A new length scale for DNS of wall-bounded turbulent flows.

Comparison of model and DNS profiles for fifth-order velocity moments in a turbulent channel flow under the strain and in a turbulent boundary layer over a flat plate revealed small discrepancies in the buffer zone of both flows. The discrepancies were found to be associated with the magnitude of velocity fluctuations in the streamwise direction, the turbulence production by the shear in the transport equations of odd (third- and fifth order) velocity moments, and with the balance errors in the DNS budgets of the odd velocity moments, all reaching their maximum in the flow buffer zone at close locations. Previous studies confirmed adequacy of convergency of the DNS data used in the study but pointed toward potential issues with the grid resolution near a wall (Poroseva et al. AIAA2016-3940). The standard practice in generating grids for DNS is the use of the Kolmogorov length scale as a guide. This criterion was also applied to obtain the DNS data used in the study. Yet, the theory of local isotropy, where this length scale comes from, is not applicable near the flow boundaries (Kolmogorov, 1941). A new length scale will be presented for the buffer zone to improve the quality of relevant statistics collected from DNS.