Gradient trajectory analysis of a scalar field with external intermittency
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Cita com:
hdl:2117/190709
Tipus de documentArticle
Data publicació2009-05-10
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Reconeixement-NoComercial-SenseObraDerivada 3.0 Espanya
Abstract
The passive scalar field of a temporally evolving shear layer is investigated using
gradient trajectories as a means to analyse the scalar probability density function and
the conditional scalar dissipation rate in the presence of external intermittency. These
results are of significance for turbulent combustion, where improved predictions of
the statistics of the conditional dissipation rate are needed in several models. First, the
variation of the conventional first and second moments of the conditional dissipation
rate across the layer is quantitatively documented in detail. A strong dependence
of the conditional dissipation rate on the lateral position and on the conditioning
value of the scalar is observed. The dependence on the transverse distance to the
centre-plane partially explains the double-hump profile usually reported when this
dependence is ignored. The variation with the scalar observed in the ratio between the
second and first moments would invalidate certain assumptions commonly done in
turbulent combustion. It is also seen that conditioning on the scalar does not reduce
the fluctuation of the dissipation rate with respect to unconditional values. Next, the
role of external intermittency in these results is investigated. For that purpose, the
flow is partitioned into different zones based on different types of gradient trajectories
passing through each point, thereby introducing non-local information in comparison
with the standard turbulent/non-turbulent separation based on the conventional
intermittency function. In addition to the homogeneous outer regions, three zones are
identified: a turbulent zone, a turbulence interface and quasi-laminar diffusion layers.
The relative contribution from each of these zones to the conventional intermittency
factor is reported. The statistics are then conditioned on each of these zones, and the
spatial variation of the scalar distribution and of the conditional scalar dissipation
rate is explained in terms of the observed zonal statistics. For the Reynolds numbers
of the present simulation, between 1500 and 3000 based on the vorticity thickness
and the velocity difference, and a Schmidt number equal to 1, it results that the major
contribution to both statistics is due to the turbulence interfaces. At the same time,
the turbulent zone shows a distinct behaviour, being approximately homogeneous but
anisotropic.
CitacióMellado, J. P.; Wang, L.; Peters, N. Gradient trajectory analysis of a scalar field with external intermittency. "Journal of fluid mechanics", 10 Maig 2009, vol. 626, p. 333-365.
ISSN0022-1120
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