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Mean-field closure parameters for passive scalar turbulence
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
Vise andre og tillknytning
2012 (engelsk)Inngår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 86, nr 1, s. 018406-Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
2012. Vol. 86, nr 1, s. 018406-
Emneord [en]
Tau-Approximation, Simulations, Model, Dynamics, Diffusion, Transport, Stresses
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-100171DOI: 10.1088/0031-8949/86/01/018406ISI: 000306245200036Scopus ID: 2-s2.0-84863753530OAI: oai:DiVA.org:kth-100171DiVA, id: diva2:542956
Forskningsfinansiär
Swedish Research Council, 621-2011-5076
Merknad

QC 20120806

Tilgjengelig fra: 2012-08-06 Laget: 2012-08-06 Sist oppdatert: 2017-12-07bibliografisk kontrollert

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Snellman, Jan EskilRheinhardt, MatthiasKäpylä, Petri J.Mantere, Marit J.Brandenburg, Axel
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