  Relativistic outflows represent one of the best-suited tools to probe the
physics of AGN. Numerical modelling of internal structure of the relativistic
outflows on parsec scales provides important clues about the conditions and
dynamics of the material in the immediate vicinity of the central black holes
in AGN. We investigate possible causes of the structural patterns and
regularities observed in the parsec-scale jet of the well known quasar 3C 273.
We compare the model with the radio structure observed in 3C 273 on parsec
scales using very long baseline interferometry (VLBI) and constrain the basic
properties of the flow. Our results show that Kelvin-Helmholtz instabilities
are the most plausible mechanism to generate the observed structures.
