ABSTRAKT

We perform Large Eddy Simulation (LES) for analysis of the dispersion in the spatially developing turbulent boundary layer over uniformly and randomly arranged roughness blocks. The objective of this paper is to pursue the detailed characteristics, of turbulence structures and dispersion process in the surface layer over the roughness blocks with various types of arrangement on the ground. We often encounter the complex arrangement of roughness such as buildings in urban area, but almost studies have dealt with uniform arrangement. Especially it can be considered that the dispersion behavior is much influenced by the pattern of arrangement. Two LES cases are carried out. One is an uniform (staggered) array for roughness blocks. The other is a random array for them in a horizontal plane without changing averaged height. But each roughness block has a different height. We obtain the profile of turbulent statistics in a statistical manner in order to avoid local effects near the roughness blocks. LES of concentration fluctuations are carried out over smooth wall at first to validate the numerical model by comparing with Fackrell's experimental results and Sykes's LES analysis data, then are done over rough wall. Consequently, near the roughness height, touchdown in the random array is earlier than that in the uniform (staggered) array because of the strong local effect in the random case. On the other hand, in the upper part, although there is no clear difference between velocity fields of uniform and random arrays, a great difference of mean as well as fluctuating concentration between both cases arises.