A comparison of numerical and physical modeling of stable atmospheric flow and dispersion around a cubical building

Our objective is to examine the potential application of a κ- model (TEMPEST) for simulating flow-field and dispersion patterns around a cubical building under stably stratified approach flow. The results of these numerical simulations are compared with available measurements from towing-tank experiments on dispersion around a cube. The modeled and measured flow and concentration patterns are compared in order to ascertain the effects of changes in the stratification, which is characterized by the Froude number Fr = U/NH (where U is the approach-flow wind speed, N is the Brunt-Väisälä frequency, and H is the building height-large Fr implies weak stability and vice versa). Both the model and experimental results suggest that the flow structure is independent of stratification when Fr 6. The model predicts a moderate increase in the length Lc of the lee-side “cavity” (recirculation region) as the Froude number is decreased from 6 to 3 (moderate stratification), then a sharp decrease in Lc as Fr is further decreased to 1 (very strong stratification). The measured results, however, show Lc to be constant as Fr is decreased to 3, then a sharp decrease as Fr is further decreased to 1. The agreement between the measured and model-predicted concentration field when a source was located within the recirculating cavity behind the building was fair under weakly stratified conditions (Fr 3), but poor under strongly stratified conditions (Fr = 1). These simulations show that, in strongly stratified conditions, mean advection plays a more 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 results suggest that stratification will rarely be a significant factor influencing the flow structure in the near-vicinity of a building.