Shark skin has long fascinated humans from fishers to physicists. That’s because shark scales have evolved into tiny three-dimensional, anvil-shaped structures called denticles that help these animals swim at furious speeds. Attacking Mako sharks, for example, have been clocked at over 70 km/h. By comparison, an Olympic sprint swimmer might reach just 5 km/h.
The thinking among hydrodynamicists is that the strange shape of shark denticles must play a crucial role in shark locomotion. And if they can somehow copy this process, it ought to be possible to make artificial shark skin that could help humans or submarines swim faster.
But nobody is quite sure how denticles work. The problem is that most experiments have been done with simplified structures or with shark skin itself, which loses its special properties when removed from the fish. So, while hydrodynamicists have made progress with certain simple skin structures, the way denticles reduce drag is largely unknown.
Now that looks set to change thanks to the work of Benjamin Savino and Wen Wu at the University of Mississippi, who have simulated the flow of fluid over denticles similar in shape to the real thing. Their results suggest that shark skin not only reduces drag but generates thrust in the direction of motion.
Physicists have been studying the hydrodynamics properties of shark skin since the 1980s. The skin differs significantly from other fish because the scales are tiny, less than half a millimeter in size, and have a three-dimensional shape depending on the species.
There is some evidence that the denticles “bristle” or stand up when a shark accelerates and that this behavior varies across the shark’s body. But nobody is sure why.
Hydrodynamicists have attempted to reproduce this behavior with varying degrees of success. The most successful have shown that denticles, in the form of tiny ridges aligned with the direction of flow, can reduce drag by up to 10 percent.
This influences the way the fluid flow behaves in the so-called “boundary layer” next to the skin. Ideally, the boundary layer should sit next to the skin, which helps to keep it smooth.
But when the boundary separates from the skin, it quickly becomes turbulent and this dramatically increases drag. The riblets work by helping to keep the boundary layer attached to the surface.
This is the kind of advantage that can make a huge difference to submarines, ships yachts, torpedoes and so on. It’s one reason the design and material specification of racing yachts and military subs is a well-kept secret. (Although one challenge is keeping the ridges clean.)
But what of the more complex denticle shapes—how do they work? To find out, Savino and Wu created a virtual mode of shark skin covered with anvil-shaped denticles, which thin pillars support the anvil heads.
The anvil heads nestle together to form a kind of secondary surface above the skin that fluid can flow across. But beneath the heads, a thin layer of fluid is free to flow in a different way in this protected region.
Denticles can channel fluid into a protected layer below the main flow (source: arxiv.org/abs/2403.14095)
When the researchers simulated the flow of fluid over these structures, they discovered that the anvil heads tend to inject a small amount of fluid into the protected region, where it turns back on itself, like a small wave. This generates a reverse flow in this region that pushes against the supporting pillars, generating thrust in the direction of motion. Savino and Wu call it reverse pore thrust.
At the same time, the process of channeling a fraction of the water into the protected region helps to keep the boundary layer attached to the surface and so reduce drag. “The results indicate a proactive, on-demand drag reduction mechanism,” they say.
A key feature of the work is that the researchers did not use a flat surface for their simulation. Instead, they simulated the flow across a bump on a surface because these tend to cause the boundary layer to detach. They placed the denticles on the leeside of the bump (the down-wind side), where they observed this effect.
Savino and Wu believe the change in pressure on the leeside of the bump is a crucial factor in injecting fluid into the protected under-region. It also explains why others trying similar simulations and experiments on flat surfaces have failed to observe a similar reduction in drag. “Sharks may have a strategy to maintain a mild curvature and thus prolong reverse pore thrust generation,” they say.
That’s interesting work with significant applications. It’s not hard to imagine how submarine and yacht designers might employ denticles on curved areas of their designs. “This offers an exciting new strategy for drag reduction,” say Savino and Wu.
But keeping synthetic shark skin free of marine fouling will still be an issue. Perhaps the next stage of research should be to determine how sharks keep their denticles clean.
Ref: Thrust Generation by Shark Denticles : arxiv.org/abs/2403.14095
