Wind tunnel experiment of tracer gas diffusion within unstable boundary layer over coastal region

Abstract

A wind tunnel experiment was carried out to simulate stack gas diffusion within an unstable atmospheric boundary layer over a coastal region. The wind tunnel floor, 4 m leeward of the entrance of the test section, was heated to 90°C over a length of 6 m in the streamwise direction, and wind tunnel experiments were performed under the flat plate condition with a prototype-to-model length scale ratio of 1200. Three similarity criteria of flow fields in the wind tunnel and in atmosphere, viz., bulk Richardson number, surface Reynolds number and the ratio of the Peclet number to the Richardson number, were considered in the wind tunnel experiment. Tracer gas was released along the coastline at a height of 10 cm, which corresponded to 120 m in height in atmosphere. The obtained wind tunnel experimental results of ground level concentration were compared with 30-min average values of the field experiments, viz., the data from the Tokai 82 field experiment. The maximum ground level concentration and its location were accurately simulated when there was close similarity between the wind tunnel and atmospheric flow conditions. The maximum concentration increased and occurred closer to the source when the level of convection was relatively stronger in atmosphere.