Effects of stratification on flow and dispersion around obstacles in turbulent boundary layers.

The effects of stability on the flow and dispersion downstream of a simple obstacle were studied experimentally. Experiments were conducted in the EnFlo atmospheric wind tunnel, where a stably stratified boundary layer is set up by heating the inlet air and cooling the floor of the wind tunnel. Neutral and stable boundary layers with different measures of stability have been successfully modelled and their measured characteristics are reported. The majority of the experiments were then conducted with a neutral boundary layer with a nominal freestream velocity of 2.5 m/s, and two stable boundary layers with nominal freestream velocities of 1.3 m/s and 1.1 m/s. The obstacle used throughout was a 100 mm cube, placed normal to the flow direction and at 45° to the flow direction. A pressure tapped version of the cube was placed in the neutral and stably stratified boundary layers and the pressure distribution was measured on the surface of the cube for each case. Flow measurements were also made in the wake of the cube for the neutral and two stable boundary layers. Concentration measurements were made for different release locations and the flow configurations described above. The source was placed at the leading edge, and source heights at ground level, equal to the obstacle height, and 50% higher than the obstacle height were used. The results and statistics reported here are vertical profiles of concentration, concentration fluctuations and concentration flux, vertical and horizontal plume spread, centreline ground level concentrations and probability distributions. A simple three dimensional wake theory typically used in dispersion modelling was evaluated and the assumptions associated with it reviewed. A Gaussian dispersion model incorporating a building effects model based on the wake theory was then implemented in a computer program. Comparisons were made between the measured and the modelled ground level concentrations. The building effects modelling was then isolated by using the experimental undisturbed plume spread results in the model