Linear waves in sheared flows. Lower bound of the vorticity growth and   propagation discontinuities in the parameters space
  This study provides sufficient conditions for the temporal monotonic decay of enstrophy for two-dimensional perturbations traveling in the incompressible, viscous, plane Poiseuille and Couette flows. Extension of J. L. Synge's procedure (1938) to the initial-value problem allowed us to find the region of the wavenumber-Reynolds number map where the enstrophy of any initial disturbance cannot grow. This region is wider than the kinetic energy's one. We also show that the parameters space is split in two regions with clearly distinct propagation and dispersion properties. 
Federico Fraternale and Loris Domenicale and Gigliola Staffilani and   Daniela Tordella
physics.flu-dyn math-ph math.MP