  We investigate the early time development of the anisotropic transverse flow
and spatial eccentricities of a fireball with various particle-based transport
approaches using a fixed initial condition. In numerical simulations ranging
from the quasi-collisionless case to the hydrodynamic regime, we find that the
onset of $v_n$ and of related measures of anisotropic flow can be described
with a simple power-law ansatz, with an exponent that depends on the amount of
rescatterings in the system. In the few-rescatterings regime we perform
semi-analytical calculations, based on a systematic expansion in powers of time
and the cross section, which can reproduce the numerical findings.
