Conditional
subgrid force and dissipation in locally isotropic and rapidly
strained turbulence
Charles Meneveau and Joseph Katz
Department of Mechanical Engineering
The Johns Hopkins University
Baltimore MD 21218
ABSTRACT: Modeling of small scales of turbulence for
Large Eddy Simulation (LES) requires sound understanding
of the relationships between large and small scales of motion.
Conditional a veraging allows to isolate the effects of specific
large-scale flow features in a statistically meaningful fashion.
We study how regions of large-scale straining, rotation,
and energy cascade rate affect SGS quantities of direct dynamical
relevance for LES, namely the subgrid-scale (SGS) force (the
divergence of the SGS stress tensor), and the SGS dissipation
rate. Conditional averages are measured from experimental
data in two flows: (i) the far-field of a turbulent jet,
and (ii) initially near-isotropic turbulence undergoing rapid
axisymmetric expansion. These data are representative of
near-equilibrium, locally isotropic turbulence, and of a
non-isotropic, non-equilibrium turbulent flow, respectively.
The separation between large and small scales is performed
with a spatially compact box filter. Results for the locally
isotropic data show that the SGS force surrounding points
of large strain-rate magnitude is nearly radial. Due to the
divergence-free condition of the velocity field, such SGS
force can only affect the resolved pressure field. Being
directed outwards, the SGS force decrease the resolved pressure
in regions of high strain-rate magnitude. Similar results
are obtained in regions of large resolved dissipation, defined
as the energy flux into the smallest resolved band of scales.
No effect of the resolved vorticity is found. In the rapidly
distorted flow, the SGS force in regions of large positive
resolved dissipation is found to affect both the resolved
pressure (again decreasing it) and the resolved velocity
(the SGS motions oppose the mean deformation). Regions of
large negative resolved dissipation (backscatter) exhibit
the opposite effect, acting in such a way as to favor the
mean deformation. The mixed model is shown to provide better
predictions of conditional SGS dissipation than the Smagorinksy
or the similarity models alone. However, observed discrepancies
in the conditional SGS force imply the need for further model
improvements.
Phys.
Fluids 11 (1999), p. 2317
full
pdf article (©AIP, see http://www.ojps.aip.org/phf)
Reused with permission from Charles Meneveau,
Physics of Fluids, 11, 2317 (1999). Copyright 1999, American
Institute of Physics.