Spatial characteristics of turbulent organized structures within the roughness sublayer over idealized urban surface wit
- PDF / 9,829,851 Bytes
- 26 Pages / 439.37 x 666.142 pts Page_size
- 95 Downloads / 175 Views
Spatial characteristics of turbulent organized structures within the roughness sublayer over idealized urban surface with obstacle‑height variability Toshiya Yoshida1 · Tetsuya Takemi2 Received: 13 December 2019 / Accepted: 19 August 2020 © Springer Nature B.V. 2020
Abstract The effects of obstacle-height variability on spatial characteristics of turbulent organized structures were investigated with the use of a large-eddy simulation technique for airflows over roughness obstacles. Two-types simulation cases were considered: one is uniformheight case in which uniform-height obstacles are aligned in streamwise direction, the other is height-variability case with staggered higher-height obstacles. Streaky structures were observed above the roughness sublayer (RSL) regardless of obstacle-height variability. When obstacles are uniform, flow fields within the RSL contain low- and high-speed regions along the streamwise streets. When obstacle heights vary, airflow within the RSL collides with the front-facing surfaces of taller obstacles. The statistical features of lowand high-speed structures were examined using the spatial correlations of flow fields centering on strong ejection and sweep, respectively. The ejection– and sweep–center spatial correlations extend forward and backward in the streamwise direction, respectively. Length scales were obtained from the ejection–center and sweep–center spatial correlations. The streamwise lengths vary significantly below the canopy height when obstacles are uniform. In contrast, the streamwise length scales remain nearly constant when obstacle heights vary. The horizontal aspect ratios below the canopy heights indicate that turbulent organized structures over obstacles with variable heights are more isotropic than those over uniform obstacles. The inclination angles of the organized structures were also deduced using the spatial correlations. The angles of sweep–center structures are steeper than those of the ejection–center structures. The angles of the ejection–center structures at the RSL heights become larger with obstacle-height variability. Keywords Turbulent organized structures · Obstacle-height variability · Roughness sublayer · Urban-like canopy · Large-eddy simulation
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1065 2-020-09764-4) contains supplementary material, which is available to authorized users. * Toshiya Yoshida [email protected] 1
Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai‑mura, Japan
2
Disaster Prevention Research Institute, Kyoto University, Uji, Japan
13
Vol.:(0123456789)
Environmental Fluid Mechanics
1 Introduction Understanding how pollutants disperse in air over an urban area is necessary for maintaining air quality and for implementing an appropriate emergency response to an accidental release of hazardous materials. The pollutant dispersion is dominated primarily by turbulent flows in the atmospheric boundary layer. One-dimensional vertic
Data Loading...
