The Structure of Shock Cell Noise from Supersonic Jets

Shock cell noise is created by supersonic jets operating at off-design conditions due to the interaction between the evolving turbulence and the quasi-fixed shock cell structure. Transonic wind tunnel tests are conducted to identify the structure of the noise produced by this process in the presence of external flow. The data is compared to a simple kinematic model that incorporates the freestream velocity. It predicts the relationship between frequency and radiation angle and also predicts the screech frequency as the limiting case of upstream propagation. Results indicate that the freestream flow has a significant effect on the noise production. The noise level decreases and the radiation angle rotates toward the upstream axis as the external flow increases. But, when convective effects are considered the direction of noise is 90 o to 100 o for all conditions.