On the nature of the amplitude modulation of jet shear layer instability waves

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Abstract
In natural jet shear layers as well as in plane mixing layers the Kelvin–Helmholtz instability waves are commonly found to be strongly amplitude modulated. In this study the nature of the modulation of the initial shear layer instability is investigated experimentally by correlating its envelope or amplitude function across the shear layer. It is shown that its origin is a receptivity of the shear layer to external disturbances which is modulated by low frequency and not a modulated growth rate. This finding is of importance for further progress in modeling feedback phenomena.