The propulsion of the majority of commercial aircraft relies on turbofan engines. A gas turbine is used to drive the fan, which produces a significant part of the thrust. Modern turbofan designs rely on high bypass-ratios, meaning that most of the air flow goes through the bypass duct instead of entering the gas turbine. The current trend for future turbofan engines is towards even higher bypass ratios. These Ultra-High Bypass Ratio (UHBR) engines have large fans rotating at relatively low speeds. As a consequence of the lower fan speed, the fuel consumption can be reduced. Another consequence is that the engine noise signature is modified. The jet exhaust velocity is reduced and the jet noise is significantly decreased. Another benefit is that the low fan speed helps reduce the noise sources due to the fan. It is becoming urgent to tackle other noise sources on UHBR engines, specifically, the core noise generated by the combustion chambers and the turbine require particular attention since it is expected to become significant at certain operating conditions and strongly increased with the next UHBR 2030+ configuration where new clean combustor design and reduced LP turbine stages will be implemented.
The CIRRUS project aims to validate advanced low noise concepts, by developing both advanced numerical and experimental tools, to enable the reduction of the core noise in future UHBR 2030+ turbofan engines. The CIRRUS consortium is made of 5 partners covering a wide range of test and numerical expertise. The overall goals of CIRRUS are to:
• Improve numerical methods to predict the noise source mechanisms and the acoustic core noise radiation up to the far field.
• Improve experimental methods, including new pressure and thermal sensors, to measure the contribution of core noise on real engines.
• Develop, test and integration of new generations of noise reduction acoustic liners made of Ceramic Matrix Composites (CMC).
• Investigate, by comparing various turbine configurations of future UHBR 2030+ architectures, the influence on the core noise sources that reducing the number of stages achieves.
As main conclusions, the following observations can be highlighted:
• 3D simulation is a requirement to address core noise analysis,
• Acoustic efficiency of CMC technology for core liner is demonstrated by test and specified for a future application on UHBR architecture,
• Direct and indirect core noise source mechanisms have been experimentally identified and analysed; composition noise is found to be negligible.
