Subgrid-scale energy transfer in the near-wall region of turbulent flows

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Abstract
Direct numerical simulation databases of turbulent channel and pipe flow have been used in order to assess the energy transfer between resolved and unresolved motions in large-eddy simulations. To this end, the velocity fields are split into three parts: a statistically stationary mean flow, the resolved, and the unresolved turbulent fluctuations. The distinction between the resolved and unresolved motions is based on the application of a cutoff filter in spectral space. Within the buffer layer a backward transfer of averaged kinetic energy from subgrid to grid-scale turbulent motions has been found to exist, which is primarily caused by subgrid-scale stresses aligned with the mean rates of strain. Such reverse transfer generally cannot be described by the simple eddy-viscosity-type subgrid models usually applied in large-eddy simulations. The use of a conditional averaging technique revealed that the reverse transfer of energy within the near-wall flow is strongly enhanced by coherent motions, such as the well-known bursting events.