Thermal Stratification Effects on Turbulence and Dispersion in Internal and External Boundary Layers

Abstract

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 ≤ 1) on turbulence and dispersion following a simulated ruralto-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) 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.

Numerical Simulations of Boundary-Layer Airflow Over Pitched-Roof Buildings

“…In addition to the above there is the question of thermal stratification effects in urban areas. These attracted little attention until one decade ago (Sessa et al 2020). The studies published since then have focused on arrays of idealised cuboid-shape buildings (e.g.…”

Neutrally- and Stably-Stratified Boundary Layers Adjustments to a Step Change in Surface Roughness

“…Numerical work includes those considering the change from a smooth surface to a rough one with 2D upstanding rods conducted by Lee (2015) using direct numerical simulation (DNS), from a urban surface to arrays of cuboids by Tomas et al (2016) and Sessa et al (2018), as well as from a smooth surface to a forest pattern by Lopes et al (2015) using large-eddy simulation (LES). Sessa et al (2020) also studied the influence of stable stratification on the IBL's growth rate developed over cuboids elements using LES and found that the thickness of IBL was significantly reduced by stable thermal stratification. A large variation of the value of n was also reported in cases of R 2 → R 1 or R → S. Rouhi et al (2019) reported n = 0.8 through simulating an open-channel flow using DNS.…”

Impacts of the side ratio of isolated buildings on airflows and pollutant dispersion under different temperature stratifications: Numerical investigations

“…Therefore, the influence of the thermal effect on air flow and pollutant dispersion had attracted extensive attention. Many researchers confirmed that atmospheric stability conditions had a strong influence on the dispersion of pollutants [8,9,11,[21][22][23][24][25][26][27]. Isolated buildings were the most basic units of complex urban structures.…”