Mixing and Ventilation in an Urban Street Canyon

Scalar mixing refers to the homogenisation of a scalar field. Although it is an important topic in fluid dynamics and atmospheric science, passive-scalar mixing has received comparatively little attention in the urban boundary-layer literature. Following work on spatially smooth flows (i.e. chaotic advection), the decay of the scalar variance is analysed for flow within a single street canyon. It is shown that the variance decay at large times (compared to the mean circulation) is controlled by ventilation or the decay of the mean concentration: sensitivity to initial conditions persists because the scalar field does not homogenise completely. At early times, by contrast, mixing is controlled by the large-scale strain or diffusion across closed streamlines. There is good agreement with analytical predictions for idealised flows. These results highlight important differences between mixing in open and closed domains and they are of relevance to urban canopy parameterisation. Application to more realistic domains is discussed.