Abstract
Large-eddy simulations (LES) are used to investigate the effect of stable stratification on rural-to-urban roughness transitions. Smooth-wall turbulent boundary layers are subjected to a generic urban roughness consisting of cubes in an in-line arrangement. Two line sources of pollutant are added to investigate the effect on pollutant dispersion. Firstly, the LES method is validated with data from wind-tunnel experiments on fully-developed flow over cubical roughness. Good agreement is found for the vertical profiles of the mean streamwise velocity component and mean Reynolds stress. Subsequently, roughness transition simulations are done for both neutral and stable conditions. Results are compared with fully-developed simulations with conventional double-periodic boundary conditions. In stable conditions, at the end of the domain the streamwise velocity component has not yet reached the fully-developed state even though the surface forces are nearly constant. Moreover, the internal boundary layer is shallower than in the neutral case. Furthermore, an investigation of the turbulence kinetic energy budget shows that the buoyancy destruction term is reduced in the internal boundary layer, above which it is equal to the undisturbed (smooth wall) value. In addition, in stable conditions pollutants emitted above the urban canopy enter the canopy farther downstream due to decreased vertical mixing. Pollutants emitted below the top of the urban canopy are 85 % higher in concentration in stable conditions mostly due to decreased advection. If this is taken into account concentrations remain 17 % greater in stable conditions due to less rapid internal boundary-layer growth. Finally, it is concluded that in the first seven streets the vertical advective pollutant flux is significant, in contrast to the fully-developed case.
Highlights
In view of the global trend of urbanization there is an increasing demand for accurate predictions of urban air quality
Use is made of large-eddy simulation (LES) to investigate a smooth-wall turbulent boundary layer exposed to a roughness transition consisting of an array of cubes in an in-line arrangement
The applied level of stratification in the stable cases is described by the bulk Richardson number which was 0.147 at the inlet of the stable D, Roughness transition (RT) and Periodic roughness (PR) simulations
Summary
In view of the global trend of urbanization there is an increasing demand for accurate predictions of urban air quality. Often the urban boundary layer is considered only for near-neutral conditions by assuming that the turbulence due to the presence of the obstacle results in a well-mixed flow with nearly uniform temperature. Use is made of large-eddy simulation (LES) to investigate a smooth-wall turbulent boundary layer exposed to a roughness transition consisting of an array of cubes in an in-line arrangement. The Reynolds number, Reτ = uτ h/ν, based on the friction velocity uτ and the obstacle height h, was between 195 at the inlet and 353 (455 in stable conditions) at the end of the domain This is in the fully-rough regime (Snyder and Castro 2002) and in addition, Cheng and Castro (2002) showed that for flow over a similar array of sharp-edged obstacles the Reynolds number dependency is small.
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