Contributions to pressure drag in rough-wall turbulent flows: Insights from force partitioning

The Force Partitioning Method is employed to decompose the hydrodynamic drag in rough-wall turbulent channel flows. The contributions of vortex and strain dominated regions on the pressure drag are quantified using an auxiliary surface-dependent potential field $\ensuremath{\phi}$. Different sources of drag are identified, and their relative importance quantified. The \ensuremath{\mathit{Q}}-induced force (where \ensuremath{\mathit{Q}} is the second invariant of the velocity gradient tensor) is responsible for about 50% of the rough-wall drag, and is mainly generated by the strain-dominated (\ensuremath{\mathit{Q}} 0) regions before each roughness element. The equivalent sand-grain height ${k}_{s}$ is also characterized using $\ensuremath{\phi}\ensuremath{-}$dependent norms.