Flow field development and energy evolution in road tunnels with unidirectional uniform traffic

Abstract

Shallow-buried road tunnel with roof openings is a green energy-saving technology. In this kind of tunnel, natural ventilation and natural smoke extraction can be realized through the roof openings. Now there are four shallow-buried road tunnels in Nanjing. Since operation, this kind of tunnel has been widely praised for the high air quality inside tunnel, but there is yet no regular design method of it. Buried section is the basic unit of this type of tunnel, while fully understanding of the development of flow field and velocity distribution is the foundation of solving the above problem. Based on simplified flow filed in the buried section and Prandtl velocity distribution law, this paper presented the formula of velocity distribution in the buried section. Besides, a 1/10 reduced-scale tunnel experiment platform was built to test the tunnel flow field on 9 kinds of traffic conditions. Energy transfer and dissipation law in the tunnel flow field were explained based on the concept of energy gradient and the formula of velocity distribution. Research showed that the velocity of the flow field increases quickly and then becomes stable with unidirectional uniform traffic. Velocity meets the logarithmic relationship with height. The air velocity in the tunnel varies approximately linearly with the vehicle speed. And the vehicle speed has much more influence on air velocity than the vehicle spacing. The energy transferred to the airflow from vehicles evolved into two parts in the flow field. One part is absorbed by the flow field and converted into the kinetic energy of the flow field itself. The other part is dissipated in turbulence. The kinetic energy increment increases with the height of the tunnel. The dissipation of energy along the tunnel presents an asymmetrical U-shaped distribution. When the flow becomes stable, the dissipation of energy also becomes stable, and the kinetic energy increment approaches zero.