Properties of the Wind Field within the Oklahoma City Park Avenue Street Canyon. Part II: Spectra, Cospectra, and Quadrant Analyses

Abstract

Velocity data were obtained within Park Avenue in Oklahoma City, Oklahoma, using three-dimensional sonic anemometers under unstable atmospheric conditions. These data are used to produce velocity spectra, cospectra, and weighted joint probability density functions at various heights and horizontal locations in the street canyon. This analysis has helped to describe a number of physically interesting urban flow phenomena. Previous research has shown that the ratio of Reynolds shear stresses to normal stresses is typically much smaller deep within the canopy than those ratios found at the top of canopy and in the roughness sublayer. The turbulence in this region exhibits significant contributions to all four quadrants of a weighted joint-probability density function of horizontal and vertical velocity fluctuations, yielding the characteristic small Reynolds shear stresses in the flow. The velocity cospectra measured at the base of the canopy show evidence of discrete frequency bands of both positive and negative correlation that yield a small correlation, as indicated by the Reynolds shear stresses. Two major peaks were often observed in the spectra and cospectra: a low-frequency peak that appears to be associated with vortex shedding off the buildings and a midfrequency peak generally associated with canyon geometry. The low-frequency peak was found to produce a countergradient contribution to the along-wind vertical velocity covariance. Standard spectral tests for local isotropy indicate that isotropic conditions occur at different frequencies depending on spatial location, demonstrating the need to be thorough when testing for local isotropy with the urban canopy.

Measurements of Turbulent Flow and Ozone at Rooftop and Sidewalk Sites in a High-Rise Building Area

“…The multiresolution spectrum of horizontal velocity exhibits a turbulence-related local peak at ~84 s (~42 s) at the rooftop (sidewalk) site. These peaks are similar in time scale to the midfrequency (0.01−0.1 Hz) peaks of premultiplied power spectra within street canyons (Nelson et al 2007). This similarity indicates that the peaks can be related to turbulence generated by nearby buildings.…”

“…Many researchers have investigated the characteristics of turbulent flow in urban areas (Roth and Oke 1993;Rotach 1995;Nelson et al 2007Nelson et al , 2011. Louka et al (2000) showed that turbulent fluctuations comprise a large portion of flow in a street canyon.…”

Spectral Coherence and the Statistical Significance of Turbulent Flux Computations

“…Using the 1% critical value as a threshold, these coherence estimates and their underlying w 0 s 0 -based momentum flux estimates would be considered indistinguishable from results that could be obtained from an equivalent-sized random number dataset producing zero flux. This is consistent with the findings of Nelson et al (2007), which showed that deep in the canopy, the off-diagonal turbulent stresses were extremely small compared with the normal turbulent stresses. In contrast, results from the second dataset (Fig.…”

Momentum and Turbulent Kinetic Energy Budgets Within the Park Avenue Street Canyon During the Joint Urban 2003 Field Campaign

“…Thus, the validity of the use of Taylor's hypothesis is highly questionable in this region. It was also found in Nelson et al (2007a) that the onset of the −5/3 slope, typical of the inertial subrange, was not indicative of the onset of local isotropy and that sonic anemometers were unable to adequately resolve the scales where local isotropy occurs. Given these factors, the dissipation rate values reported here should be viewed with caution.…”