A New Anechoic Facility for Supersonic Hot Jet Noise Research at Lockheed-Georgia

Abstract

Much confusion about jet noise has come about as a direct result of inadequate facilities and insufficient knowledge and control of test conditions, in many cases giving rise to completely erroneous conclusions. The facility described here has been carefully designed, accounting for the shortcomings of other facilities and being guided by the stringent demands of ongoing jet noise research at Lockheed. The design goal was the capability of testing model jets up to 2000°F at pressure ratios as high as 8, in a free-field environment anechoic at all frequencies above 200 Hz. A comprehensive series of flow visualization and temperature mapping experiments in a one-sixth scale-model anechoic room was conducted. The results dictated the design of the exhaust collector/muffler to provide entrainment and room cooling air in the quantities demanded by the jet operating conditions. In order to optimize the choice of material and anechoic wedge design to achieve the 200-Hz requirement, a special impedance tube was used extensively in performance evaluations. Some features of the facility are (1) an exhaust collector providing air in quantities dictated by the particular jet operating condition with no special forced-air injection or fan installation and (2) a “cherry-picker” crane used to gain access to instrumentation, etc., for maintenance, calibration, and setup, thus eliminating the need for access platforms, etc. The crane is stowed by remote control under an anechoic cover during all test operations. The superior performance of the room is described by intensity/distance plots obtained from a fixed loudspeaker source and moving microphone installation.

Designing Clean Jet-Noise Facilities and Making Accurate Jet-Noise Measurements

“…In regards to the effect of the wedge-mounting layout, the three layouts shown earlier in Figure 4 seemed to have the same performance in the tests performed by Burrin et al 16 . They also found that this arrangement, mounted on a 1/2 inch thick plywood panel, provided the same results as when mounted to an aluminum plate.…”

“…This length will provide at least one pressure maximum and minimum in the duct for it to be used as a standing wave tube for impedance measurements. Burrin et al 16 carried out such a test using an array of four wedges with a square duct of a length of 15 ft with 5/8-inch thick polished aluminum walls and with an end termination of 1-inch thick aluminum plate. Note that the duct dimensions place a frequency limitation for the (0,0) mode (plane wave) operation, which are 374 Hz for the 180´180 duct and 187 Hz for the 360´360 duct, as at the higher frequencies the (0,1) mode is cut on.…”

“…Burrin et al 16 measured the absorption coefficient of a collection of wedges by mounting four microphones on a trolley that could be translated manually along the axis of the duct to locate the pressure minima and maxima. A four-armed "spider" made from 1/2 inch steel tubing was used.…”

“…The picture is a view of the wedges looking at the tips of the wedges. A study by Lockheed 16 on the effect of air gap indicated that the gaps helped increase the sound absorption at frequencies lower than 100 Hz. Unfortunately, at the same time, they provided an unacceptable decrease in the absorption between 100 and 200 Hz, with no change at higher frequencies.…”

“…As shown in Figure 5, for a fixed length of 18 inches, a 4-inch deep base provided the Sound rays almost grazing the wedge tip appear to produce reflections. Absorption performance of different wedge arrangements 16 .…”