  Viscous depletion of vorticity is an essential and well known property of
turbulent flows, balancing, in the mean, the net vorticity production
associated with the vortex stretching mechanism. In this letter we however
demonstrate that viscous effects are not restricted to a mere destruction
process, but play a more complex role in vorticity dynamics that is as
important as vortex stretching. Based on results from particle tracking
experiments (3D-PTV) and direct numerical simulation (DNS) of homogeneous and
quasi isotropic turbulence, we show that the viscous term in the vorticity
equation can also locally induce production of vorticity and changes of its
orientation (viscous tilting).
