Assessment of Different Methods for Drag Penalty Predictions in Rough-Wall Boundary Layers

Accurate predictions of the drag penalty in rough-wall flows require careful characterisation of surface roughness to determine the equivalent sand-grain roughness height (k(s)). The procedure involves measuring wall-shear stress (tau(w)) using direct or indirect methods and analyzing velocity profiles. However, indirect methods often rely on assumptions whose validity cannot always be guaranteed. In this paper (partly based on the study [1]), wind tunnel measurements on a realistic rough surface scanned from a fouled ship hull are carried out to evaluate drag penalty predictions. Current data enabled the evaluation of k(s) and associated wake parameters using several methods, which were then used for full-scale drag penalty predictions at high Reynolds numbers, with results showing the drag penalty could vary by nearly 15% among methods, highlighting the importance of exercising caution when employing such methods.