Instability of diverging and converging flows in an annulus
  The stability of two-dimensional diverging and converging flows in an annulus between two permeable cylinders is examined. The basic flow is irrotational and has both the radial and azimuthal components. It is shown that for a wide range of the parameters of the problem, the basic flow is unstable to small two-dimensional perturbations. The instability is inviscid and oscillatory and persists if the viscosity of the fluid is taken into consideration. 
Konstantin Ilin and Andrey Morgulis
physics.flu-dyn math-ph math.MP