Effect of a boundary layer about a nozzle on jet noise in flight

It is demonstrated that the presence of a relatively thick boundary layer about the outside of a nozzle can explain observed differences in jet noise between flight simulation experiments (usually thick boundary layers) and flyover experiments with engines mounted in nacelles (thin boundary layers). To study the boundary layer influence, the uniform velocity profile of a single stream static jet is replaced by a coannular jet in flight. The momentum loss in flight due to the external boundary layer is considered by reducing the velocity of the annular portion of the jet properly. A scaling law that relates flyover to static coannular jet noise (A. Michalke and U. Michel, AIAA Paper 80-1031) and prediction methods for static coannular jet noise as implemented in the NASA Aircraft Noise Prediction Program are then used to study the effect of the external boundary layer on the flyover jet noise directivities. [Research was accomplished while the first author held an NRC Senior Research Associateship.]