There is a growing need to significantly reduce the noise generated by high-performance, supersonic military aircraft. The noise generated during takeoff and landing on aircraft carriers has direct impact on shipboard health and safety issues. Noise complaints are increasing as communities move closer to military bases or when there are changes due to base closures and realignment. Furthermore, U.S. and international noise regulations and policies will have an impact on military operations and training unless effective steps are taken to reduce the noise.
The specific objectives of this research were to develop physical understanding of the mechanisms of noise production and identify noise sources when a military aircraft engine is operating at ideal (perfectly expanded) and non-ideal conditions (over and under-expanded nozzle exhaust). Additionally, the project aimed to develop three fluidically-based noise reduction techniques that can: a) enhance effectiveness of mechanical chevrons, or b) induce virtual modification of the nozzle area ratio to ensure continuous adaptation to design conditions, or c) use fluidic jets in a regular nozzle to virtually mimic mechanical elements such as chevrons.
The technical approach for this project was to design and fabricate representative nozzles, conduct experiments and acquire data, compare this information and validate numerical simulations, and then use both experiments and simulations to understand the sources of the noise and investigate three fluidics-based approaches to reduce the noise.
The research from this project led to the development of a validated computational capability for predicting the near-field noise generated by supersonic military aircraft jets under a variety of operating conditions, including non-ideally expanded jet nozzle conditions. It was observed that even when ideally expanded the flow field from typical military type jet exhaust nozzles are shock containing and produce shock associated noise because of the sharp throat present in these nozzles (due to other operational, manufacturing and maintenance requirements).
This work determined that mechanical chevrons are effective in reducing the noise level of supersonic jets under a variety of operating conditions. Fluidically enhancing mechanical chevrons was shown to improve noise reduction while only employing a small amount of injection air (1-2%). For a substantially over-expanded jet, fluidic enhancement shows a reduction in noise of nearly 2dB. The same level of noise reduction as that is achieved using mechanical chevrons can also be obtained using modest amounts (1-2 %) of fluidic injection of air. The further advantage of fluidic injection is that it can be turned on or off while mechanical chevrons are always present, once installed on the nozzle. Fluidic injection can be effectively used to vary the area-ratio between the throat and exit areas and does modify the external shock-cell structure.
Results to date have shown only modest reduction in the near-field noise. The injection geometries and chevrons employed in the study were chosen to ensure that significant effects would be present. The geometries were in no way optimized. Further systematic studies using the developed computational tool and complimentary experiments are needed to optimize mechanical chevrons and fluidic injection for noise reduction under various scenarios.
In general, fluidic injection in place of mechanical chevrons shows promise. For a constant injection mass flow, the effectiveness of fluidic injection decreases with increasing jet Mach number. Fluidic injection in general enhances chevron effectiveness, especially at lower jet Mach numbers where chevrons alone tend to be less effective. Therefore, fluidic injection does provide a complementary technique to mechanical chevrons for noise reduction. A key advantage of fluidic injection is that it can be easily turned on and off unlike mechanical chevrons, which are always present once installed on the nozzle and hence will modify the flow field under all operating conditions. A disadvantage of fluidic injection is that more changes are required for retrofitting to existing engines than a chevron nozzle modification since an additional stream of air is required.