The Influence of External Wind in Tunnels

A model tunnel (approximately ten hydraulic diameters) with different designs of the tunnel mouth has been placed in a wind tunnel and has been subjected to the effects of external wind by varying the wind direction at the mouth of the tunnel. In the experimental oriented study pressures have been measured and the airflow has been made visible with smoke and by the sand erosion method (semolina). The relation between the flow ratio and the direction of the wind has been explored. When the wind is blowing parallel to the tunnel, the tunnel flow is about 70% of the reference flow (the undisturbed flow due to the wind through an area corresponding to the tunnel cross section). This result holds for the length of this tunnel. For longer tunnels it will decrease due to increased friction. When the angle at which the wind is blowing increases, the tunnel flow decreases. The pressure measurements made it possible to quantify some of the phenomena which were observed in the visual trials. A large under-pressure was measured just outside the mouth of the model tunnel on the side of the tunnel corresponding to the separation in the visual trials. In front of the tunnel, a pressure increase due to the braking of the airflow was measured. The position of the pressure increase moved depending on the internal resistance in the tunnel. If the resistance in the model tunnel was high, the over-pressure in front of the mouth of the tunnel was higher and further from the opening. When the internal resistance was reduced (corresponding to e.g. a very short tunnel) the over-pressure decreased and moved closer to the tunnel opening.