Investigation of Turbulent Skin Friction Behaviour over Coated, Biofouled, and Laser-Cleaned Marine Surfaces

This study investigates the turbulent skin friction behaviour of original coated, slime/algae-covered, and laser-cleaned marine surfaces through a combined experimental and numerical approach under fully developed channel flow conditions. Numerical validation was performed against established smooth-wall experimental datasets over the investigated Reynolds number range using Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES) approaches. LES provided the closest agreement with the reference data, while the two-dimensional wall function RANS k-ω SST model was selected for subsequent rough-wall simulations based on its balance between predictive accuracy and computational cost. The selected RANS approach was independently validated against rough-wall experimental data, showing good agreement in predicted skin friction behaviour. Measured surface elevation data were used to estimate equivalent sand grain roughness for the original coated, slime/algae-covered, and post-cleaning surfaces. The post-cleaning surfaces exhibited lower measured roughness and predicted skin friction coefficients than the slime/algae-covered surfaces, while the three investigated laser configurations produced broadly similar roughness characteristics and hydrodynamic responses. However, the original coating was characterised before immersion, whereas the fouled and post-cleaning surfaces had undergone water exposure. Because the coating is hydrolysing/self-polishing, the observed post-cleaning roughness reduction may be partly or entirely associated with immersion-induced coating changes and cannot be attributed uniquely to laser treatment. The results therefore demonstrate an association between the post-cleaning surface condition and reduced roughness and predicted skin friction, but do not establish a causal relationship between laser treatment and these reductions.