Simulation of a Taylor-Couette flow

Simulations of a Taylor-Couette flow were tried with the CFD code Fluent 5. A simple 3D grid done with Gambit is used to mesh the problem.
Here for the current case, constants  have been chosen such as R1= 4cm, R2= 6cm and h= 10cm. Boundary conditions are symmetry on the top and on the bottom, and wall conditions for the cylinders.
Grid of the problem
Results are calculated for a laminar case. Classical results are quiet easy to obtain.
Laminar results
Rq: A quick calculation enable to check that the situation is clearly laminar.
Taking a lengh such a L=a=2cm, a velocity quantity v=Omega1*R1=0.2*0.04=8.e-3 m/s and Nu=1.e-6m2/s, the Reynolds number is so: Re=160 that is a 'laminar value'.
Instabilities vizualisations