Most sharks are heavier than the water they swim in and therefore must generate hydrodynamic lift to compensate for the lack of buoyancy. To this end, they can use many possible combinations of fins, including the tail. This multitude of options is responsible for a long-standing debate about how the sharks distribute the lift along their body and fins. It also raises a question of stability: are sharks stable?
In this study, we address both questions using a combined theoretical/numerical/ experimental approach. We use wind tunnel experiments and CFD simulations to construct a hydrodynamic model of the Great Hammerhead shark (Sphyrna mokarran). We use this model in a standard ‘flight mechanics’ type analysis to assess its dynamic stability and identify its trim options.
We show that the shark’s body cannot provide the necessary lift by itself and most of the lift comes from its fins (mostly the pectoral fins) with the tail providing a sizeable (10 to 30%) portion of the lift for balance. We also show that the shark is unstable, and needs an active control to keep its trim.
