The dominant sources of noise are usually fans and jets, where the noise reduction can be achieved by various reduction measures. The modelling of the noise sources is necessary to develop, while the tastings are typically provided in wind tunnels and free jets.
Generally, fan noise is a function of the rotational tip speed and fan pressure ratio. In this case in order to reduce the fan noise it is better to reduce the rotational tip speed and fan pressure ratio, but consequently, the increase of an engine diameter is to be observed. An optimal value for the rotational tip speed is under Mach =1 for tackling the noise. The reason for noise has been described by (Huff, 2007) that ''the source for this noise is the turbulence in the fan wakes striking the stators and the resulting unsteady pressure field that becomes acoustic waves and radiates from the engine fan duct'' (p.3).
It became evident after several tests, conducted by Pratt & Whitney, that noise reduction can be achieved by decreasing the fan tip speed and pressure ratio. It is also beneficial to utilize such measures, as acoustic treatment, casing treatment for stall control and decrease the fan tip speed under constant fan pressure ratio.
Figure 3b illustrates a concept of decreasing inlet fan noise by changing the direction of the radiated sound away from community. The tests have shown the positive results of this method. A noise control method has been developed and advanced by means of a fan rig, called the ''Advanced Noise Control Fan'', which was used in numerous experiments, and shown in Figure 3c. Regarding ''Active Noise Control'' it was described by (Huff, 2007) that ‘'actuators designed to cancel fan tones were mounted in fan duct walls and inside stator vanes. A series of tests has been performed to show single and multiple duct mode cancellation from both the inlet and exhaust of the fan duct'' (p.3). However, a concept of hybrid systems, which involves an integration of acoustic treatment, remains to be under ongoing research.
Figure 1 – Noise reduction techniques (Huff, 2007)
Figure 3d depicts another way to reduce noise, which is produced by forward-swept fans by delaying the onset of ''multiple pure tones''. To decrease aerodynamic losses it is reasonable to install fan sweep near the tip, which will also improve the stall margin. Swept stators in Figure 3e are also utilized for reducing the fan noise, as stated in (Huff, 2007), by ''increasing the phase changes from hub-to-tip of the unsteady aerodynamics producing the sound and by increasing the effective distance from the fan to the stator vanes'' (p.4).
Recently NASA has discovered the way to reduce fan noise, which was earlier the way to reduce the jet noise, after several conducted tests as a part of Quiet Aircraft Technology project. It has been proved that using variable area nozzles can be employed for noise diminishing and thrust increasing. It was observed by (Huff, 2007) that the variable area nozzles are achieved by ‘'controlling the incidence angle of the flow near the rotor and stator'' (p.4).
At NASA Glenn Research Center another perspective technique was tested to promote other fan noise reduction methods. It was tried to increase an acoustic treatment area over the tip of the rotor. Usually acoustic liners are utilized in the inlet and aft fan ducts of the engine, where honeycomb materials with metal face are employed in the design. NASA has discovered metal foams to provide necessary bulk liner characteristics with maintaining the needed range of temperatures. The result of these experiments was nose reduction.
Apart of the fan noise reduction, Figure 2 illustrates examples of jet noise reduction.
Figure 2 – Chevron nozzles (Huff, 2007)
Generally, the target is to achieve jet noise reduction by lowering the jet exhaust velocity. In the newer engines extracting energy from the engine core is widely used, which in turn reduces the velocity of the core and fan ducts. The target is to reduce the jet noise without compromising the engine cycle, which appeared to be challenging under NASA research tests. Mixing the core and bypass flows are employed for reducing a low frequency mixing noise from the turbulent flow. This technique of the jet noise reduction has utilized ‘’chevron nozzles’’, which are depicted in the Figure 2. As a result, noise reduction has reached 2.5 EPNdB without compromizing an engine cycle. Consequently, this led to the commercial applications of chevron nozzles on the aircrafts.
References
Huff, D. L. (2007). Noise Reduction Technologies for Turbofan Engines. National Aeronautics and Space Administration, Glenn Research Center, Cleveland, Ohio: NASA Center for Aerospace Information.
