Predicting the global drag of turbulent channel flow over roughness strips

Predicting the global drag of heterogeneous rough surfaces remains one of the great challenges in roughness research (Chung et al., 2021). In the limit where patch sizes are much larger than the boundary layer thickness, predictive formulas can be derived under the assumption that the flow is in local equilibrium with the surface properties (Neuhauser et al., 2022, Hutchins et al., 2023). The present work extends this concept to predict the drag behavior of turbulent channel flows over surfaces with spanwise heterogeneous roughness properties which vary over length scales comparable to the boundary layer thickness. The drag predictions are compared with high-fidelity measurements obtained in an air channel flow facility (Frohnapfel et al., 2024). As an outlook to future work, it is discussed how heat transfer over rough surface strips can be modeled under the local equilibrium assumption.