NEAR NOISE FIELD OF A JET-ENGINE EXHAUST Ⅰ – SOUND PRESSURES

摘要

The acoustical near field produced by the exhaust of a stationary turbojet engine having a high pressure ratio was measured for a single operating condition without afterburning. The maximum over-all sound pressure without afterburning was found to be about 42 pounds per square foot along the jet boundary in the region immediately downstream of the jet-nozzle exit. With afterburning the maximum sound pressure was in-creased by 50 percent. The largest sound pressures without afterburning were obtained in the frequency range from 350 to 700 cps.nAdditional tests were made at a few points to find the effect of jet velocity on near-field sound pressures and to determine the differ¬ence in value between sound-pressure levels at rigid surfaces and cor-responding free-field values. Near the jet nozzle, over-all sound pres-sures were found to vary as a low power (approx. unity) of the jet velocity. Over-all sound-pressure levels considerably greater than the corresponding free-field levels were recorded at the surface of a rigid plate placed along the jet boundary.nThe downstream locations of the maximum sound pressure at any given frequency along the jet-engine-exhaust boundary and the longitudinal turbulent-velocity maximum of the same frequency along a small cold-air jet at 1 nozzle-exit radius from the jet axis were found to be nearly the same when compared on a dimensionless basis. Also, the Strouhal number of the corresponding spectra maximums was found to be nearly equal at similar distances downstream.