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

建物高さのばらつきを伴う理想都市地表面上におけるroughness sublayer内の乱流組織構造の空間特性

吉田 敏哉   ; 竹見 哲也*

Yoshida, Toshiya; Takemi, Tetsuya*

本研究では、建物高さのばらつきが乱流組織構造の空間特性に及ぼす影響を調べた。そのために、高さのばらつきがない場合、中程度の場合、大きい場合の3ケースの建物群上乱流を対象としたLarge-eddy simulationを実行した。乱流組織構造の統計的特徴を調べるため、強い運動量輸送を伴う低速流および高速流の空間相関分布を算出した。これらの相関分布から組織構造の長さスケールを見積もったところ、高さのばらつきのない建物群では、建物高さ以下における組織構造の主流方向長さは鉛直方向に大きく変化することが分かった。一方、高さのばらつきを伴う建物群では、その主流方向長さはほぼ一定の値を示した。また、長さスケールから算出した水平方向のアスペクト比により、高さのばらつきを伴う建物群上の乱流組織構造は、高さ一定の場合に比べて、より等方的な形状を有することが分かった。

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.