Viscous Flow Instability of Inflectional Velocity Profile
  Rayleigh showed that inviscid flow is unstable if the velocity profile has an inflection point in parallel flows. However, whether viscous flows is unstable or not is still not proved so far when there is an inflection point in the velocity profile. Fluid viscosity has showed dual role to the flow instability. In this paper, it is demonstrated for the first time that viscous parallel flow with inflectional velocity profile is sufficient for flow instability. 
Hua-Shu Dou