Coherent Vortices and Subharmonic Interactions in the Two-Dimensional Navier-Stokes Equations

2D Navier-Stokes solutions are generated for a periodic shear layer with a fundamental ($m=0$) and a single subharmonic ($m>0$) disturbance. For m=2, 3, and 4 we find clusters with $m+1$ vortices without the formation of vortex pairs. This implies enhanced spreading in a spatially growing mixing layer. However, when $m=5$ we find a different dynamical clustering due to nondirect energy transfer from the fundamental to the subharmonic. Additionally, if we force the layer with a wavelength smaller than the fundamental we find a new mechanism called "collective interaction."