The Experts below are selected from a list of 303 Experts worldwide ranked by ideXlab platform
Parviz Moin - One of the best experts on this subject based on the ideXlab platform.
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a dynamic global coefficient subgrid Scale model for large Eddy simulation of turbulent scalar transport in complex geometries
Physics of Fluids, 2009Co-Authors: Donghyun You, Parviz MoinAbstract:The dynamic global-coefficient subgrid-Scale Eddy-viscosity model by You and Moin [Phys. Fluids 19, 065110 (2007)] is generalized for large-Eddy simulation of turbulent flow with scalar transport. The model coefficient for subgrid-Scale scalar flux which is constant in space but varies in time is dynamically determined based on the “global conservation” of the transport equation for scalar variance. Large-Eddy simulations of turbulent flow with passive scalar transport through a channel and over a backward-facing step show that the present model has a similar predictive capability as the dynamic Smagorinsky model. The present dynamic model is especially suitable for large-Eddy simulation of turbulent flow with scalar transport in complex geometries since it does not require any spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and requires only a single-level test filter. The present model is not more complicated in implementation and not more expensive in ter...
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a dynamic global coefficient subgrid Scale Eddy viscosity model for large Eddy simulation in complex geometries
Physics of Fluids, 2007Co-Authors: Donghyun You, Parviz MoinAbstract:An improvement of the dynamic procedure of Park et al. [Phys. Fluids 18, 125109 (2006)] for closure of the subgrid-Scale Eddy-viscosity model developed by Vreman [Phys. Fluids 16, 3670 (2004)] is proposed. The model coefficient which is globally constant in space but varies in time is dynamically determined assuming the “global equilibrium” between the subgrid-Scale dissipation and the viscous dissipation of which utilization was proposed by Park et al. Like the Vreman model with a fixed coefficient and the dynamic-coefficient model of Park et al., the present model predicts zero Eddy-viscosity in regions where the vanishing Eddy viscosity is theoretically expected. The present dynamic model is especially suitable for large-Eddy simulation in complex geometries since it does not require any ad hoc spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and more importantly, requires only a single-level test filter in contrast to the dynamic model of Park et al., whi...
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application of a dynamic global coefficient subgrid Scale model for large Eddy simulation in complex geometries
ASME JSME 2007 5th Joint Fluids Engineering Conference, 2007Co-Authors: Donghyun You, Parviz MoinAbstract:The application of a dynamic global-coefficient subgrid-Scale Eddy-viscosity model for large-Eddy simulation in complex geometries is presented. The model employs a dynamic procedure for closure of the subgrid-Scale Eddy-viscosity model developed by Vreman [Phys. Fluids 16 , 3670 (2004)]. The model coefficient which is globally constant in space but varies in time is dynamically determined assuming the “global equilibrium” between the subgrid-Scale dissipation and the viscous dissipation of which utilization was proposed by Park et al. [Phys. Fluids 18 , 125109 (2006)]. Like the Vreman’s model with a fixed coefficient and the dynamic-coefficient model of Park et al., the present model predicts zero Eddy-viscosity in regions where the vanishing Eddy viscosity is theoretically expected. The present dynamic model is especially suitable for large-Eddy simulation in complex geometries since it does not require any ad hoc spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and requires only a single-level test filter.Copyright © 2007 by ASME
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a dynamic localization model for large Eddy simulation of turbulent flows
Journal of Fluid Mechanics, 1995Co-Authors: Sandip Ghosal, Thomas S. Lund, Parviz Moin, Knut AkselvollAbstract:In a previous paper, Germano, et al. (1991) proposed a method for computing coefficients of subgrid-Scale Eddy viscosity models as a function of space and time. This procedure has the distinct advantage of being self-calibrating and requires no a priori specification of model coefficients or the use of wall damping functions. However, the original formulation contained some mathematical inconsistencies that limited the utility of the model. In particular, the applicability of the model was restricted to flows that are statistically homogeneous in at least one direction. These inconsistencies and limitations are discussed and a new formulation that rectifies them is proposed. The new formulation leads to an integral equation whose solution yields the model coefficient as a function of position and time. The method can be applied to general inhomogeneous flows and does not suffer from the mathematical inconsistencies inherent in the previous formulation. The model has been tested in isotropic turbulence and in the flow over a backward-facing step.
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a dynamic subgrid Scale Eddy viscosity model
Physics of Fluids, 1991Co-Authors: Massimo Germano, Ugo Piomelli, Parviz Moin, William H CabotAbstract:One major drawback of the Eddy viscosity subgrid‐Scale stress models used in large‐Eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new Eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a p r i o r i. The model is based on an algebraic identity between the subgrid‐Scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐Scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐Eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.
Carsten Eden - One of the best experts on this subject based on the ideXlab platform.
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A global map of meso-Scale Eddy diffusivities based on linear stability analysis
Ocean Modelling, 2013Co-Authors: Lukas Vollmer, Carsten EdenAbstract:Abstract Using a hydrographic climatology, global maps of meso-Scale Eddy kinetic energy (EKE), diffusivities for mixing along isopycnals (isopycnal diffusivity) and for the advective effect of meso-Scale eddies (skew diffusivity) are created using properties of the fastest growing unstable baroclinic waves and a simple ad hoc scaling of the amplitudes from linear stability theory. Amplitudes of EKE compare well with near-surface observational estimates based on satellite data and results of an Eddy-permitting model, but show a low bias in regions where eddies are not generated locally but propagate into, which will likely transfer both to the diffusivities. In agreement with previous studies we find largest diffusivities in the deep Antarctic Circumpolar Current, and in the shallow western boundary and low latitude westward currents. In agreement with analytical consideration, we find that isopycnal diffusivities are increased at the depth of the steering level where unstable waves and mean flow propagate at the same speed, while skew diffusivities exhibit less vertical dependency, and that isopycnal diffusivities are roughly three times larger than skew diffusivities at the steering level. It is shown that the vertical structure of the diffusivities can be explained to a large extent by the effect of the planetary vorticity gradient which leads to a decrease of skew diffusivities at the surface (bottom) and to a downward (upward) shift of the steering level, and thus the maximum of isopycnal diffusivities, for eastward (westward) flow.
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Implementing diffusivities from linear stability analysis in a three-dimensional general circulation ocean model
Ocean Modelling, 2012Co-Authors: Carsten EdenAbstract:Abstract Linear stability analysis is used to predict the vertical and lateral structure for the diffusivity related to meso-Scale Eddy buoyancy and potential vorticity fluxes in three-dimensional primitive equation models, following a suggestion by Peter D. Killworth. Using two idealized numerical models as example, it is shown that the linear stability analysis yields a consistent lateral and vertical structure for both lateral diffusivities. Parameterizations based on isopycnal thickness or potential vorticity diffusion are shown to be equivalent for constant diffusivities in quasi-geostrophic approximation. For spatially varying diffusivities they yield similar results in the model experiments, although the corresponding diffusivities show different vertical structure.
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Implementing diffusivities from linear stability analysis in a three-dimensional general circulation ocean model
Ocean Modelling, 2012Co-Authors: Carsten EdenAbstract:Linear stability analysis is used to predict the vertical and lateral structure for the diffusivity related to meso-Scale Eddy buoyancy and potential vorticity fluxes in three-dimensional primitive equation models, following a suggestion by Peter D. Killworth. Using two idealized numerical models as example, it is shown that the linear stability analysis yields a consistent lateral and vertical structure for both lateral diffusivities. Parameterizations based on isopycnal thickness or potential vorticity diffusion are shown to be equivalent for constant diffusivities in quasi-geostrophic approximation. For spatially varying diffusivities they yield similar results in the model experiments, although the corresponding diffusivities show different vertical structure. (c) 2012 Elsevier Ltd. All rights reserved
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Parameterising meso-Scale Eddy momentum fluxes based on potential vorticity mixing and a gauge term
Ocean Modelling, 2010Co-Authors: Carsten EdenAbstract:Meso-Scale fluctuations are known to drive large-Scale zonal flows in the ocean, a mechanism which is currently missing in non-Eddy-resolving ocean models. A closure for meso-Scale Eddy momentum fluxes is evaluated in a suite of idealised Eddying channel models, featuring Eddy-driven zonal jets. It is shown how the appearance of zonal jets, which act as mixing barriers for turbulent exchange, and reduced lateral diffusivities are linked in a natural way by implementing mixing of potential vorticity and using a gauge term to insure that no Spurious forces are introduced. It appears, therefore, possible to parameterise the appearance of zonal jets and its effect on the ventilation of interior ocean basins in non-Eddy-resolving, realistic ocean models. (C) 2009 Elsevier Ltd. All rights reserved
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Parameterising meso-Scale Eddy momentum fluxes based on potential vorticity mixing and a gauge term
Ocean Modelling, 2009Co-Authors: Carsten EdenAbstract:Meso-Scale fluctuations are known to drive large-Scale zonal flows in the ocean, a mechanism which is currently missing in non-Eddy-resolving ocean models. A closure for meso-Scale Eddy momentum fluxes is evaluated in a suite of idealised Eddying channel models, featuring Eddy-driven zonal jets. It is shown how the appearance of zonal jets, which act as mixing barriers for turbulent exchange, and reduced lateral diffusivities are linked in a natural way by implementing mixing of potential vorticity and using a gauge term to insure that no spurious forces are introduced. It appears, therefore, possible to parameterise the appearance of zonal jets and its effect on the ventilation of interior ocean basins in non-Eddy-resolving, realistic ocean models.
Marc B Parlange - One of the best experts on this subject based on the ideXlab platform.
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on the magnitude and variability of subgrid Scale Eddy diffusion coefficients in the atmospheric surface layer
Journal of the Atmospheric Sciences, 2003Co-Authors: Jan Kleissl, Charles Meneveau, Marc B ParlangeAbstract:Eddy-viscosity closures for large Eddy simulations (LES) of atmospheric boundary layer dynamics include a parameter (Smagorinsky constant cs), which depends upon physical parameters, such as distance to the ground, atmospheric stability, and strain. A field study [Horizontal Arrays Turbulence Study (HATS)] specifically designed to measure turbulence quantities of interest in LES, such as the parameter cs, is conducted. The instrumentation consists of two vertically separated horizontal arrays of 3D sonic anemometers, placed in the atmospheric surface layer. From 2D filtering and differentiating the velocity fields, subgrid-Scale (SGS) and resolved quantities are computed. The parameter cs is obtained from the data by matching measured and modeled SGS dissipations under various flow conditions. Results indicate that cs is reduced near the ground, and also decreases rapidly with increasing stability in stable atmospheric conditions. A simple fit that parameterizes the data is proposed. The variability from one sample to another is studied by means of the probability density function (pdf ) of cs. The pdfs show a most preferred value, which is essentially independent of the timeScale used for statistical averaging. The width of the pdfs decreases with increasing averaging time, for unstable and neutral stability conditions. For stable conditions, the relative variability of the coefficient remains strong even for long averaging times, indicative of strong intermittency. In unstable conditions, cs is fairly independent of local strain-rate magnitude, supporting the basic scaling of the Smagorinsky Eddy viscosity. For stable conditions, a transition occurs between small local strain-rate magnitudes, where cs is nearly constant, and high local strain-rate magnitudes, where cs decreases appreciably. The results suggest that when the filter Scale approaches the local integral Scale of turbulence (height above the ground or Obukhov length), one needs to include the friction velocity as relevant velocity to Scale the Eddy viscosity, in addition to the standard velocity Scale of the Smagorinsky model based on filtered strain-rate magnitude. The analysis is repeated for the SGS heat flux, and for the associated Eddy-diffusion coefficient ( ) and Prandtl number (PrT). The latter is found to depend only very weakly 2 12 Pr c Ts on stability, but it increases with decreasing distance from the ground.
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LOCAL ISOTROPY AND ANISOTROPY IN THE SHEARED AND HEATED ATMOSPHERIC SURFACE LAYER
Boundary-Layer Meteorology, 1995Co-Authors: Gabriel G. Katul, Marc B Parlange, John D. Albertson, Chia R. ChuAbstract:Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of Eddy-motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-Scale and small-Scale Eddy motion, and (2) thermal effects on the small-Scale Eddy motion. In order to explore these two anisotropy sources, we developed statistical measures that are sensitive to such interactions. It was found that the large-Scale/small-Scale interaction was significant in the inertial subrange up to 3 decades below the production Scale, thus reducing the validity of the local isotropy assumption. The anisotropy generated by thermal effects was also significant and comparable in magnitude to the former anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large-Scale/smallScale interaction. The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements. The impact of this anisotropy cancellation on the dynamics of inertial subrange Eddy motion was also considered. For that purpose, the Kolmogorov-Obukhov structure function equation, as derived from the Navier-Stokes equations for locally isotropic turbulence, was employed. The Kolmogorov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closure scheme, which is also based on the local isotropy assumption, was verified and was found to be in good agreement with the measurements. The accepted 0.4 constant skewness value derived from grid turbulence experiments overestimated our measurements. A suggested 0.26 constant skewness value, which we derived from Kolmogorov's constant, was found to be adequate.
Haecheon Choi - One of the best experts on this subject based on the ideXlab platform.
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Dynamic global model for large Eddy simulation of transient flow
Physics of Fluids, 2010Co-Authors: Haecheon Choi, Noma ParkAbstract:In the present study, the dynamic subgrid-Scale Eddy viscosity models with a global model coefficient by Park et al. [Phys. Fluids 18, 125109 (2006)] (called dynamic global models hereafter) are applied to large Eddy simulation of decaying isotropic turbulence to examine their performances in transient flow. The dynamic global model based on the global equilibrium between the subgrid-Scale dissipation and the viscous dissipation fails to predict the temporal behavior of decaying isotropic turbulence. On the other hand, the dynamic global model based on the Germano identity shows an excellent agreement with the experimental data of decaying isotropic turbulence.
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A dynamic subgrid-Scale Eddy viscosity model with a global model coefficient
Physics of Fluids, 2006Co-Authors: Noma Park, Sungwon Lee, Jungil Lee, Haecheon ChoiAbstract:In the present study, a dynamic subgrid-Scale Eddy viscosity model is proposed for large Eddy simulation of turbulent flows in complex geometry. A subgrid-Scale Eddy viscosity model recently proposed by Vreman [Phys. Fluids 16, 3670 (2004)] which guarantees theoretically zero subgrid-Scale dissipation for various laminar shear flows, is considered as a base model. A priori tests with the original Vreman model show that it predicts the correct profile of subgrid-Scale dissipation in turbulent channel flow but the optimal model coefficient is far from universal. A dynamic procedure of determining the model coefficient is proposed based on the “global equilibrium” between the subgrid-Scale dissipation and the viscous dissipation. An important feature of the proposed procedure is that the model coefficient determined is globally constant in space but varies only in time. A posteriori tests of the proposed dynamic model are conducted through large Eddy simulations of forced isotropic turbulence at Reλ=103, tur...
Ugo Piomelli - One of the best experts on this subject based on the ideXlab platform.
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high reynolds number calculations using the dynamic subgrid Scale stress model
Physics of Fluids, 1993Co-Authors: Ugo PiomelliAbstract:The dynamic subgrid‐Scale Eddy viscosity model has been used in the large‐Eddy simulation of the turbulent flow in a plane channel for Reynolds numbers based on friction velocity and channel half‐width ranging between 200 and 2000, a range including values significantly higher than in previous simulations. The computed wall stress, mean velocity, and Reynolds stress profiles compare very well with experimental and direct simulation data. Comparison of higher moments is also satisfactory. Although the grid in the near‐wall region is fairly coarse, the results are quite accurate: the turbulent kinetic energy peaks at y+≂12, and the near‐wall behavior of the resolved stresses is captured accurately. The model coefficient is o(10−3) in the buffer layer and beyond, where the cutoff wave numbers are in the decaying region of the spectra; in the near‐wall region the cutoff wave numbers are nearer the energy‐containing range, and the resolved turbulent stresses become a constant fraction of the resolved stresses....
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a dynamic subgrid Scale Eddy viscosity model
Physics of Fluids, 1991Co-Authors: Massimo Germano, Ugo Piomelli, Parviz Moin, William H CabotAbstract:One major drawback of the Eddy viscosity subgrid‐Scale stress models used in large‐Eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new Eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a p r i o r i. The model is based on an algebraic identity between the subgrid‐Scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐Scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐Eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.
