Towards Prediction of Dual-Stream Jet Noise: Database Generation

Abstract

The accurate prediction of jet noise from dual-stream nozzle exhaust geometries typical of high bypass ratio turbofan engines has practical relevance for aircraft design studies and in aircraft noise certification. Existing empirical methods do not provide good predictions against measured data; in addition, they are restricted to lower area ratio (secondary to primary) nozzle geometries and lower bypass ratios (BPR). The area ratio and BPR of newer engines have increased substantially, well beyond the range of validity of existing prediction methods. Therefore, there is a dire need to develop an accurate empirical prediction method that is valid over a wider range of jet operating conditions and nozzle geometries that are representative of current engines. A wind tunnel test has been completed and a comprehensive database from realistic geometries and operating conditions has been generated. The data have been analyzed and investigated with an eye towards modeling that would lead to an empirical prediction method. The area ratio and BPR cover wide ranges that encompass all current engines and potential ultra-high BPR engines of the future. The spectral characteristics have been examined and the effects of the following parametric variations are reported in this paper: (1) impact of area ratio with fixed, but over a range of, power settings; (2) at a given As/Ap, fixed primary jet and systematically varying secondary conditions; (3) at a given As/Ap, fixed secondary jet and systematically varying primary conditions; and (4) the effects of forward flight for the above situations. It is established that the characteristics of dual-stream jets with velocity ratio less than 0.5 are similar to those for a single jet, with the velocity ratio being the main controlling parameter and the area ratio playing a lesser role. At higher velocity ratios, contributions from the secondary shear layer is dominant at higher frequencies and the contributions from the mixed jet component controls the peak spectral levels in the peak radiation sector at large aft angles. At low inlet angles, the spectral shape remains invariant for all jet and geometric conditions.