Noise Reduction by Lowering the Jet Mach Number and Using Inverted Velocity Profile at Fixed Thrust and Mass Flow

Noise reduction concepts of lowering the jet Mach number and using the inverted velocity profile at fixed thrust and mass flow have been assessed by large-eddy simulations. This assessment involves a baseline jet that has a relatively high jet Mach number near 1.5, and Low-M_j jets that have jet Mach numbers barely above 1.0. The Low-M_j jets have either a uniform or inverted inlet velocity profile. Both noise reduction concepts can reduce the noise in the downstream direction. The combination of these two methods can achieve a noise reduction up to 3.5dB near the peak radiation direction. In addition, because the Low-M_j jets generate very low shock-associated noise, they produce a substantial noise reduction in the upstream forward) direction. However, the inverted velocity profile elevates the noise level at both the upstream and small aft angles. This noise elevation gets worse as the velocity ratio increases. Lowering the jet Mach number at the fixed thrust and mass flow reduces the turbulent kinetic energy. The inverted velocity profile further reduces the turbulent kinetic energy, but it elevates the high-frequency noise source intensities near the nozzle exit. These two competing effects produce a double peak pattern in the far-field spectral sound pressure levels in the peak noise direction.