Passive control of pipe-jet noise using trailing-edge castellations

In the current study, geometric modifications in the form of trailing edge castellations are explored for the reduction of pipe-jet noise. The number of castellations determines the interaction between the streamwise vortices, which alters the sound produced. The noise levels in pipe-jets with and without castellations are compared by measuring the polar and azimuthal directivities. The parameters, namely the number of castellations and the nozzle pressure ratio, are varied in this study. The castellations work best in the underexpanded flow regime of nozzle pressure ratio 3–5. The spectral analysis of the sound revealed that the screech tones are eliminated for all castellated pipes except the one with two castellations. The broadband shock associated noise is found to increase with the number of castellations, and screech tones reappear at a higher nozzle pressure ratio. The maximum noise mitigating and amplifying cases are identified by acoustic power analysis for all castellated pipes with equidistant teeth and gap. As the number of castellations increases, the modified pipes attain the noise characteristics of the base model, which can be considered as an infinitely castellated pipe. These results are supported by the flow visualization using schlieren imaging, which showed the differential expansion in teeth and gap plane, leading to asymmetry in flow. Proper Orthogonal Decomposition of the schlieren images indicated the presence of symmetric and anti-symmetric mode of oscillation in gap and teeth plane, respectively, of the model with two castellations.