2,576 research outputs found
Optimal model parameters for multi-objective large-eddy simulations
A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are "near optimal," and refer to such regions as "multi-objective optimal parameter regions." We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study
Proper Orthogonal Decomposition Closure Models For Turbulent Flows: A Numerical Comparison
This paper puts forth two new closure models for the proper orthogonal
decomposition reduced-order modeling of structurally dominated turbulent flows:
the dynamic subgrid-scale model and the variational multiscale model. These
models, which are considered state-of-the-art in large eddy simulation,
together with the mixing length and the Smagorinsky closure models, are tested
in the numerical simulation of a 3D turbulent flow around a circular cylinder
at Re = 1,000. Two criteria are used in judging the performance of the proper
orthogonal decomposition reduced-order models: the kinetic energy spectrum and
the time evolution of the POD coefficients. All the numerical results are
benchmarked against a direct numerical simulation. Based on these numerical
results, we conclude that the dynamic subgrid-scale and the variational
multiscale models perform best.Comment: 28 pages, 6 figure
Large-eddy simulation of the flow in a lid-driven cubical cavity
Large-eddy simulations of the turbulent flow in a lid-driven cubical cavity
have been carried out at a Reynolds number of 12000 using spectral element
methods. Two distinct subgrid-scales models, namely a dynamic Smagorinsky model
and a dynamic mixed model, have been both implemented and used to perform
long-lasting simulations required by the relevant time scales of the flow. All
filtering levels make use of explicit filters applied in the physical space (on
an element-by-element approach) and spectral (modal) spaces. The two
subgrid-scales models are validated and compared to available experimental and
numerical reference results, showing very good agreement. Specific features of
lid-driven cavity flow in the turbulent regime, such as inhomogeneity of
turbulence, turbulence production near the downstream corner eddy, small-scales
localization and helical properties are investigated and discussed in the
large-eddy simulation framework. Time histories of quantities such as the total
energy, total turbulent kinetic energy or helicity exhibit different evolutions
but only after a relatively long transient period. However, the average values
remain extremely close
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