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# Thermal stratification effects on turbulence and dispersion in internal and external boundary layers

Sessa, Vincenzo, Xie, Zheng-Tong and Herring, Steven (2020) Thermal stratification effects on turbulence and dispersion in internal and external boundary layers. Boundary-Layer Meteorology, 176 (1), 61-83.

Record type: Article

## Abstract

A synthetic turbulence and temperature fluctuation generation method embedded in large-eddy simulations (LES) was developed to investigate the effects of weakly stable stratification (i.e. with Richardson number $Ri\leq1$) on turbulence and dispersion following a rural-to-urban transition region. The work was based on firstly validating predictions of mean velocity, turbulent stresses, and point-source dispersion against wind-tunnel experiments of a stable boundary layer approaching a regular array of uniform cuboid elements at $Ri=0.21$. The depth of the internal boundary layer (IBL) formed at the leading edge of the uniform array was determined using the method which we have previously proposed. Vertical profiles of wall-normal turbulent stress showed that the height and the growth rate of the IBL were sensitive to the thermal stability prescribed at the inlet. We found that the IBL height was reduced when the inflow turbulence kinetic energy ($TKE$) was reduced while maintaining the same stratification condition. Lastly, scalar fluxes and mean concentrations within and above the canopy from a ground-level line source were simulated and analyzed. It was found that increasing the stable stratification level reduced the vertical transport of the pollutant, which increased the volume-averaged concentration within the canopy. For a given level of stable stratification, the effect on the total scalar fluxes within and above the canopy, and on the volume-averaged mean concentration within the lateral streets is more pronounced when the $TKE$ is reduced.

Text
blm2019 - Author's Original

Submitted date: 12 August 2019
Accepted/In Press date: 13 April 2020
Published date: 22 May 2020
Keywords: IBL, dispersion, inflow turbulence, stable stratification

## Identifiers

Local EPrints ID: 436815
URI: http://eprints.soton.ac.uk/id/eprint/436815
ISSN: 0006-8314
PURE UUID: 2bbceef1-a83f-479a-b695-97279d4ae486
ORCID for Zheng-Tong Xie: orcid.org/0000-0002-8119-7532

## Catalogue record

Date deposited: 10 Jan 2020 17:31

## Contributors

Author: Vincenzo Sessa
Author: Zheng-Tong Xie
Author: Steven Herring