CONCENTRATION MEASUREMENTS AROUND AN ISOLATED BUILDING - A COMPARISON BETWEEN WIND-TUNNEL AND FIELD DATA

Field experiments have been conducted in flat terrain using a model building, which represents real structures at a scale of between 1/3 and 1/10. The building is rotatable, and thus may be positioned in any orientation relative to the wind direction. A tracer gas is released upwind of the building and detectors are used to measure concentration values at a frequency of 10 Hz. Similar experiments were undertaken at 3/50 of the field scale using a wind tunnel. The measured concentration values are non-dimensionalised to enable the two sets of results to be compared directly. The highest mean concentrations found around the building tend to be over-estimated by the wind tunnel, whilst the minimum concentrations tend to be underestimated. This suggests that there is more dispersion of the plumes in the atmosphere than in the wind tunnel. A likely explanation for this is the differences in turbulence scales relative to the size of the model building. Data sets from the field experiments are also divided into sections each of 5 min duration. This indicates that there is a great deal of variation at a single location throughout each experiment. This is caused by the continual changes in the wind direction (both mean and standard deviation) leading to continual changes in the direction of travel of the plume. Intermittency (the proportion of time for which no gas is observed) and conditional means (which exclude zero sections of data) are calculated. Both parameters exhibit considerable variation between each 5 min period at a single location.