Despite these limitations, when the velocity profile u(z) is plotted as a function of In(z-d) in these short arrays, the resulting data usually follows a reasonably straight line in theinternal boundary layer, and a value of can be is extracted from the slope.
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