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Mean-field closure parameters for passive scalar turbulence
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2012 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 86, no 1, 018406Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNSs) of isotropically forced homogeneous stationary turbulence with an imposed passive scalar concentration gradient are compared with an analytical closure model which provides evolution equations for the mean passive scalar flux and variance. Triple correlations of fluctuations appearing in these equations are described in terms of relaxation terms proportional to the quadratic correlations. Three methods are used to extract the relaxation timescales tau(i) from DNSs. Firstly, we insert the closure ansatz into our equations, assume stationarity and solve for tau(i). Secondly, we use only the closure ansatz itself and obtain tau(i) from the ratio of quadratic and triple correlations. Thirdly, we remove the imposed passive scalar gradient and fit an exponential law to the decaying solution. We vary the Reynolds (Re) and Peclet numbers (while fixing their ratio at unity) and the degree of scale separation and find for large Re a fair correspondence between the different methods. The ratio of the turbulent relaxation time of the passive scalar flux to the turnover time of the turbulent eddies is of the order of 3, which is in remarkable agreement with earlier work. Finally, we make an effort to extract the relaxation timescales relevant for the viscous and diffusive effects. We find two regimes that are valid for small and large Re, respectively, but the dependence of the parameters on scale separation suggests that they are not universal.

Place, publisher, year, edition, pages
2012. Vol. 86, no 1, 018406
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-80412DOI: 10.1088/0031-8949/86/01/018406ISI: 000306245200036OAI: diva2:556316


Available from: 2012-09-25 Created: 2012-09-19 Last updated: 2016-07-01Bibliographically approved

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Brandenburg, Axel
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Department of AstronomyNordic Institute for Theoretical Physics (Nordita)
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