Simulation of the Flow around the Stratospheric Observatory for Infrared Astronomy SOFIA Using URANS and DES

The Stratospheric Observatory For Infrared Astronomy SOFIA, a joint project between NASA and DLR, is a 2.5 m reflecting telescope housed in an open cavity on board of a Boeing 747 aircraft. The observatory operates in the stratosphere at an altitude above 13 km to observe objects in the universe in the infrared region of the electromagnetic spectrum. The flow over the open port during the observation presents some challenging aerodynamic and aeroacoustic problems. Pressure fluctuations inside the cavity excite structural vibrations and deteriorate the image stability. NASA successfully designed a passive control device by means of extensive wind-tunnel studies to mitigate the unsteady pressure fluctuations inside the cavity. The present study focuses on the findings of several URANS/DES-simulations to gain a profound insight into the complex physics of the cavity flow and the excitation of acoustic resonance. Pressure spectra computed with URANS and DES methods show good agreement with experimental data. Vortex generators, located upstream of the cavity, reveal to be an effective means for a further attenuation of unwanted pressure fluctuations inside the cavity.