Sneakier Submarines with Actuators
Submarines are already pretty silent and deadly. Most are either nuclear powered or have one or more diesel engines. Some use fuel cell technology to reduce the number of moving parts. Once the engines power the onboard batteries, that power spins the propeller allowing the craft to go under water for long periods at a time. German U-Boats may no longer be the enemy under water but it’s still crucial that submarines are able to go silent at depth and be capable of quick and quiet escapes whenever possible. Everything that goes into the shape and power of the submarine is meant to reduce its noise.
Researchers modelled a submarine hull as a cylindrical shell with two cone shells on either end. They converted the displacement components of the cylinder to wave propagation equations and used a point force representation for the propeller as a Fourier series. The goal was to reduce the sound generated in the radial direction from propeller movement both from the propeller itself and vibration and displacement of the hull as a result of the propeller movement.
The researchers proposed an array of 60 commercial off the shelf linear actuators in a ring around the aft cone forward of the propeller.
The actuators act as a mass-spring dampening system. Their effectiveness would depend on what kind of controls were used in the feedback system to power the actuators. They examined both Active Vibration Control (AVC) and Active Structural Acoustic Control (ASAC).
The researchers found actuators could be effective at reducing radial sound generated from hull vibration. Both systems of controls they used can be seen in the above figures from their study. The actuators were especially effective at lower frequencies. That’s probably a good thing as propellers typically have lower RPMs than the engines that power them. Though it’s hard to see the Navy attaching a ring of off the shelf actuators to the hull of a submarine it is interesting to think what the implications might be or how actuators might reduce noise in similar applications. Or if actuators can be proven to reduce the noise what more practical mechanisms might mimic actuators successfully enough to reduce the sound profile of a submarine even further.
Caresta, M. (2011) Active control of sound radiated by a submarine in bending vibration. Journal of Sound and Vibration 330 (2011) 615–624.