Large-eddy and Gaussian simulations of downwind dispersion from large building HVAC exhaust

We have simulated the dispersion from elevated HVAC vents of a large single building. A massless tracer was continuously released for 10 minutes from 16 vents at a height of 35 meters. The simulated ambient meteorological conditions are representative of a mid-latitudes winter day: neutral stratification, a surface temperature of 0 deg-C, and a power-law inflow velocity profile with a velocity of 2.6 m/s at the building roof top. The LES code used in this simulation was HIGRAD, which has a meteorological framework and uses a Smagorinsky subgrid closure method. The simulation was run over a 1.5km by 840m by 250m domain, using 10m horizontal and 5m vertical resolution.

The results of the simulation indicate that a significant amount of the tracer is trapped in the building wake and produces high ground-level concentrations and dosages within the recirculation zone. The downwind concentration and dosage patterns are distinctly non-Gaussian, showing a double-lobe of maximum concentration downstream of the building edges. The results to be presented will include a comparison of instantaneous and time-averaged concentration and dosage patterns, and tracer flux through planes normal to the mean wind. The simulation output is also being used to characterize a source term for a large-scale puff dispersion model. We will present the development of the source term from the LES for input to the puff dispersion model, and show the differences in dispersion from the puff model using this new source term.