  We analyze particle number fluctuations in the crossover region near the
critical endpoint of a first-order phase transition by utilizing molecular
dynamics simulations of the classical Lennard-Jones fluid. We extend our
previous study [V.A. Kuznietsov et al., Phys. Rev. C 105, 044903 (2022)] by
incorporating longitudinal collective flow. The scaled variance of particle
number distribution inside different coordinate and momentum space acceptances
is computed through ensemble averaging and found to agree with earlier results
obtained using time averaging, validating the ergodic hypothesis for
fluctuation observables. Presence of a sizable collective flow is found to be
essential for observing large fluctuations from the critical point in momentum
space acceptances. We discuss our findings in the context of heavy-ion
collisions.
