The jet facility and anechoic chamber at the Gas Dynamics and Turbulence Laboratory (GDTL) at The Ohio State University have been redesigned and rebuilt to significantly improve their capabilities. The new jet facility is capable of jets of 2-inch diameter—twice the size of the old jets. The new and much larger anechoic chamber can handle the larger jet and enables the measurements of shock noise generated by the jet of tactical aircraft. Free-field qualification requirements of ISO 3745 standard are met, and the chamber has a cutoff frequency of 160 Hz. A few improvements were incorporated into the new facility including thicker, acoustically-treated walls and an acoustically transparent grating floor above the floor anechoic wedges. Tests showed that very minor variations in the spectra are introduced by the grating floor panels.
Two additional microphones were added to the new facility with three within the upstream region of the acoustic field (a maximum polar angle of 130° compared to the maximum of 90° of the old facility). The radial distances of the microphones were increased, and far-field tests show that the microphones are safely within the far-field of 1-inch and 1.5-inch jets. For a 2-inch jet, some microphones are likely within the transition region of the acoustic field but could be moved farther outward to locate them within the far-field, as there is more room within the chamber. The stagnation chamber diameter was increased from 3.068 inches to 5.047 inches to handle the larger mass flow rate of a 2-inch jet. Initially, spectra suffered from narrowband cavity tones generated by ports upstream. The ports were modified, and a second perforated plate was added to eliminate these tones.
Acoustic data of the new and old jets are compared, and some minor differences in the high frequency content of the spectra are found. Early guesses point to internal rig noise created by flow through the second perforated plate. Work will continue to remove these differences. Finally, PIV results of the old and new jets are compared. The Mach number decay and spreading rates of a new Mach 0.9 jet compare well to an old Mach 0.9 jet. The old Mach 0.9 jets had slightly lower levels of turbulent kinetic energy. A new Mach 1.3 jet compares well with an old Mach 1.3 jet all these statistics.