Modelling the dispersion of a tracer gas in the wake of an isolated low-rise building

Mean concentrations of ammonia gas released as a tracer from an isolated lo w-rise building have been measured and predicted. Predictions were calculat ed using computational fluid dynamics (CFD) and two dispersion models: a di ffusion model and a Lagrangian particle tracking technique. Explicit accoun t was taken of the natural variation of wind direction by a technique based on the weighted summation of individual steady state wind direction result s according to the probability density function of the wind direction. The results indicated that at distances >3 building heights downstream the weig hted predictions from either model are satisfactory but that in the near wa ke the diffusion model is less successful. Weighted solutions give signific antly improved predictions over unweighted results. Lack of plume spread is identified as the main cause of inaccuracies in predictions and this is li nked to inadequate resolution of flow features and mixing in the CFD model. Further work on non-steady state simulation of wake flows for dispersion s tudies is recommended.