UCL Discovery
UCL home » Library Services » Electronic resources » UCL Discovery

Micro structure and Lagrangian statistcs of the scalar field with a mean gradient in isotropic turbulence

Brethouwer, G; Hunt, JCR; Nieuwstadt, FTM; (2003) Micro structure and Lagrangian statistcs of the scalar field with a mean gradient in isotropic turbulence. Journal of Fluid Mechanics , 474 193 - 225. 10.1017/S0022112002002549. Green open access

[thumbnail of download1.pdf]
Preview
PDF
download1.pdf
Available under License : See the attached licence file.

Download (1MB)

Abstract

This paper presents an analysis and numerical study of the relations between the small-scale velocity and scalar fields in fully developed isotropic turbulence with random forcing of the large scales and with an imposed constant mean scalar gradient. Simulations have been performed for a range of Reynolds numbers from Reλ = 22 to 130 and Schmidt numbers from Sc = 1/25 to 144. The simulations show that for all values of Sc [gt-or-equal, slanted] 0.1 steep scalar gradients are concentrated in intermittently distributed sheet-like structures with a thickness approximately equal to the Batchelor length scale η/Sc[fraction one-half] with η the Kolmogorov length scale. We observe that these sheets or cliffs are preferentially aligned perpendicular to the direction of the mean scalar gradient. Due to this preferential orientation of the cliffs the small-scale scalar field is anisotropic and this is an example of direct coupling between the large- and small-scale fluctuations in a turbulent field. The numerical simulations also show that the steep cliffs are formed by straining motions that compress the scalar field along the imposed mean scalar gradient in a very short time period, proportional to the Kolmogorov time scale. This is valid for the whole range of Sc. The generation of these concentration gradients is amplified by rotation of the scalar gradient in the direction of compressive strain. The combination of high strain rate and the alignment results in a large increase of the scalar gradient and therefore in a large scalar dissipation rate. These results of our numerical study are discussed in the context of experimental results (Warhaft 2000) and kinematic simulations (Holzer & Siggia 1994). The theoretical arguments developed here follow from earlier work of Batchelor & Townsend (1956), Betchov (1956) and Dresselhaus & Tabor (1991).

Type: Article
Title: Micro structure and Lagrangian statistcs of the scalar field with a mean gradient in isotropic turbulence
Open access status: An open access version is available from UCL Discovery
DOI: 10.1017/S0022112002002549
Publisher version: http://dx.doi.org/10.1017/S0022112002002549
Language: English
Additional information: © 2003 Cambridge University Press
Keywords: FIELD, gradient, Micro, Structure
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/29879
Downloads since deposit
244Downloads
Download activity - last month
Download activity - last 12 months
Downloads by country - last 12 months

Archive Staff Only

View Item View Item