Subgrid-scale backscatter in transitional and turbulent flows

Most subgrid-scale (SGS) models for large-eddy simulations are absolutely dissipative (that is, they remove energy from the large scales at each point in the physical space). The actual SGS stresses, however, may transfer energy to the large scales (backscatter) at a given location. Direct numerical simulations of turbulent channel flow and compressible isotropic turbulence are used to study the backscatter phenomena. In all flows considered roughly 50 percent of the grid points were experiencing backscatter when a Fourier cutoff filter was used. The backscatter fraction was less with a Gaussian filter, and intermediate with a box filter in physical space. Moreover, the backscatter and forward scatter contributions to the SGS dissipation were comparable, and each was often much larger than the total SGS dissipation. The SGS dissipation (normalized by total dissipation) increased with filter width almost independently of filter type and Reynolds number. The amount of backscatter showed an increasing trend with Reynolds numbers. In the near-wall region of the channel, events characterized by strong Reynolds shear stress correlated fairly well with areas of high SGS dissipation (both forward and backward). In compressible isotropic turbulence similar results were obtained, independent of fluctuation Mach number.