Jet noise is an important concern for both commercial and military aviation. It has been identified as one of the primary obstacles to increasing commercial aviation capacity. Thus, it has been the focus of many research studies over the past six decades. Many innovative fluidic injection based noise reduction systems have been proposed recently. However, the high mass flow rates and momentum flux needed for such injection systems have made practical implementation of these systems a bit difficult. In this study a fluid injection scheme consisting of multiple radial microjets located downstream from the nozzle exhaust is analyzed for its ability to suppress far field jet noise. Microjets in cross flow are known to enhance turbulent mixing in the shear layer due to the induced stream-wise vortices. This enhanced mixing can be used for reorganizing the spatial distribution of acoustic energy and reducing the far-field noise. Contrary to other previous research studies, which injected fluid either inside the nozzle or just at the nozzle exhaust, this injection scheme uses a coaxial injector tube to inject multiple microjets perpendicular to the jet axis at an axial location downstream from the nozzle exhaust. Microjet injection closer to the jet axis leads to the formation of a counter rotating vortex pair (CVP) close to the injection location which subsequently breaks down into stream-wise vortices further downstream as the microjet bends and follows the flow direction.
A parametric nozzle-injector setup is constructed and various design and operational parameters of the system are outlined. Detailed Large Eddy Simulations are performed for a nozzle-injector setup operating at Mach 0.9 jet with Reynolds number ≈ 106 to help understand the aerodynamic and acoustic features of the interaction of this fluid injection scheme with the main jet. Permeable Ffowcs Williams-Hawkings based formulation is used for computing the far field acoustic spectra at two microphone locations for nozzle-injector setups with various values of these parameters. It is observed that an increase in the number of microjets, leads to the reduction in the far field noise. However, beyond np > 4 there is a lower acoustic benefit per additional port due to spatial interference between the induced CVPs. Microjet injection pressure and angle of injection play a crucial role in determining the jet trajectory, penetration and the strength of the streamwise vortices. The results show that as the injection pressure is increased the microjet trajectory and the induced streamwise vorticity both increase. Similarly, as the injection angle increases, the transverse momentum of the microjet also increases leading to better microjet penetration and effective mixing enhancement. Mean flow measurements suggest that jet core lengths are shorter due to the enhanced mixing resulting from fluidic injection. Peak noise reduction of ≈ 4.5 dB is observed for both locations for the nozzle-injector setup analyzed.
Moreover, targeted reduction in the downward-emitted turbulent mixing noise can be achieved by strategically injecting high momentum fluid downstream of the jet exhaust. A similar setup with asymmetric configuration is utilized for enhancing turbulent mixing in a particular part of the jet plume and it is observed that significant noise reduction can be obtained in a particular direction of interest. The induced asymmetry due to the asymmetric fluidic injection gives rise to a corresponding asymmetric acoustic field leading to targeted directional noise reduction (≈ 3.5dB) in the far field. The main advantage of this type of injection scheme is that it allows a certain degree of operational flexibility by allowing the user to switch it on or off, as well as tune it for a higher noise reduction during night time and lower during the day time. When operating in the asymmetric mode, it also allows the user to choose a preferred direction of noise reduction and injecting fluid accordingly. This helps to cut down the injection requirements and the thrust penalty associated with downstream injection thus making the setup economically viable for practical implementation.
|Commitee:||Donev, Aleksander, Sahin, Iskender, Thorsen, Richard|
|School:||New York University Tandon School of Engineering|
|Department:||Mechanical and Aerospace Engineering|
|School Location:||United States -- New York|
|Source:||DAI-B 81/2(E), Dissertation Abstracts International|
|Subjects:||Mechanical engineering, Acoustics, Aerospace engineering|
|Keywords:||Aeroacoustics, Fluid injection, Jet noise, Large eddy simulation, Noise reduction, Openfoam|
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