Joint PDF modelling of turbulent flow and dispersion in an urban street canyon

TL;DR

This paper introduces a novel particle-in-cell Monte Carlo method to compute the full joint PDF of turbulent velocity and scalar concentration in urban street canyons, capturing complex flow and dispersion characteristics.

Contribution

It presents the first computation of the full joint PDF of velocity and scalar concentration in an urban environment using advanced PDF modeling techniques.

Findings

Method accurately captures turbulence and dispersion statistics.

Comparison with experimental data and LES shows good agreement.

Boundary layer modeling impacts the results significantly.

Abstract

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.