Jet noise is amplified at low frequencies when the jet is installed under a wing. Serrations are designed at the nozzle exit to mitigate installation noise generation. In this paper, LES-RANS is performed to investigate the serration effects on installed jet flows and acoustics in an industrially relevant configuration, an Ultra-High-Bypass Ratio (UHBR) aeroengine mounted under a three-dimensional wing. The results show that the serrations are able to weaken jet and wing interactions and consequently reduce far-field installation noise. The noise reduction mechanisms have been sought through the quadrupole sources in the jet plume and evanescent pressure wave scattering at the wing trailing edge. Nomenclature c Sound speed, m/s Cp Surface pressure fluctuation coefficient, p /(ρ a u 2 a ) D Bypass nozzle diameter, m d wall distance, m f Frequency, Hz m Azimuthal mode number ṁ Mass flow rate, kg/s M a Mach number, |u|/c a p Pressure, Pa St Strouhal number (nondimensional frequency), f D/U