The role of viscosity in AGN outflows in relation to jet periodicities G. Lanzafame, P. Cassaro, F. Schilliró, V. Costa, G. Belvedere and R. A. Zappalá A&A, 482 2 (2008) 473-482 Published online: 14 February 2008 DOI: 10.1051/0004-6361:20077958