Theoretical acoustic benefit of high bypass ratio and variable-area nozzle in turbofan engines

Abstract

This paper documents the acoustic benefit that can be expected from a variable-area nozzle (VAN) implemented in turbofan engines with different design fan pressure ratio. To address this question, a fast meanline approach provides an aerodynamic design and the location of the acoustic certification points within the fan map; then, the overall sound power levels from the jet and from the fan stage are calculated based on well-established empirical correlations and an analytical method. Both models predict an increasing overall acoustic benefit of the VAN as the engine bypass ratio gets larger, which is essentially related to the aerodynamic unloading of the fan stage as the nozzle is opened. The jet noise reduction is confirmed, and it is found out that fan tonal noise may slightly increase while fan broadband noise may be substantially reduced. This is of particular interest, since fan broadband noise dominates in ultra high-bypass-ratio engines.