Modelling The Mean Velocity Profile In The Urban Canopy Layer

A simple model originally derived for meanwind speed profiles in vegetative canopy flows ismodified for application to arrays ofthree-dimensional surface obstacles (cubes), whichcould be representative of a simple urban-typesurface. It is shown that for cube arrays that arenot too densely packed, the predicted exponentialvelocity profile provides an adequate fit to thespatially averaged velocity profile (u(z))within the obstacle canopy. Application of the model to a set of wind-tunnel dataallows for the evaluation of an empirical fittingparameter called the attenuation coefficient. This isrelated to the turbulence length scale, which can befound by manipulating the results of thegradient-diffusion model used to derive the velocityprofile. The results show a reduction of theturbulence length scale with increasing obstaclepacking density. By assuming a linear transition fromthis length scale at the top of the canopy to theclassical Prandtl length scale in the overlyinginertial sublayer, an acceptable model is obtained forthe full velocity profile within simple obstaclearrays, from the ground up to the overlyingsemi-logarithmic region.

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