Effect of tab shape on the acoustic characteristcs of jets

This study experimentally investigates the influence of tab shape on the acoustic characteristics of jet flows. Tabs introduce streamwise vorticity, with width affecting the vortex size and significantly impacting jet noise. To achieve substantial jet noise reduction, tab shapes such as triangular, trapezoidal, and inverted trapezoidal (with low base width) are compared to the conventional rectangular shape. A short-length pipe model generates an under-expanded free jet for comparison. The tab pipe models consist of two tabs inclined outward at 45°, placed diametrically opposite each other at the pipe exit, with varying shapes. Far-field acoustic measurements and schlieren images are captured in two planes, one aligned with the tabs (tab plane) and the other perpendicular to it (gap plane).

Measurements revealed the presence of screech tones specifically in the gap plane for rectangular and trapezoidal tab cases. Further, noise reduction is improved at all pressure ratios when the tab shape is varied from rectangular to other shapes. Notably, the inverted trapezoidal shape resulted in the highest noise reduction among others, with an 11 dB decrease in overall noise level. Similarly, it produced the lowest acoustic power among all the tab shapes tested. These results are attributed to differences in jet development governed by vorticity distribution associated with different tab shapes and highlight the importance of tab shape in effective noise control.