UNSTEADY FLOWFIELD AND CAVITY ACOUSTICS OF THE STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY

The Stratospheric Observatory for Infrared Astronomy is a 2.5-m-apertu re Cassegrain telescope housed in an open cavity onboard a Boeing 747 aircraft, This flying observatory operates in the Earth's stratosphere , at an altitude above 12.5 km, to view objects in the universe in the infrared region of the electromagnetic spectrum, The location of the telescope in an open cavity during its operation presents some challen ging aerodynamic and aeroacoustic problems, A combined computational f luid dynamics (CFD) and experimental investigation has been initiated to understand and resolve these issues. Different aircraft platforms, cavity apertures, aft ramp shapes, and telescope configurations have b een considered in these studies. The present study focuses on the find ings from a CFD study of a circular aperture cavity in the Boeing 747- 200 aircraft platform and a tub telescope, In particular, numerical so lutions of Navier-Stokes equations on overset grid systems are present ed at wind-tunnel and cruise flight conditions for a freestream condit ion of M-infinity = 0.85 and alpha = 2.5 deg. Comparison of CFD result s for wind-tunnel conditions show good agreement with experimental dat a for drag and time-averaged surface pressures and fair agreement for sound pressure levels and power spectra at various locations within th e cavity and on the telescope, The open-cavity condition produces an i ncrease in drag of 2%. However, the open-cavity impact on the effectiv eness of aircraft control surfaces appears minimal.