Jet Flow Field during Screech

Measurements were made of the flow field structure and the near field parameters of a jet exhausting from a sonic nozzle with a 1.27 cm exit diameter. Compressed air was used for obtaining stagnation pressures up to ~ 5 atmospheres. The jet exhausted vertically from a settling chamber into an acoustically insulated room and through an insulated duct out through the roof. Measurements were made with several different reflecting surfaces at the nozzle exit as well as an insulating surface. Schlieren pictures at 500,000 frames/s were taken. Overall sound pressure level, impact pressure level downstream, and sound frequency analyzer measurements were made. It was found that with a reflecting surface there was a radial oscillation of the jet which had the same frequency as the dominant sound (screech) frequency emitted by the jet. No axial motion of the inviscid part of the flow structure was detected. The insulated surface at the nozzle exit appeared to shift the dominant frequencies of the sound generated into the region above the audible ( > 16 KHz). A reflecting surface yielded "pure tones" (screech) with one or two harmonics. The dominant (screech) frequency decreased as the stagnation pressure increased. The screech frequency was found to be approximately inversely proportional to the length of the first shock cell. Nomenclature Co speed of sound in ambient gas D diameter of nozzle exit f frequency of pure tone (screech frequency) L1 length of first cell, distance between nozzle exit plane and intersection of shock with shear layer M Mach number based on isentropic expansion to ambient pressure Po stagnation chamber pressure P~ ambient pressure Pi impact pressure R~,B distance from nozzle centerline to left boundary of jet RRB distance from nozzle centerline to right boundary of jet t time T period of screech, 1If