Adaptation of the Beveled Nozzle for High-Speed Jet Noise Reduction

Abstract

The noise of high-speed jets, generated by nozzle exhaust systems that resemble typical jet engine installations on fighter aircraft, has been investigated with the main objective of developing noise reduction designs. To this end, the aeroacoustic characteristics of beveled nozzles have been examined in model scale tests and their performance relative to a conventional round nozzle system has been established. Two different physical models with appropriate area ratios to simulate the engine operating conditions at different power settings were tested; these correspond to 96% N1 (Military or MIL power) and 91% N1 (cutback power). Aeroacoustic measurements and analyses have been carried out for nozzles at bevel angles of 20o, 24o, 28o and 35o for MIL power, and 24o and 32o for cutback power. For the same plenum conditions, the mass flow rates for the beveled nozzles are identical to those of the baseline round nozzle, for a wide range of NPR. The thrust coefficients are higher for the beveled nozzles for both the nozzle geometries at typical MIL power and cutback power, respectively. The increase in the thrust coefficient ranges from ~0.75% for the bevel35 to ~2% for bevel28, in spite of the slight deflection of the jet plume. The reasons for this phenomenon are examined. The beveled nozzles produce at least the same or greater absolute thrust as the baseline nozzle. The beveled nozzles specifically reduce the noise in the peak polar radiation angles; the maximum noise benefit is observed in the azimuthal direction of the longer lip. The magnitude of noise reduction increases with increasing bevel angle, with the largest reduction for bevel35. An examination of the dBA metric, which is used in noise exposure studies, indicates that there is a noise benefit of ~3 dBA to ~4 dBA in the peak radiation sector. The noise benefit in the azimuthal direction of the shorter lip is only slightly lower than that in the direction of the longer lip. I. Introduction he noise of high-speed jets, as from the engines that power fighter aircraft, is a major concern for both military personnel and civilians living close to military bases. Military jets are powered by turbojets or very low bypassratio turbofan engines (BPR ≤ 0.3). The attendant high jet velocities produce extremely high levels of jet noise. This problem is severe for operations on aircraft carrier decks because military personnel are stationed very close to the aircraft during the launch and the landing of carrier-based fighter aircraft. Pilots perform training missions, called Field Carrier Landing Practice, at military bases that mimic the actual flights at the appropriate engine power settings, thereby creating a huge noise problem for the surrounding communities. The magnitude of the problem is first highlighted with a comparison of the noise footprints from the F/A-18 A/B and the Boeing 737-800 aircraft. The noise foot print is calculated as follows: first, A-weighted spectra are calculated from the measured spectra over a wide range of radiation angles. The A-weighting adjusts the full-scale spectrum measured by the microphone to account for the response of the human ear, with the low frequency levels decreased by several decibels, and the high-frequency portion of the spectrum relatively unmodified. The energy levels in the resulting spectrum at the different frequencies are logarithmically summed to produce one dBA number at each angle. The maximum dBA