Abstract:
Jet engine exhaust is one of the main noise sources during airplane take-off. There is a strong shear layer formed between the flow outside the nacelle and the circular jet flow in its bypass duct downstream of the trailing edge. The large vortex structure due to the instability of the shear layer is an important noise source. Recently, some modern jet engines have chevrons at their trailing edge, which is believed to be a measure for noise reduction. In this paper, from the perspective of flow instability, a simplified model, i.e. the shear layer downstream of the trailing edge of a flat plate splitting two uniform streams, is proposed to explore the possible cause of the noise reduction. It was found that for the case without chevron, unsteady vortex roll-up appears downstream of the trailing edge, whose frequency is actually chosen to match the most unstable 2-D shear mode by the stability characteristics of the mean flow. For the case with chevron, there is no 2-D layer mode. Biglobal stability analysis of the 3-D mean flow shows that the most unstable mode is significantly smaller compared with the case without chevron, which is also verified by the numerical simulation. Therefore, the mechanism of noise reduction for using chevron can be concluded as the following. The existence of chevron changes the stability characteristics of the mean flow, so that the amplification rate and the amplitude of large scale vortices are reduced. So are the second derivatives of Reynolds stress, which is one of source terms in noise generation and the noise is thus reduced.