A synthetic-turbulence and temperature-fluctuation-generation method is developed and embedded in large-eddy simulations to investigate the effects of weak stable stratification (i.e. Richardson number $$Ri\le 1$$ R i ≤ 1 ) on turbulence and dispersion following a simulated rural-to-urban transition. The modelling approach is validated by comparing predictions of mean velocity, turbulent stresses, and point-source dispersion against data from a wind-tunnel experiment that simulates a stable atmospheric boundary layer ( $$Ri=0.21$$ R i = 0.21 ) approaching a regular array of uniform rectangular blocks. The depth of the internal boundary layer (IBL) that develops from the leading edge of the block array is determined using the wall-normal turbulent stress method proposed by Sessa et al. (J Wind Eng Ind Aerodyn 182:189–291, 2018). This shows that the depth and growth rate of the IBL are sensitive to the thermal stability and the turbulence kinetic energy (TKE) prescribed at the inlet, such that the IBL depth reduces as the TKE of the inflow is reduced while maintaining the same Ri , or as the Ri is increased while maintaining the same inflow TKE. When a ground level line source is introduced it is found that increasing Ri evidently reduces the vertical scalar fluxes at the canopy height, while increasing the mean concentrations within the streets. Furthermore, as with IBL development it is found that for a given value of Ri the effect of stratification becomes more pronounced as the inflow level of TKE is reduced, affecting scalar fluxes within and above the canopy, and volume-averaged mean concentrations within the streets.

中文翻译：

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热分层对内外边界层湍流和色散的影响

开发了一种合成湍流和温度波动生成方法并将其嵌入到大涡模拟中，以研究弱稳定分层（即理查森数 $$Ri\le 1$$R i ≤ 1 ）对湍流和色散的影响模拟从农村到城市的转变。通过将平均速度、湍流应力和点源色散的预测与模拟稳定大气边界层 ($$Ri=0.21$$R i = 0.21 ) 的风洞实验数据进行比较来验证建模方法。均匀矩形块的规则阵列。从块阵列的前沿发展的内部边界层 (IBL) 的深度是使用 Sessa 等人提出的壁法向湍流应力方法确定的。(J Wind Eng Ind Aerodyn 182:189–291, 2018)。这表明 IBL 的深度和生长速率对入口处规定的热稳定性和湍动能 (TKE) 敏感，因此 IBL 深度随着流入 TKE 的减小而减小，同时保持相同的 Ri，或随着 Ri 增加，同时保持相同的流入 TKE。当引入地面线源时，发现增加 Ri 明显降低了冠层高度的垂直标量通量，同时增加了街道内的平均浓度。此外，与 IBL 的发展一样，发现对于给定的 Ri 值，随着 TKE 流入水平的降低，分层的影响变得更加明显，影响冠层内和上方的标量通量，以及街道内的体积平均平均浓度.