The dynamics of concentration fluctuations within passive scalar plumes in a turbulent neutral boundary layer

Autor(en)

Andreas Stohl, Massimo Cassiani, Hamidreza Ardeshiri, Ignacio Pisso, P Salizzoni, M Marro, Kerstin Stebel, S.-Y. Park

Abstrakt

We investigate the concentration fluctuations of passive scalar plumes emitted from small, localised (point-like) steady sources in a neutrally stratified turbulent boundary layer over a rough wall. The study utilises high-resolution large-eddy simulations for sources of varying sizes and heights. The numerical results, which show good agreement with wind-tunnel studies, are used to estimate statistical indicators of the concentration field, including spectra and moments up to the fourth order. These allow us to elucidate the mechanisms responsible for the production, transport and dissipation of concentration fluctuations, with a focus on the very near field, where the skewness is found to have negative values – an aspect not previously highlighted. The gamma probability density function is confirmed to be a robust model for the one-point concentration at sufficiently large distances from the source. However, for ground-level releases in a well-defined area around the plume centreline, the Gaussian distribution is found to be a better statistical model. As recently demonstrated by laboratory results, for elevated releases, the peak and shape of the pre-multiplied scalar spectra are confirmed to be independent of the crosswind location for a given downwind distance. Using a stochastic model and theoretical arguments, we demonstrate that this is due to the concentration spectra being directly shaped by the transverse and vertical velocity components governing the meandering of the plume. Finally, we investigate the intermittency factor, i.e. the probability of non-zero concentration, and analyse its variability depending on the thresholds adopted for its definition.

Organisation(en)

Institut für Meteorologie und Geophysik

Journal

Journal of Fluid Mechanics

Band

1001

Anzahl der Seiten

39

ISSN

0022-1120

DOI

https://doi.org/10.1017/jfm.2024.861

Publikationsdatum

12-2024

Peer-reviewed

Ja

ÖFOS 2012

105206 Meteorologie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/ae0d47ab-c7c2-41e3-b363-27c30aabfdb5