Roughness and the effect of streamers motion on drag in biofilm-like surfaces

Hard and soft biofouling in marine environments can cause material degradation and substantial drag penalties, posing significant operational and economic challenges. Understanding how compliance, roughness, and streamer morphology influence the flow is essential for developing effective mitigation strategies. Soft biofilms, in particular, are difficult to study experimentally due to sloughing and non-uniform growth. Fur-like synthetic surfaces provide a practical analogue for controlled studies of streamer–flow interactions. In this work, two faux-fur surfaces were tested in the Skin-Friction Flow Facility (SF3), measuring their coefficient of friction (Cf) for Re_h ranging between 6000 and 36000. To isolate the effects of unsteady streamer motion on drag, high-resolution 3D laser scans were performed under various flow conditions to create rigid replicas approximating the time-averaged morphology of streamers. Downstream planar Particle Image Velocimetry (PIV) was conducted enabling channel-height corrections and smooth-wall drag subtraction. The rigid replicas produced only a fraction of the drag observed with the compliant fur, highlighting the dominant role of streamer dynamics. Roughness function estimates from PIV and Granville’s model reveal both mean roughness and drag due to fur motion.