This research is concerned with experimental studies of the effects of moving vehicles (piston-effect) and jet fans (jet fan-effect) on the tunnel ventilation in rotating-belt model vehicle tunnels. Specifically, the aerodynamics of the flow induced by moving vehicles and effects of jet fans on the distribution of vehicle exhausts were investigated under different traffic conditions. The results of these studies show that the distributions of flow velocities inside the tunnel induced by moving vehicles weakly depend upon the vehicle speed, vehicle spacing and vehicle size. In addition, the vehicle speed plays a more important role than the vehicle spacing and vehicle size in the piston effect inside the tunnel. The results also show that the piston effect in a tunnel is not confined to the vicinity of vehicles. In general, the piston effect in the upper region of the tunnel is 40% of that around the vehicle. Furthermore, a piston-effect still exists in each lane of a two-way traffic tunnel but it is only 35% of that in a one-way traffic tunnel. The mean flow velocities induced by the moving vehicles are largely dependent upon the traffic conditions. The ratio of the mean flow velocity to the vehicle speed, U/V, is close to 1/3 for normal traffic conditions, and mostly is smaller than 1/3 for the other traffic conditions examined in this research. The results of the jet fan-effect study show that a larger fan velocity results in a higher exhaust concentration in the lower region of the tunnel, which would be harmful to drivers, though it causes higher flow velocity inside the tunnel. (C) 1998 Elsevier Science B.V. All rights reserved.
