Numerical Simulation and Field Validation of Airflow Field and CO Dispersion Characteristics during Tunnel Construction under Dynamic Vehicle Movement

During tunnel construction, a substantial amount of pollutants, such as carbon monoxide, are generated. Compared with the blasting phase, the CO produced by internal combustion engines during the mucking phase exhibited continuity owing to temporal accumulation and had a wider range of influence. Therefore, it is necessary to investigate CO dispersion characteristics under different ventilation modes at various construction stages. In this study, to reveal the influence mechanism of the coupled effect of traffic-induced wind and mechanical ventilation on the airflow field and pollutant migration under dynamic vehicle movement during the tunnel mucking phase, a construction ventilation model was established based on the overset grid method. The effects of mucking truck speed, mechanical wind velocity, and truck travel direction on CO dispersion patterns under different ventilation modes were systematically analyzed. The results show that under forced ventilation conditions, a stable traffic-induced wake was generated by the dynamic movement of the vehicle in the mechanically ventilated stable zone, whereas airflow field fluctuations and multiple turbulent structures were induced in the recirculation zone near the tunnel face, where the peak airflow velocity was reduced by 47.33%. The CO concentration exhibited an annular spatial distribution pattern, characterized by higher concentrations in the central region and lower concentrations toward the periphery, and residual accumulation was formed along the vehicle travel path, with the CO dispersion range extending to 256 m after 45 s of travel. Both the peak CO concentration and the residual time of CO were effectively reduced by increasing the travel speed of the mucking truck or mechanical wind velocity. For every 2.5 km/h increment in travel speed, the peak CO concentration was reduced by 8.13% to 20.4%, and for every 5 m/s increase in wind velocity, the peak CO concentration was reduced by 0.47% to 3.74%. More favorable CO dispersion conditions were observed when the truck traveled in the downwind direction. The peak CO concentration was found to increase initially and then stabilize with prolonged ventilation time, and the CO diffusion coefficient was enhanced by increasing either the truck speed or mechanical wind velocity. This study provides a basis for optimizing ventilation parameters and controlling vehicle emissions during the tunnel mucking phase.