| Summary: | Flow characteristics and pollutant dispersion characteristics of a mechanical upper vent of a tunnel under various discharge patterns and ambient wind were studied by using the computational fluid dynamics (CFD) numerical method and an experimental model. On this basis, variations in the environmental impact radius <i>r</i><sub>criti</sub> changing with the ambient wind velocity <i>U</i> for pollutants from vertical and horizontal outlets were also analyzed with the set discharge capacity. According to our findings, the pollutant emission of the upper vent under a vertical discharge pattern can be considered as free buoyant jet diffusion under the impact of the momentum of ambient wind, while the emission under the horizontal discharge pattern can be regarded as semi-confined buoyant jet diffusion under the impact of the momentum of ambient wind. Moreover, the <i>r</i><sub>criti</sub> of the vertical outlet increased with the increase of <i>U</i> when <i>U</i> ≤ 3 m/s and was minimally affected by <i>U</i> for <i>U</i> > 3 m/s. Additionally, the <i>r</i><sub>criti</sub> of the horizontal outlet decreased with the increase of <i>U</i>. When the discharge capacity was constant, the <i>r</i><sub>criti</sub> of the upper vent under both vertical and horizontal discharge patterns declined with the increase of the discharge velocity <i>V</i><sub>0</sub>. Note that the <i>r</i><sub>criti</sub> of the vertical outlet was smaller than that of the horizontal one for the range of parameters considered. These results can provide a theoretical reference for optimizing the design of a vent on the top of an urban tunnel.
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