Abstract
Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l / h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for l/h=2 the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.
Highlights
Because there is a worldwide increase in urbanization, more pollutant emission sources, such as from power generation, households and traffic, are present near populated areas
All profiles are normalized with the velocity at obstacle height at the start of the measurement/simulation domain, Uh ≡ u|z=h, which proved to be the best scaling for the rural-to-urban flows discussed in the subsequent sections
The results show similar large turbulent structures arising from the top of the obstacles, which grow in downstream direction and that have a larger magnitude than the velocity fluctuations in the approaching
Summary
Because there is a worldwide increase in urbanization, more pollutant emission sources, such as from power generation, households and traffic, are present near populated areas. Investigations of the urban boundary layer are often made for fully-developed flow over areas with uniform properties. Cheng and Castro (2002b) carried out experiments on turbulent boundary layers where the complete bottom wall of the wind tunnel was covered with cubical roughness elements, thereby representing the flow over an urban area. There are only a few recent numerical studies on turbulent flow over an explicitly resolved roughness transition (Lee et al 2011; Cheng and Porté-Agel 2015), where mostly cubical roughness elements or riblets are considered. Experimental investigations with results up to the roughness elements are scarce, and if a roughness transition is considered it is to determine when the boundary layer retains an equilibrium state, without investigating pollutant dispersion (Cheng and Castro 2002a)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
