The Prediction of STOVL Noise- Current Semi-Empirical Methods and Comparisons with Jet Noise Data 901058
Current empirical models of several turbojet acoustic sources have been incorporated in a scheme for prediction of conventional or STOVL jet aircraft noise. The acoustic sources modeled were jet mixing noise, core noise, and broadband shock noise. The free-jet noise was then coupled with a new empirical equation for ground interaction noise generated by a vertically impinging jet. The modification of out-of-ground free-jet acoustic directivity pattern by a Harrier type nozzle installation was incorporated in the prediction of STOVL noise.
The jet/ground interaction noise prediction is the result of a flight test of the NASA Ames Harrier jet aircraft that was operated in vertical takeoff and landing. Acoustic data measured with an array of ground level microphones showed ground amplification of jet noise that peaked at a jet height equal to 18 nozzle diameters. At jet heights below 18 nozzle diameters, far-field ground level noise decreased. It is suggested that the noise decrease was caused by refraction of sound upward by the jet ground sheet. Near-field noise on the airframe was not measured, but published near-field data are examined.
Unlike numerous small-scale studies of jet impingement on a hard surface, in this study, no tones were found in the Harrier spectra. Implications for improved laboratory simulations of jet/ground interactions are discussed.
The acoustic prediction method described here gives fairly good agreement with measured far-field noise of the Harrier aircraft during hover in and out of ground effect.