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 (τ_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.

Full-scale drag penalty predictions of flows over rough walls require surface roughness characterisation from laboratory experiments or numerical simulations. In either approach, it is necessary to determine the so-called equivalent sand-grain roughness height (k_s ). There are several steps involved in determining this aerodynamic roughness lengthscale, but its procedure typically includes a combination of measurement of wall-shear stress (τ_w ) using direct or indirect methods as well as analysis of velocity profiles. Indirect methods usually rely on assumptions made about flow and its scaling including the validity of universal outer-layer similarity. However, the implications of the underlying assumptions involved in full-scale drag prediction are unclear. In this work, we carry out wind tunnel measurements over a realistic rough surface (from a fouled ship-hull) to evaluate the impact of different methods with an emphasis on using the outer-layer similarity hypothesis for full-scale drag predictions. Wall-shear stress is measured using an in-house floating-element drag balance (DB), and velocity profiles are obtained using particle image velocimetry (PIV), allowing the evaluation of k_s , and the associated wake parameters through several methods. The aerodynamic roughness parameters hence obtained are used for full-scale drag penalty calculations. It is observed that the predicted drag penalty can vary by over 15 % among the different methods highlighting the care that should be taken when employing such methods.