Noise Reduction by Lowering the Jet Mach Number at Fixed Thrust and Mass Flow

View Video Presentation: https://doi.org/10.2514/6.2023-4515.vid It has been observed that lowering the jet Mach number can reduce jet noise when the jet velocity is fixed. Large-eddy simulations (LES) have been used to assess this concept and a scaling analysis has been conducted to provide guidance to the design of the LES test matrices. Since a lower jet Mach number at fixed jet velocity requires a higher nozzle temperature, this phenomenon sometimes is referred to as the “heating effect”. Three test matrices have been made and each test includes two jets that have either higher or lower jet Mach numbers with similar mass flow rates and thrusts. LES predictions have confirmed that a lower jet Mach number generates a lower peak noise level when the thrust and mass flow rate are fixed. However, the noise reduction is less than the scaling law prediction. In jets with lower jet Mach numbers, the turbulence kinetic energies are higher and the pressure fluctuation intensities are lower. In addition, the shear-layer growth is increased and the convection velocity is reduced. These observations indicate that the noise reduction is not caused by changes in turbulence intensities, rather, it is more likely due to the enhanced shear-layer mixing. The cross correlations between the far-field noise and the pressure fluctuations in the jet plume have also been examined to identify the major noise source locations for the far-field noise in different far-field radiation directions.