Wind tunnel experiment on investigating the times of ventilation in case of pollutant dispersion in an urban area*

The times of ventilation, which is defined as how long pollutants take to arrive at and then vanish away in a distant place from the source point, were investigated in wind tunnel. In this study, rectangular shaped blocks were allocated in equally spaced interval in a wind tunnel. In order to cut off the disturbance of drift effect, which happens in symmetrical block alignment, the blocks array were tipped at 5-25 degrees. We defined semi-opened space between the blocks in the center of the modeled area as ``Void.'' Tracer gas was discharged at the center of the ``Void.'' The times of ventilation were measured in a boundary face of the ``Void.'' In addition, the distributions of velocity and concentration were measured in the `Void' simultaneously. We considered the results of times measurement based on the distributions of velocity and concentration. Concentration was disproportionately dispersed in the `Void' by unsymmetrical flow. With increasing angle, two factors of ventilation, horizontal transportation and vertical transportation, varied their features. Influenced by these feature, the times of ventilation shifted their peaks and distributions.