Experimental analysis of jet noise and thrust in nozzles with zigzag-shaped vortex generators

This paper reports an experimental analysis of a vortex generator adopted to reduce the noise emitted by subsonic jets of aero-engines. This noise reduction device consists of a tape adhered to the inner surface of the nozzle near its exit, covering the entire nozzle circumference and featuring a zigzag pattern, with a thickness approximately 1% of the nozzle diameter. Acoustic and hot-wire measurements were conducted in an anechoic chamber to quantify the thrust reduction due to the vortex generator and correlate the observed noise reduction with changes in flow turbulence. The device increases the turbulence intensity in the shear layer near the nozzle exit whilst reducing it further downstream. These changes in the flow turbulence reduce the noise emission at low to mid frequencies but increases at higher frequencies. The results for a jet at an acoustic Mach number of 0.5 showed that noise reduction achieved with the vortex generator is sustained, even when the nozzle exit velocity is increased to compensate for a minor decrease in thrust. This finding is significant, as previous studies involving vortex generators did not assess the thrust penalty.