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Spatial characteristics of turbulent organized structures within the roughness sublayer over idealized urban surface with obstacle-height variability

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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 uniform-height 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 low- and 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.

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Acknowledgements

The comments from the two anonymous reviewers were greatly acknowledged to improve the original version of the manuscript. This research partly used computational resources under the Collaborative Research Program for Young Scientists provided by the Academic Centre for Computing and Media Studies, Kyoto University. This study was supported by the Environment Research and Technology Development Fund (2-1905) of the Environmental Restoration and Conservation Agency of Japan and Japan Society for Promotion of Sciences Kakenhi 18H01680.

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Correspondence to Toshiya Yoshida.

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Yoshida, T., Takemi, T. Spatial characteristics of turbulent organized structures within the roughness sublayer over idealized urban surface with obstacle-height variability. Environ Fluid Mech 21, 129–154 (2021). https://doi.org/10.1007/s10652-020-09764-4

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  • DOI: https://doi.org/10.1007/s10652-020-09764-4

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