Turbulent 3D MHD dynamo model in spherical shells: Regular oscillations   of the dipolar field
  We report the results of three-dimensional numerical simulations of convection-driven dynamos in relatively thin rotating spherical shells that show a transition from an strong non-oscillatory dipolar magnetic field to a weaker regularly oscillating dipolar field. The transition is induced primarily by the effects a stress-free boundary condition. The variation of the inner to outer radius ratio is found to have a less important effect. 
R.D. Simitev, F.H. Busse, A.G. Kosovichev
physics.flu-dyn