###### Abstract  
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 \(r_{\text{criti}}\) changing with the ambient wind velocity \(U\) 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 \(r_{\text{criti}}\) of the vertical outlet increased with the increase of \(U\) when \(U\leq 3\) m/s and was minimally affected by \(U\) for \(U>3\) m/s. Additionally, the \(r_{\text{criti}}\) of the horizontal outlet decreased with the increase of \(U\). When the discharge capacity was constant, the \(r  
In fact, many scholars have been highly concerned about the problem of ventilation and pollution discharge from tunnels' upper vent. [PERSON] et al. [10] analyzed the formation and consequences of pressure at the upper vent using a one-dimensional constant flow model and revealed that natural ventilation derives from the exchange of air inside the tunnel with that outside, due to oscillating pressure fluctuations at the upper vent caused by the traffic wind generated by moving vehicles. Further, [PERSON] et al. [11] analyzed the impact of the number of upper vent groups on the efficiency in pollutant emission using the subway environment simulation software (SES). According to the simulation, the best pollutant emission efficiency can be observed with two groups of upper vents, achieving an emission of roughly 50% of the pollutants, on the premise that the number of upper vents, overall emission coverage, and spacing between the vents are constant. [PERSON] et al. [6,12] established a pulsation model for a highway tunnel with natural upper vents combined with a experimental model; after studying the velocity distribution and flow field characteristics of the upper vent for three values of vehicle velocities, they established that the average inlet wind velocity at the upper vent increases linearly with the increase of vehicles' speed. In addition, various scholars have studied the impact of height, cross-sectional area, distance from the combustion source, inclination, quantity, and other parameters of the upper vent on the smoke exhaust of the upper vent using a combination of theoretical analysis and numerical simulation. They found that a large cross-sectional area of the upper vent and a longer longitudinal distance from the combustion source contribute more favorably to flue gas emission than the height of the vent, which has a limited contribution [13, 14, 15]. Moreover, an upper vent that is low in position and slightly inclined can enhance emission efficiency, the best inclination being 76\({}^{\circ}\)[16]. The total mass flow rate of the vent increases with the increase of the number of upper vents when the vent area is constant [17]. Most of the existing studies focused on the impact of factors, such as the location and structural parameters of the upper vent and tunnel traffic's flow conditions, on the discharge or smoke emission efficiency of the upper vent. On the contrary, the interaction of the upper vent with the outside environment of the tunnel has been rarely discussed in a systematic paper. Therefore, this paper was designed to study the flow characteristics and the pollutant dispersion law in the case of a mechanical vent on the top of a tunnel under various pollutant discharge patterns and types of ambient wind by using a computational fluid dynamics (CFD) numerical method and an experimental model.