The role of coherent structures in subgrid-scale energy transfer within the log layer of wall turbulence

Natrajan, Vinay K.; Christensen, Kenneth T.

doi:10.1063/1.2206811

Cited by 46 publications

(54 citation statements)

References 38 publications

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“…Numerical results (3D LES) by Taylor et al (2005) also support the previously described trends in <uw>. (2000) and Natrajan and Christensen (2006). These packets align coherently in the streamwise direction to generate large regions of uniform streamwise momentum deficit, which maintain coherence over large streamwise distances.…”

Section: Investigation Of the Intermittent Motions With Quadrant Decosupporting

confidence: 67%

“…This dynamical arrangement creates multiple uniform momentum zones and lower velocity toward the wall. The vertical extent of these structures, or packets, is about 0.6H, with H the water depth, and the streamwise length is about 1.5H (Natrajan and Christensen 2006). The size of the individual hairpin vortices varies.…”

Section: Investigation Of the Intermittent Motions With Quadrant Decomentioning

confidence: 99%

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An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

Nijs

Pietrzak

2011

Ocean Dynamics

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Measurements of turbulent fluctuations of velocity, salinity, and suspended particulate matter (SPM) are presented. The data show persistent countergradient buoyancy fluxes. These countergradient fluxes are controlled by the ratio of vertical turbulent kinetic energy (VKE) and available potential energy (APE) terms in the buoyancy flux equation. The onset of countergradient fluxes is found to approximately coincide with larger APE than VKE. It is shown here that the ratio of VKE to APE can be written as the square of a vertical Froude number. This number signifies the onset of the dynamical significance of buoyancy in the transport of mass. That is when motions driven by buoyancy begin to actively determine the vertical turbulent transport of mass. Spectral and quadrant analyses show that the occurrence of countergradient fluxes coincides with a change in the relative importance of turbulent energetic structures and buoyancydriven motions in the transport of mass. Furthermore, these analyses show that with increasing salinity-induced Richardson number (Ri), countergradient contributions expand to the larger scales of motions and the relative importance of outward and inward interactions increases. At the smaller scales, at moderate Ri, the countergradient buoyancy fluxes are physically associated with an asymmetry in transport of fluid parcels by energetic turbulent motions. At the large scales, at large Ri, the countergradient buoyancy fluxes are physically associated with convective motions induced by buoyancy of incompletely dispersed fluid parcels which have been transported by energetic motions in the past. Moreover, these convective motions induce restratification and enhanced settling of SPM. The latter is generally the result of salinity-induced convective motions, but SPM-induced buoyancy is also found to play a role.

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Section: Investigation Of the Intermittent Motions With Quadrant Decosupporting

confidence: 67%

Section: Investigation Of the Intermittent Motions With Quadrant Decomentioning

confidence: 99%

An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

Nijs

Pietrzak

2011

Ocean Dynamics

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“…The presence of backscatter is thought to play an important role on the local dynamics of the turbulent flow. For example, recent experimental studies have shown that in turbulent boundary layers strong backscatter and forward-scatter events occur in the leading and trailing edges, respectively, of inclined vortical (hairpin-like) structures (Carper and Porté-Agel 2004;Natrajan and Christensen 2006). In LES, SFS transfer rates of resolved kinetic energy are especially important since the reproduction of their mean values has been shown to be a necessary condition for a simulation to produce accurate higher-order velocity statistics (Meneveau and Katz 2000).…”

Section: Les and Sfs Modellingmentioning

confidence: 98%

Subfilter-Scale Fluxes over a Surface Roughness Transition. Part II: A priori Study of Large-Eddy Simulation Models

Carper

Porté‐Agel

2008

Boundary-Layer Meteorol

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The ability of subfilter-scale (SFS) models to reproduce the statistical properties of SFS stresses and energy transfers over heterogeneous surface roughness is key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. In this study, several SFS models are evaluated a priori using experimental data acquired downwind of a rough-to-smooth transition in a wind tunnel. The SFS models studied include the eddy-viscosity, similarity, non-linear and a mixed model consisting of a combination of the eddy-viscosity and non-linear models. The dynamic eddy-viscosity model is also evaluated. The experimental data consist of vertical and horizontal planes of high-spatial-resolution velocity fields measured using particle image velocimetry. These velocity fields are spatially filtered and used to calculate SFS stresses and SFS transfer rates of resolved kinetic energy. Coefficients for each SFS model are calculated by matching the measured and modelled SFS energy transfer rates. For the eddy-viscosity model, the Smagorinsky coefficient is also evaluated using a dynamic procedure. The model coefficients are found to be scale dependent when the filter scales are larger than the vertical measurement height and fall into the production subrange of the turbulence where the flow scales are anisotropic. Near the surface, the Smagorinsky coefficient is also found to decrease with distance downwind from the transition, in response to the increase in mean shear as the flow adjusts to the smooth surface. In a priori tests, the ability of each model to reproduce statistical properties of the SFS stress is assessed. While the eddy-viscosity model has low spatial correlation with the measured stress, it predicts mean stresses with the same accuracy as the other models. However, the deficiency of the eddy-viscosity model is apparent in the underestimation of the standard deviation of the SFS stresses and the inability to predict transfers of kinetic energy from the subfilter scales to the resolved scales. Overall, the mixed model is found to have the best performance.

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“…It may be said that the reduction of the turbulent shear stress and the turbulent energy by filtering is related more to losing of the vortex structure than the streak structure. Natrajan & Christensen 43) also showed that the vortex structures are important in energy transfer between the filtered and sub-filter scale flow.…”

Section: Filtering Of Flow Over Flat Smooth Surfacementioning

confidence: 99%

Direct Numerical Simulation and Filtering of Turbulent Flows over Model Rough Surfaces

Nakayama¹

2009

gseku.e

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The role of coherent structures in subgrid-scale energy transfer within the log layer of wall turbulence

Cited by 46 publications

References 38 publications

An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

Subfilter-Scale Fluxes over a Surface Roughness Transition. Part II: A priori Study of Large-Eddy Simulation Models

Direct Numerical Simulation and Filtering of Turbulent Flows over Model Rough Surfaces

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