  We analyse the tumbling of small non-spherical, axisymmetric particles in
random and turbulent flows. We compute the orientational dynamics in terms of a
perturbation expansion in the Kubo number, and obtain the tumbling rate in
terms of Lagrangian correlation functions. These capture preferential sampling
of the fluid gradients which in turn can give rise to differences in the
tumbling rates of disks and rods. We show that this is a weak effect in
Gaussian random flows. But in turbulent flows persistent regions of high
vorticity cause disks to tumble much faster than rods, as observed in direct
numerical simulations [Parsa et al., Phys. Rev. Lett. 109 (2012) 134501]. For
larger particles (at finite Stokes numbers), rotational and translational
inertia affects the tumbling rate and the angle at which particles collide, due
to the formation of rotational caustics.
