Assessment of aerodynamic roughness parameters of turbulent boundary layers over barnacle-covered surfaces

• Published in: Experiments in fluids 2023-11, Vol.64 (11), Article 169
• Main Authors: Medjnoun, Takfarinas, Aguiar Ferreira, Manuel, Reinartz, Ralf, Nugroho, Bagus, Monty, Jason, Hutchins, Nicholas, Ganapathisubramani, Bharathram
• Format: Article
• Language: English
• Subjects: Aerodynamic balance; Drag; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Evaluation; Fluid- and Aerodynamics; Fouling; Heat and Mass Transfer; Lift drag ratio; Parameters; Particle image velocimetry; Research Article; Shear stress; Ship hulls; Similarity; Surface roughness; Turbulent boundary layer; Velocity distribution; Wall shear stresses; Wind tunnels
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-023-03709-5

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.