Shark-inspired high-low alternating riblets by inkjet printing of UV-curable ink*

Riblets reducing fluid friction drag were originally inspired by ridges of dermal denticles of sharks. In some shark species including a great white shark (Carcharodon Carcharias), ridge height in a denticle is not uniform: the middle ridge is high and the side ridges are low. We hypothesized that the high-low alternating ridges adapt to wide range of swim speed in the sharks. In this study, we realized the riblets with high-low alternating ribs by inkjet printing of UV-curable ink on flat plates, followed by drag measurement in a water tunnel. The printing method enabled us to fabricate and compare different riblet designs with low cost. The height and spacing of the riblets were determined by our micro X-ray CT observation of the denticles of a great white shark for different locations in the body. Friction speed of the each location was estimated by modeling the body as a flat plate. The size scale of the riblets and range of flow speed of the water tunnel were decided to simulate the wide range of swim speed of the shark. The results of the measurement support our hypothesis and suggest that the high ribs are effective for low speed and the low ribs work for high speed.