US researchers have developed a jet engine noise silencer that uses electrical arcs to control turbulence in engine exhaust airflow.
The researchers’ ‘plasma actuators’ are pairs of electrodes fitted around the rear of the engine exhaust that manipulate pressure gradients in the air flow by fast local heating of air pockets. Turbulence in exhaust airflow is one of the chief causes of engine noise.
Prof Mohammad Samimy and colleagues at Ohio State University tested the actuators on air streams simulating both commercial and high-speed military aircraft. ‘Noise is generated because of dynamic interaction of flow structures. You can play with these to reduce the noise,’ Samimy said. ‘When you generate a plasma arc that creates an obstruction in the flow that changes the pressure gradient.’
Manipulation of turbulence is used by other noise-reducing devices called chevrons, zigzag-shaped cutouts at the nozzle exit that introduce longitudinal vortex structures into the exhaust flow. But chevrons protrude into the fan exhaust and cause thrust loss.
The plasma actuators can be switched off when not needed. Noise reduction is only necessary during take-off and landing, but many chevrons developed so far are permanent fixtures and cannot be disengaged at higher altitudes.
Plasma actuators can also change the frequency characteristics of the airflow, and increase or decrease mixing of flow patterns in the exhaust. Frequency instabilities, like high-frequency shear layer instability and low-frequency jet column instability, which result in turbulence and noise, are weaker closer to the engine and can be manipulated before they grow larger and potentially noisier, Samimy said.
Increasing turbulence and mixing could also reduce noise – if the frequency is manipulated, he said. ‘In general increasing mixing will increase noise. However, the human ear is much more sensitive to certain frequencies, so if you can shift the noise to higher frequencies it will reduce perceived noise.’
Military stealth aircraft could benefit too. Plasma actuators could create arcs in certain patterns to promote mixing between hot gases and cool ambient air to reduce thermal radiation picked up on infrared tracking systems, the team claimed.
Power consumption of the electrodes is low, the researchers said, but a high voltage is needed to create the arc. ‘That aspect needs to be worked out on a plane, but having spoken to aircraft designers this can easily be done,’ Samimy said. ‘It’s very rich technology, but we’ve only scratched the surface.’
The NASA-funded researchers hope to optimise the electrode position and number and collaborate with engine makers within six months. But Boeing announced earlier this year that all its future aircraft will have chevrons that straighten out at cruise altitudes without reducing thrust.