A COMPARISON OF NUMERICAL AND PHYSICAL MODELING OF STABLE ATMOSPHERICFLOW AND DISPERSION AROUND A CUBICAL BUILDING

Our objective is to examine the potential application of a k-epsilon m odel (TEMPEST) for simulating flow-field and dispersion patterns aroun d a cubical building under stably stratified approach Row. The results of these numerical simulations are compared with available measuremen ts from towing-tank experiments on dispersion around a cube. The model ed and measured Row and concentration patterns are compared in order t o ascertain the effects of changes in the stratification, which is cha racterized by the Froude number Fr = U/NH (where U is the approach-Row wind speed, N is the Brunt-Vaisala frequency, and H is the building h eight-large Fr implies weak stability and vice versa). Both the model and experimental results suggest that the Row structure is independent of stratification when Fr greater than or equal to 6. The model predi cts a moderate increase in the length L(c) of the lee-side ''cavity'' (recirculation region) as the Froude number is decreased from 6 to 3 ( moderate stratification), then a sharp decrease in L(c) as Fr is furth er decreased to 1 (very strong stratification). The measured results, however, show L(c) to be constant as Fr is decreased to 3, then a shar p decrease as Fr is further decreased to 1. The agreement between the measured and model-predicted concentration field when a source was loc ated within the recirculating cavity behind the building was fair unde r weakly stratified conditions (Fr greater than or equal to 3), but po or under strongly stratified conditions (Fr = 1). These simulations sh ow that, in strongly stratified conditions, mean advection plays a mor e important role than does turbulent diffusion because the turbulence is very weak in such flows. And because the Froude number will rarely be less than about 3 in the nighttime stable boundary layer, these res ults suggest that stratification will rarely be a significant factor i nfluencing the flow structure in the near-vicinity of a building.