Flow noise reduction using bio-inspired structured surfaces*

Turbulent flow over bio-inspired surface features such as shark denticles or riblets has been studied extensively in attempts to reduce drag. Another consequence, noted in aeroacoustics, is a decrease in associated flow noise. Recent advances in 3D printing using polyjet and stereolithography, facilitate accurate fabrication of such complex surface geometries down to 200 μm over large areas (>100 cm²). Streamwise riblets with a characteristic height of 8-20 δ_ν are known to cause a 5-8% reduction in skin-friction drag, while those outside this range result in a drag increase. This study aims to quantify noise reduction by 3D printed riblets of varying properties across this range in water. To maximize the contribution of skin friction to flow noise production, a NACA0012 foil section is towed by a 0.93m long rotating arm at chord Reynolds numbers (Re_c) from 0.29 to 3.2×10⁵. A spanwise trip at x/c=0.1 fixes flow transition, ensuring a turbulent boundary layer downstream. The modular foil design, allows for easy replacement of test surfaces, located between x/c=0.15 and 0.7. Wall pressure and noise measurements are performed by pressure sensor and hydrophone arrays respectively, housed internally, to evaluate flow noise reduction in comparison to a smooth wall.