Acoustics and Unsteady Flow of Telescope Cavity in an Airplane

The Stratospheric Observatory for Infrared Astronomy consists of a 2.5-m aperture Cassegrain telescope housed in an open cavity onboard a Boeing 747-SP aircraft cruising at around 13 km altitude. The open cavity of this airborne observatory presents many aerodynamic and aeroacoustic challenges that are being resolved for its design using results from computational and experimental investigations. Results are reported from one such Navier-Stokes computational fluid dyanamics (CFD) investigation on an overset grid system at wind-tunnel and cruise flight conditions. Numerical results of sound pressure levels, spectra of unsteady pressures on the telescope and bulkheads, spectra of telescope moments, and time-averaged surface pressures on the empennage show fair comparisons with experiments. An acoustic tone at 440 Hz has been identified for this cavity from CFD calculations at wind-tunnel conditions, and this acoustic tone is in agreement with experimental observation. The good agreement of scaled wind-tunnel results with computational results at cruise conditions indicates the scaling relations used are accurate. The open cavity produces a drag increase of less than 2% of the aircraft drag and is found to have negligible effect on the aircraft controls in cruise flight.

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