'Fluid cloak' to help submarines leave no wake

SUPER-STEALTHY submarines may one day glide through the water without creating a wake, if a plan to channel fluid intelligently around objects can be made to work.

A vehicle moving through a fluid normally disturbs the medium in two ways. First, some of the fluid gets dragged along with the vehicle, sapping its energy and slowing it down. Second, a turbulent wake forms behind it where fluid rushes in to fill the vacant space. The churning fluid in the wake in turn creates noise that reveals the vehicle's presence.

But channelling the fluid around the object in just the right way could solve both problems at once.

To do this, Yaroslav Urzhumov and David Smith of Duke University in Durham, North Carolina, propose encasing it in a mesh shell.

Crucially, the permeability of this mesh casing should vary from place to place to alter the speed of fluid flowing through it. This means that the shell and the object it contains would leave no lasting impression in the fluid - the fluid would exit the shell at exactly the same speed and in the same direction as it entered.

They modelled the pattern of permeability needed to make a sphere undetectable in fluid. The pattern was complex, with some spots having to accelerate the fluid flowing through it. To do that, the researchers propose embedding tiny pumps in the material to boost the flow rate. Pumps that are mere millimetres across already exist for biomedical devices.

The overall effect of their pattern is to initially accelerate the incoming fluid near the front of the shell, then to let it slow back down to its original speed at the back of the shell before it exits.

Since there is no net change to the motion of the fluid when the vehicle passes through it, there is no drag and no turbulent wake. The fluid closes seamlessly around the vehicle, as if it had never been there. "It's possible to have this structure glide through the fluid without disturbing it at all," says Urzhumov.

For the pattern in the mesh to work, there is a trade-off between the sphere's size and its speed. Steven Ceccio of the University of Michigan in Ann Arbor cautions that the "fluid cloaking" is only complete for small and slow-moving objects. For example, a vehicle 1-centimetre across could only stay drag and wake-free at speeds of less than 1 centimetre per second, he says: "If the object gets bigger, the [limiting] speed goes down even more."

But Urzhumov says it might be possible to develop mesh patterns that will work for larger objects or different shapes. And he argues that the fluid-cloaking pattern in this study could still reduce drag and weaken the wakes of larger and faster vehicles, even if it does not completely eliminate them.