Effects of Engine Acoustic Waves on Aerooptical Environment in Subsonic Flight

Spatially and temporally resolved wave fronts were collected in flight using Airborne Aero-Optics Laboratory at varying subsonic Mach numbers through both the naturally developing boundary layer and an artificially generated shear layer. Dispersion analysis was used to separate the wave fronts into the downstream and upstream-traveling components. The upstream propagating component was associated with sound waves, originating from the aircraft’s engine, located downstream of the measurement location, with convective speeds that are consistent with theoretical predictions. The spectra of the acoustic-related component were dominated by the tonal frequency associated with the engine’s fan blade pass frequency. Modal analysis also revealed the streamwise spatially periodic nature of the acoustic component of the wave fronts, further confirming the acoustic nature of these distortions. An analytical model was derived which established the relationship between overall levels of acoustic-related optical distortions and the acoustic pressure fluctuations. The model was used to estimate the acoustic pressure spectra and sound pressure levels at the measurement location.