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The dynamics of subgrid-scale energy transfer in turbulence is investigated in a database of a planar turbulent jet at Reλ ≈ 110, obtained by direct numerical simulation. In agreement with analytical predictions (Kraichnan 1976), subgrid-scale energy transfer is found to arise from two effects: one involving non-local interactions between the resolved scales and disparate subgrid scales, the other involving local interactions between the resolved and subgrid scales near the cutoff. The former gives rise to a positive, wavenumber-independent eddy-viscosity distribution in the spectral space, and is manifested as low-intensity, forward transfers of energy in the physical space. The latter gives rise to positive and negative cusps in the spectral eddy-viscosity distribution near the cutoff, and appears as intense and coherent regions of forward and reverse transfer of energy in the physical space. Only a narrow band of subgrid wavenumbers, on the order of a fraction of an octave, make the dominant contributions to the latter. A dynamic two-component subgrid-scale model (DTM), incorporating these effects, is proposed. In this model, the non-local forward transfers of energy are parameterized using an eddy-viscosity term, while the local interactions are modelled using the dynamics of the resolved scales near the cutoff. The model naturally accounts for backscatter and correctly predicts the breakdown of the net transfer into forward and reverse contributions in a priori tests. The inclusion of the local-interactions term in DTM significantly reduces the variability of the model coefficient compared to that in pure eddy-viscosity models. This eliminates the need for averaging the model coefficient, making DTM well-suited to computations of complex-geometry flows. The proposed model is evaluated in LES of transitional and turbulent jet and channel flows. The results show DTM provides more accurate predictions of the statistics, structure, and spectra than dynamic eddy-viscosity models and remains robust at marginal LES resolutions.

To investigate dynamics of vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Re τ = 1270. In the outer layer, the vortex clusters are composed of coherent fine-scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kolmogorov length and velocity. The large-scale structure in the outer layer is composed of these clusters of the CFSEs, which contributes to the streamwise velocity deficit (i.e. low-momentum region). The CFSE clusters are observed in the low-momentum regions of the outer layer, and the scale of those clusters tends to be enlarged with the increase of a distance from the wall. The dynamics of large-scale structures reveals that the cluster structure generated in the bottom of the logarithmic region moves downstream and its scale increases with the increase of the low-momentum region. The CFSE clusters in the low-momentum regions of u ≤ −u rms consist of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The process of destruction of the CFSE cluster is also clarified in the outer layer.

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To investigate a relation between vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Re = 1270. The vortex clusters in the outer layer consist coherent fme scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kohnogorov length and the Kohnogorov velocity. The CFSE clusters are inside the large-scale structure, which contributes to the streamwise velocity deficit. The scale of those clusters tends to be enlarged with the increase of a distance from the wall. The CFSE clusters are composed of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The most expected maximum azimuthal velocity of the CFSEs in these low-momentum regions of the outer layer is 30~70% faster compared with those of the CFSEs in unconditioned regions (i.e. all regions of the outer layer), while the most expected diameter of the CFSEs is not changed greatly.

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