Effect of lip thickness on the noise emitted by underexpanded castellated jets

The paper studies the effect of lip thickness on the flow and noise characteristics of castellated pipe jets in underexpanded jet conditions. By castellated, we refer to rectangular cutouts along the nozzle wall toward the jet exit. The study demonstrates that the lip thickness (height of a castellation) is an important parameter for optimizing noise reduction in pipes with longer arc lengths of castellation elements, notably two and three castellations. In the case of a two-castellated model, the reduction in lip thickness is observed to be favorable for noise mitigation within a narrow nozzle pressure ratio range of 3.2–3.62. This outcome results from a delay in the onset of screech tones in the gap section, despite the presence of trapped waves in the near-field. Conversely, for a three-castellated model, the reduction in lip thickness leads to an unfavorable acoustic spectral composition. Although the overall acoustic power remains unchanged, the model with a thickness-to-diameter (t/d) ratio of 0.1 exhibits more dominant tones than the model with t/d = 0.2. This difference in the three castellated model is attributed to the variation in the positioning of streamwise vortices and wider expansion fans at the exit. Even though the overall sound pressure levels do not vary much with lip thickness variation, spectral characteristics are altered.