  In hydrodynamical modeling of ultrarelativistic heavy-ion collisions, the
freeze-out is typically assumed to take place at a surface of constant
temperature or energy density. A more physical approach is to assume that
freeze-out takes place at a surface of constant Knudsen number. We evaluate the
Knudsen number as a ratio of the expansion rate of the system to the pion
scattering rate, and apply the constant Knudsen number freeze-out criterion to
ideal hydrodynamical description of heavy-ion collisions at RHIC
($\sqrt{s_\mathrm{NN}}=200$ GeV) and the LHC ($\sqrt{s_\mathrm{NN}}=2760$ GeV)
energies. We see that once the numerical values of freeze-out temperature and
freeze-out Knudsen number are chosen to produce similar $p_T$ distributions,
the elliptic and triangular anisotropies are similar too, in both
event-by-event and averaged initial state calculations.
