Directionally-targeted jet noise supression: Benefits of asymmetric downstream fluidic injection

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. The main aim of this computational study is to analyze directionally-targeted jet noise reduction using asymmetric downstream fludic injection scheme for a Mach 0.9 nozzle. Previous investigations have shown significant mixing enhancement and subsequent far field noise reduction in the case of symmetric fluidic injections. Parametric studies have been performed previously for symmetric injection scheme and feasible design and operational parameters were outlined. Targeted reduction in the downward-emitted turbulent mixing noise can be achieved by strategically injecting high momentum fluid downstream of the jet exhaust. In this study, a similar setup is utilized for enhancing turbulent mixing in a particular part of the jet plume. The effect of this localized asymmetric mixing on the far field noise is analyzed and it is observed that significant noise reduction can be obtained in a particular direction of interest. Detailed Large Eddy Simulations are performed on a hybrid block structured- unstructured mesh to generate the flow field which is then used for near-field and far-field noise computation. Aeroacoustic analogy-based formulation is used for computing far-field noise estimation. Benchmark cases are validated with pre-existing experimental data sets. Mean flow measurements suggest that jet core lengths are shorter due to the enhanced mixing resulting from fluidic injection. The induced asymmetry due to the fluidic injection gives rise to an asymmetric acoustic field leading to targeted directional noise reduction (≈ 5dB) in the far field. The advantage of this type of injection scheme is that it allows a certain degree of operational exibility by allowing 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 makeing the setup economically viable for practical implementation.