Disturbance evolution in rotating boundary layers

The global linear stability of the family of rotating boundary layers (that includes discs and cones) is reviewed. Using a velocity-vorticity form of the linearised Navier-Stokes equations, disturbance evolution is impulsively excited for a variety of flow geometries and perturbation parameter settings. For azimuthal mode numbers below a fixed threshold value, disturbance development is dominated by convectively unstable characteristics, even though the flow might be locally absolutely unstable. As the azimuthal mode number is increased to larger values a form of global linear instability emerges that is characterised by a faster than exponential temporal growth. However, this is only observed when the azimuthal mode number is taken to be significantly greater than the conditions necessary for the onset of absolute instability to occur.