On the influence of coherent structures upon interscale interactions in turbulent plane jets (original) (raw)

2002, Journal of Fluid Mechanics

The influence of the coherent structures on grid/subgrid-scale (GS/SGS) interactions in free shear layers is analysed through the application of a top-hat filter to several plane jet direct numerical simulations (DNS). The Reynolds number based on the plane jet inlet slot width is Re h = 3000. The study deals with energy containing (Kelvin-Helmholtz) and inertial range (streamwise) vortices, from the far field of the turbulent plane jet. The most intense kinetic energy exchanges between GS and SGS occur near these structures and not randomly in the space. The GS kinetic energy is dominated by GS advection and GS pressure/velocity interactions which appear located next to the Kelvin-Helmholtz rollers. Surprisingly, GS/SGS transfer is not very well correlated with the coherent vortices and GS/SGS diffusion plays an important role in the local dynamics of both GS and SGS kinetic energy. The so-called 'local equilibrium assumption' holds globally but not locally as most viscous dissipation of SGS kinetic energy takes place within the vortex cores whereas forward and backward GS/SGS transfer occurs at quite different locations. Finally, it was shown that SGS kinetic energy advection may be locally large as compared to the other terms of the SGS kinetic energy transport equation. Another aspect that is beginning to receive increased attention is the effect and relation of coherent structures on GS/SGS interactions. Coherent structures arise naturally in many turbulent flows and it has been argued for a long time that they govern most of the energy of the turbulent flow and are responsible for most of the transfers of mass and momentum, whence their key role in turbulence. Much work concerning coherent structures and local turbulence dynamics was carried out by ) but only recently have the first studies on the role of coherent structures in GS/SGS interactions appeared. analysed the correlation of typical events from wall flows (sweeps and ejections) and GS/SGS transfer. For instance, in the buffer layer, forward scatter is associated with ejections, backward scatter with sweeps. Since upward and downward fluid motions are connected with streamwise vortices existing in the buffer layer, a strong correlation exists between these coherent structures and regions of positive/negative GS/SGS transfer. Regions of forward transfer are located in the upwash side of a streamwise vortex leg, backward transfer appears in the downwash region. Lin (1999) used LES to analyse the same phenomena in a planetary convective boundary layer, arriving at similar conclusions. Jiménez (1999) explained how the presence of vortices of different sizes at different wall locations, could promote a mechanism of backward energy cascade. In a series of articles, Horiuti (1995, 1996, 1997) studied in great detail the phenomena of spatial correlation between forward/backward scatter and coherent vortices in a mixing layer. He observed that the 'SGS production', taking place in the streamwise vortices, is positive (forward cascade) in the regions corresponding to the first and third quadrants of the rib vortices and negative (backward cascade) in the second and fourth quadrants. This was explained by the particular field of deformation rate caused by these streamwise structures. Horiuti analysed the types of vortical structure which are mostly responsible for the forward and backward cascade, in homogeneous isotropic turbulence. He saw that forward scatter primarily occurs along the flat sheet region, similar to the Burgers vortex layer, whereas the backward cascade mainly arises in the vortex tube core regions. Using turbulent wake laboratory experiments, O'Neil & Meneveau (1997) observed a strong influence between the von Kármán-street vortices and the so-called 'surrogate' (τ 11 S < 11 ) subgrid-scale dissipation. The higher values of this quantity always appear ahead of the big rollers. The results of this study put in question the hypothesis of small scales universality, since the observed strong correlation between large and much smaller scales implies the existence of some level of anisotropy at small scales. This point was previously suggested by Piomelli, Coleman & Kim (1997) and Liu, who studied the evolution of the subgrid-scales in non equilibrium turbulent flows. They showed that although it is true that subgrid-scales are more isotropic and adjust more rapidly from non-equilibrium situations than the large scales (Piomelli et al. 1997), they can be highly anisotropic and require a finite time to adjust themselves from external (large scale) effects. Recently, some attention was given to the influence