Robotic submarines are poised to become a major addition to the fleets of the world's major navies and as the technology matures more attention is being paid by the likes of the European Defence Agency (EDA) to making them quieter and stealthier.
If you look at policy proposals that will shape what future navies will look like, they are a far cry from the old days of battleships and vast numbers of patrolling frigates. The Royal Navy's Atlantic Bastion plan, for example, aims at monitoring the security of the North Atlantic with eight Type 26 frigates commanding a much larger fleet of autonomous underwater vehicles (AUV).
That requires some pretty heavy technology. Not only do you need to build robotic submarines that can operate for months without human supervision, you need some sophisticated artificial intelligence systems as well. Then you have the same problem that every naval submarine engineer has had since the Turtle took to sea during the American Revolution – how to make the sub stealthy enough to keep the hunter from becoming the hunted.
And stealth for a submarine means quiet. The goal is to make a boat that is so silent that it can cruise undetected or sit and wait in ambush like a barracuda.
That's the goal of the EDA's €4.8-million (US$5.6-million) Submarine Hull/Rudder/Propeller Hydrodynamics Interaction and Hydroacoustics (SPHYDA) project. This four-year endeavor will look at how to reduce the acoustic noise of robotic submarines, both to protect them from hostile forces and to protect sea life from them.
Part of the project concentrates on studying how noises are generated by the robotic sub. These can be caused by the flow of water over the hull and control surfaces, the propulsion system, machinery inside the craft, and even the flow of water through pipes. By gathering detailed information about these in regard to UAVs, it will be possible to build digital models to better predict and control these effects.
The other side of the sopping wet coin is how to counter these noises. The good news is that engineers have been working on this problem for over a century. The bad news is that it's been an arms race between more stealth and better sonar, which is why you should never buy a secondhand Cold War Russian submarine. You can almost hear them through the bottom of a rowboat.
One of the most complex problems is dealing with hydrodynamic noise. That is, the interaction of hull, rudders, and propellers, which is extremely complicated and requires advanced numerical models. Propulsion systems are the worst because they not only generate turbulence but cavitation, which is tiny vacuum bubbles that collapse with an extremely loud pop. This can be countered with skewed propeller blades to distribute pressure evenly, pump jets where a rotor is set inside a shroud to muffle noises, or low-RPM, high-torque designs to cut down on vibrations. Or you can go for more exotic designs like robotic subs that use changes in buoyancy by shifting oil between a set of bladders to move forward by rising and sinking.
Meanwhile, machinery noises can be dealt with by setting equipment on pallets separated from the hull by rubber mounts. Both listening and active sonar can be foiled by coating the boat with rubber tiles, which absorb noise coming from inside the hull and soak up sonar beams striking the vessel. Another machinery noise reducer involves cutting out the gurgling of flowing liquids by using special pipes that send the fluids on a tortuous path that minimizes turbulence.
Then there's optimizing the hull design to cut down on drag and turbulence. This has been the mainstay of stealth since the Second World War and has advanced to the state where it's often difficult to tell one generation of submarines from another. However, there's always room for improvement, so SPHYDA wants to find ways to integrate hull, rudder, and propulsion into a single, smooth shape.
Currently, the project is focusing on models, but the next stages will go on to tank testing followed by sea trials.
"SPHYDA is a crucial step toward developing the capability to diagnose and predict the complex hydrodynamic mechanisms responsible for the generation and propagation of noise from underwater vehicles in real operating conditions," said Riccardo Broglia, Research Director at the Institute of Marine Engineering of the Italian National Research Council, the project manager.
Source: EDA
