Turbulence statistics of flow over a drag-reducing and a drag-increasing riblet-mounted surface

Turbulence statistics of turbulent flow over a drag-reducing and a drag-increasing riblet-mounted surface are investigated by using a DNS dataset. With a generalized FIK identity (Fukagata et al. (2002) [42]), we find that the suppression of the weighted Reynolds shear stress and spatial heterogeneity are both responsible for the drag reduction with riblets; while the drag increase in the other case is mostly related to the increasing contribution of spatial heterogeneity. From the analysis of anisotropy-invariant mapping, we observed that the local drag reductions near the riblet valley are accompanied by increasing turbulent anisotropy and forcing the near-wall turbulent structures at small scales to restructure and satisfy constraint of local oneor two-dimensionality, and the local drag increases near the riblet tip are accompanied by enhancing the turbulent anisotropy. (c) 2020 Elsevier Masson SAS. All rights reserved.