3,354 research outputs found
A physical-space version of the stretched-vortex subgrid-stress model for large-eddy simulation
A physical-space version of the stretched-vortex subgrid-stress model is presented and applied to large-eddy simulations of incompressible flows. This version estimates the subgrid-kinetic energy required for evaluation of the subgrid-stress tensor using local second-order structure-function information of the resolved velocity field at separations of order the local cell size. A relation between the structure function and the energy spectrum is derived using the kinematic assumptions of the stretched-vortex model for locally homogeneous anisotropic turbulence. Results of large-eddy simulations using this model are compared to experimental and direct numerical simulation data. Comparisons are shown for the decay of kinetic energy and energy spectra of decaying isotropic turbulence and for mean velocities, root-mean-square velocity fluctuations and turbulence-kinetic energy balances of channel flow at three different Reynolds numbers
Navier-Stokes-alpha model: LES equations with nonlinear dispersion
We present a framework for discussing LES equations with nonlinear
dispersion. In this framework, we discuss the properties of the nonlinearly
dispersive Navier-Stokes-alpha model of incompressible fluid turbulence ---
also called the viscous Camassa-Holm equations and the LANS equations in the
literature --- in comparison with the corresponding properties of large eddy
simulation (LES) equations obtained via the approximate-inverse approach.
In this comparison, we identify the spatially filtered NS-alpha equations
with a class of generalized LES similarity models. Applying a certain
approximate inverse to this class of LES models restores the Kelvin circulation
theorem for the defiltered velocity and shows that the NS-alpha model describes
the dynamics of the defiltered velocity for this class of generalized LES
similarity models. We also show that the subgrid scale forces in the NS-alpha
model transform covariantly under Galilean transformations and under a change
to a uniformly rotating reference frame. Finally, we discuss in the spectral
formulation how the NS-alpha model retains the local interactions among the
large scales, retains the nonlocal sweeping effects of large scales on small
scales, yet attenuates the local interactions of the small scales amongst
themselves.Comment: 15 pages, no figures, Special LES volume of ERCOFTAC bulletin, to
appear in 200
Leray and LANS- modeling of turbulent mixing
Mathematical regularisation of the nonlinear terms in the Navier-Stokes
equations provides a systematic approach to deriving subgrid closures for
numerical simulations of turbulent flow. By construction, these subgrid
closures imply existence and uniqueness of strong solutions to the
corresponding modelled system of equations. We will consider the large eddy
interpretation of two such mathematical regularisation principles, i.e., Leray
and LANS regularisation. The Leray principle introduces a {\bfi
smoothed transport velocity} as part of the regularised convective
nonlinearity. The LANS principle extends the Leray formulation in a
natural way in which a {\bfi filtered Kelvin circulation theorem},
incorporating the smoothed transport velocity, is explicitly satisfied. These
regularisation principles give rise to implied subgrid closures which will be
applied in large eddy simulation of turbulent mixing. Comparison with filtered
direct numerical simulation data, and with predictions obtained from popular
dynamic eddy-viscosity modelling, shows that these mathematical regularisation
models are considerably more accurate, at a lower computational cost.Comment: 42 pages, 12 figure
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