Subgrid-scale, SGS

It is then assumed that due to this separation in scales, the so-called subgrid scale (SGS) modeling is largely geometry independent because of the universal behavior of turbulence at the small scales. The SGS eddies are therefore more close to the ideal concept of isotropy (according to which the intensity of the fluctuations and their length scale are independent of direction) and, hence, are more susceptible to the application of Boussinesq s concept of turbulent viscosity (see page 163). 

Reynolds, 27, 28, 81, 84 subgrid-scale (SGS), 33, 34 surface shear, 114 stretch, 69, 90 flame, 69 structure... 

Due to the importance of turbulence in spray systems, this topic is treated in some detail. It includes a summary of time averaging and spatial filtering, followed by a description of RANS and LES turbulence modeling. The RANS model that is presented is the k—s turbulence model, and the LES SGS models that are outlined include the Smagorinsky model and the one-equation subgrid scale (SGS) model. 

In LESs (see, e.g., Crowe et al, 1996 Van den Akker, 2006), most of the turbulent motions of the flow field of the carrier phase are resolved. A particle being tracked in an LES flow field feels all the associated velocity fluctuations. Now, just the subgrid scale (SGS) fluctuations are not resolved while they still may have an effect on the particle motions modeling their effect is usually done by means of a Gaussian stochastic procedure (Derksen et al, 2008), on the basis of the idea that these SGS velocity fluctuations (in the absence of particles) are more or less isotropic. It may be anticipated, however, that the resolved velocity fluctuations are much stronger than the SGS fluctuations. When, in addition, the particle relaxation time is larger than the time step applied in the LES, Derksen (2003) argued that particle motion is hardly influenced by the SGS velocities. 

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