Direct modelling of subgrid scales of turbulence in large eddy simulations

Subgrid scale (SGS) models for large eddy simulations of turbulent flows fall into two general categories. One category consists of the models that provide expressions for the SGS terms such as a stress tensor or a heat flux and usually employ eddy viscosity concepts. The other category models the unresolved primitive variables such as a velocity or a temperature and the SGS stresses are secondary quantities that are computed directly from the definitions. We discuss the fundamentals of the latter category of SGS models and review several specific approaches, including the defiltering or deconvolution procedures and various estimation models.