Effect of complex terrain on turbulent diffusion

The diffusion from sources placed near a step is studied through a quantitative visualization system. The geometry of the flow consists of a step of 0.15 m in a boundary layer 0.6 m thick. Two angles of the step relative to the free-stream velocity are studied (90-60 deg). The mean velocity and the turbulence intensity are measured with hot wire far from the step, above the step, and downstream. The concentration field is measured by a new quantitative visualization technique using a source of smoke particles, the diameter of which ranges from 0.1 to 1.0 micron. The root mean square and third order moment of the concentration fluctuation show that the presence of the step induces a great variability on the concentration field compared to the diffusion on a flat plate. Two heights of sources have been studied.