Wave-induced wake dynamics of cylinders with surrogates of marine biofouling

The rapid growth of offshore wind energy, motivated by the demand for sustainable energy solutions and the aim of achieving greenhouse gas neutrality, has led to increased attention to the impact of marine biofouling on substructures such as monopiles and jacket structures. Although the effects of hard biofouling have been studied, soft biofouling remains underexplored. This study investigates flow dynamics and vorticity patterns around eight cylindrical structures subjected to wave loading, with hard and soft biofouling surrogates. Soft biofouling is further divided into stiff and flexible models. Physical experiments are conducted with slender piles ( D / L = 0 . 07 ) in a mid-scale wave flume, covering Reynolds numbers of R e = 2 ⋅ 1 0 4 − 2 ⋅ 1 0 5 and Keulegan–Carpenter numbers of K C = 2 − 50 . Volumetric flow velocities are measured using Particle-Tracking Velocimetry with the Shake-the-Box method. Results show that biofouling alters flow patterns, creating recirculation zones with reverse flow velocities. Vorticity analysis reveals vortex formation in the wake, expanding with wave period and roughness. For the same fibre lengths, flexible biofouling models allow high levels of vorticity to spread further downstream (up to 133%), while stiff models create distinctive recirculation zones with a 18% larger recirculation length. These findings improve understanding of wave-induced wake development for rough surfaces. • Particle-Tracking-Velocimetry reveals wake dynamics of flexible and rigid biofouling. • Recirculation length during wave crest passage shrinks for flexible biofouling. • Flexible biofouling extends downstream vorticity, rigid biofouling produces confined recirculation.