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

Abstract

Mean concentrations of ammonia gas released as a tracer from an isolated low-rise buildingrnhave been measured and predicted. Predictions were calculated using computational fluid dynamics (CFD)rnand two dispersion models: a diffusion model and a Lagrangian particle tracking technique. Explicit accountrnwas taken of the natural variation of wind direction by a technique based on the weighted summation ofrnindividual steady state wind direction results according to the probability density function of the windrndirection. The results indicated that at distances >3 building heights downstream the weighted predictionsrnfrom either model are satisfactory but that in the near wake the diffusion model is less successful. Weightedrnsolutions give significantly improved predictions over unweighted results. Lack of plume spread isrnidentified as the main cause of inaccuracies in predictions and this is linked to inadequate resolution ofrnflow features and mixing in the CFD model. Further work on non-steady state simulation of wake flowsrnfor dispersion studies is recommended.