Inner--outer interactions in turbulent channel flow over rough surfaces

Real internal and external aerodynamic surfaces lose their original smooth features over time. This, in turn, can decrease the performance of a vehicle over its lifespan. I will present recent fully-resolved direct numerical  simulations (DNS) of turbulent channel flow over rough hemispherical surfaces. The friction Reynolds numbers of these simulations range from low to moderate and include up to 33,000 hemispheres. We use unstructured grids to ensure all flow features are well resolved with over a billion grid points, making these  simulations among the largest to date. The flows are analyzed to investigate the interactions between outer and inner  (near surface) scales. The discussion is framed around amplitude modulation (AM) analysis, that has shown its ability to highlight which features of the inner flow scales are directly correlated with the driving flow away from the wall. The objective of these studies is the construction of physically-informed advanced models for large-eddy simulation (LES). We will discuss how to extend existing models using principal component analysis (PCA) to predict both velocity moments of low and high order as well as their intercomponent cross-moments of different orders.