High resolution measurements of billfish skin roughness

Biological surfaces often exhibit roughness features across a wide range of scales, serving a multitude of functions from protective armour through to controlling heat and mass transfer rates at the surface of the organism. The significant positive impact that skin surface roughness can have on hydrodynamic performance of marine species has been demonstrated through numerous studies of sharks. These natural roughness patterns offer an ideal template for mankind to develop engineering surfaces with improved efficiencies
for applications involving, for example, the transport of fluids through pipelines and channels. In this respect, the Billfish offer an interesting source to draw inspiration from. Their bodies are covered in distinctive roughness patterns yet, despite continuing efforts of researchers, the role that this surface roughness plays on their exceptional swimming capabilities remains poorly understood. The subsequent practical bio-inspired engineering applications that could follow also remain to be investigated. Two closely related reasons for this are identified. First, is a lack of detailed information on the three-dimensional morphology of billfish roughness, and second, is a lack of rigorous experimental
investigation of the hydrodynamic properties of the roughness. An overview of existing knowledge of roughness in the Billfish is provided first. Strategies aimed at addressing these outstanding issues are then discussed, including recent developments in measurement capabilities. The discussions are supported with some novel measurements of sailfish, marlin and swordfish skin.