Quiescent core in turbulent channel flow with spanwise-uniform roughness

This study examines drag and outer-layer similarity in a rough-wall channel with spanwise-uniform bars at pitch-to-height ratios 6, 12 and 24, using direct numerical simulations at italic Re Subscript tau Baseline equals 395 Re τ = 395 $ \textit{Re}_{\tau } = 395$ . The quiescent core (QC) is detected via kernel density estimation of the streamwise velocity probability density function (PDF). The QC modal velocity remains nearly identical for rough and smooth walls, providing a basis for outer-layer similarity, whereas the exterior modal velocity shifts to lower values and broadens. Over rough walls, the QC becomes thinner, exhibits larger wall-normal excursions, and interrupts the centreline more frequently. These geometric changes alter the extent and stability of the highest-speed zone, increasing contact between the core and slower fluid in the roughness sublayer, and producing larger velocity jumps. Joint PDFs weighted by Reynolds shear stress indicate that strong shear at the QC boundary shifts momentum-carrying events towards larger amplitudes. Linear stochastic estimation shows that near-wall structures follow the roughness geometry, while structures at the QC boundary differ systematically across cases, becoming progressively more spatially compact as QC disruption intensifies. Overall, QC geometry and interfacial metrics appear to provide a compact, structure-informed framework for interpreting the non-monotonic drag variation and the partial breakdown of outer-layer similarity observed in the present configuration.