EVOLUTION OF COHERENT STRUCTURES IN A PLANE SHEAR-LAYER

A two-dimensional plane shear layer perturbed by small-amplitude plane acoustic waves was studied experimentally. The perturbation consists of two different modes with equal amplitude, a fundamental mode and its first subharmonic. The evolution of the shear layer is influenced substantially by the phase difference between the two perturbation modes, resulting in two distinct vortex merging processes. For the pairing process, two neighboring vortices rotate around one another and gradually amalgamate. In a certain range of the phase difference, every other vortex weakens and vanishes within a few wavelengths while all the vortices remain aligned. In this tearing process of amalgamation, the subharmonic mode does not develop as large amplitude as during the pairing process. The energy distributions as well as the growth of the shear layer are different for these two kinds of vortex amalgamation.