  We study the anisotropic, elliptic expansion of a thermal atomic Bose gas
released from an anisotropic trapping potential, for a wide range of
interaction strengths across a Feshbach resonance. We show that in our system
this hydrodynamic phenomenon is for all interaction strengths fully described
by a microscopic kinetic model with no free parameters. The success of this
description crucially relies on taking into account the reduced thermalising
power of elastic collisions in a strongly interacting gas, for which we derive
an analytical theory. We also perform time-resolved measurements that directly
reveal the dynamics of the energy transfer between the different expansion
axes.
