The path from homogeneous to heterogeneous roughness drag models
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While surface roughness in many real-world applications, including on ship hulls, is strictly heterogeneous, the vast majority of existing correlations and models for prediction of skin-friction drag in turbulent flows are developed for homogeneous rough walls. This calls for an extension of the state of the art. The present contribution is an attempt to discuss the way ahead based on some recent direct numerical simulation results. Specifically we are interested to identify the most suitable set of inputs for roughness models starting from recent attempts at a generalizable data-driven model for homogeneous surfaces and extending towards heterogenous 'patchy' roughness.