Wind tunnel simulations of urban dispersion in stable and convective conditions

Predicting dispersionin urban areas can assist in preventing health hazards and planning emergency procedures. One of the main problems that affects models used for this purpose is the way they treat atmospheric stratification, very often present in environmental flows. Artificially thickened stable and unstable boundary layers were simulated in the EnFlo wind tunnel over a very rough surface. The effect of different parameters was investigated (among them, inlet temperature profile, capping inversion and surface roughness). These boundary layers were then employed as approaching flow for idealised urban models.

An array of rectangular blocks was used, where a pollutant tracer was also released from a point source at ground level. Mean and fluctuating velocities, temperatures and concentrations were sampled, together with heat and pollutant fluxes. The analysis of the data revealed that even in case of weak stratification there are important modifications inside and above the canopy on both the urban boundary layer and the plume characteristics.

In stable stratification the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length. The roughness length and displacement height were also affected, with a reduction for the former and an increment for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratification cases, the friction velocity appears increased by both the effect of roughness and unstable stratification. The increased roughness causes a reduction in the surface stratification, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching flow. The effect on the aerodynamic roughness length and displacement height are opposite to the SBL case, with an increase of the former and a reduction of the latter.

The results for pollutant dispersion show that the stratification effect on the plume horizontal width is significantly lower than the effect on the vertical profiles. Stable stratification did not affect the plume central axis inside the canopy, but in the unstable case the axis appeared to deviate from the neutral case direction. Above the canopy both stratification types caused an increase in the plume deflection angle compared to the neutral case. Measured concentrations in stable stratification were up to two times larger in the canopy compared to the neutral case, the opposite for the convective stratification (which were up to three times lower). Vertical turbulent pollutant fluxes have been found to be only slightly affected by stratification, but without significant changes in the general trend. Mean pollutant fluxes in the canopy remain predominant close to the source, even though at roof level and above turbulent and mean fluxes have the same order of magnitude. The proportionality between the vertical turbulent fluxes and the vertical mean concentration gradient (base of the K-theory) is confirmed also in the stratified cases.

This work helps in gaining new knowledge on the effects of stratification and encourages further work on the topic. The experimental database produced during the project is unique and of high quality. It can assist in developing, improving and validating numerical models, as well as developing parametrisations for simpler models.