Properties of the Wind Field within the Oklahoma City Park Avenue Street Canyon. Part I: Mean Flow and Turbulence Statistics

Abstract

Velocity data were obtained from sonic anemometer measurements within an east-west-running street canyon located in the urban core of Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. These data were used to explore the directional dependence of the mean flow and turbulence within a real-world street canyon. The along-canyon vortex that is a key characteristic of idealized street canyon studies was not evident in the mean wind data, although the sensor placement was not optimized for the detection of such structures. Instead, surface wind measurements imply that regions of horizontal convergence and divergence exist within the canopy, which are likely caused by taller buildings diverting the winds aloft down into the canopy. The details of these processes appear to be dependent on relatively small perturbations in the prevailing wind direction. Turbulence intensities within the canyon interior appeared to have more dependence on prevailing wind direction than they did in the intersections. Turbulence in the intersections tended to be higher than was observed in the canyon interior. This behavior implies that there are some fundamental differences between the flow structure found in North American-style cities where building heights are typically heterogeneous and that found in European-style cities, which generally have more homogeneous building heights. It is hypothesized that the greater three-dimensionality caused by the heterogeneous building heights increases the ventilation of the urban canopy through mean advective transport as well as enhanced turbulence.

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

“…The airflow inside the street canyons was found to be intermittent, suggesting occurrences of flapping (Belcher 2005) and the KelvinHelmholtz instability (Chandrasekhar 1981) of the roof-level shear layer. On the other hand, measurements in Oklahoma City showed that an array of heterogeneous buildings promotes turbulence levels (Nelson et al 2007). The building geometry is thus a major factor affecting the ventilation and pollutant removal in street canyons.…”

Analysis of Urban Atmosphere Plume Concentration Fluctuations

“…Brown et al (2004b) and Ramamurthy et al (2004) reported that wind direction relative to street canyon orientation significantly affected the turbulent features present and how that related to the spatial variability of turbulence (e.g., vortex formation, shear stress). Nelson et al (2007) found that turbulence intensities in the Park Avenue canyon were greater during south and south-east flows compared to south-west flows. South-east flows were more common during the nighttime IOPs than during the daytime IOPs (Table 1).…”

“…There were also no clear differences in the overall intensities of turbulence between day and night during JU03 (Ramamurthy et al 2007;Nelson et al 2007;Klein and Clark 2007). Klein and Clark (2007) and Lundquist and Chan (2007) concluded that the in-canyon flows and turbulence in JU03 were driven primarily by the boundary-layer flow at the level of the average roof height and that stability effects were minor.…”

“…Concentrations would have abruptly increased/decreased as lateral meander swept contaminated/clean air across an analyser with corresponding changes in variability and mean concentration. Klein and Clark (2007) and Nelson et al (2007) documented how flow in crosswind-oriented street canyons was commonly channelled in the along-canyon direction with the direction keenly sensitive to small changes in wind direction during JU03. Flaherty et al (2007a) also found greater variability in wind direction during the day and linked it to the variations observed in tracer concentration profiles 1 km from the release.…”

“…An analysis of urban dispersion in the traditional framework of plume centreline, crosswind distance, and downwind distance is more complicated due to the uncertainties that exist as to what the relationship is between the mean wind direction aloft and the actual direction of plume transport at the surface. For example, flow in crosswind-oriented street canyons was commonly channelled in the along-canyon direction with the direction keenly sensitive to small changes in wind direction during JU03 (Klein and Clark 2007;Nelson et al 2007). This had the effect of shifting the plume laterally in a direction offset from the centreline suggested by the wind direction as it passed through the CBD (Flaherty et al 2007a).…”