  In this study, the wake flow around a circular nano-cylinder is numerically
investigated with molecular dynamics simulation to reveal the micro/nano size
effect on the wake flow. The cavitation occurring when Reynolds number (Re) >
101 can effectively influence the wake flow. The Strouhal number (St) of the
wake flow increases with the Re at low Re, but steadily decreases with the Re
after the cavitation appears. The dominant frequency of the lift force
fluctuation can be higher than that of the velocity fluctuation, and be drowned
in the chaotic fluctuating background of the Brownian forces when Re {\geq}
127. Also because of the strong influence of the Brownian forces, no dominant
frequency of the drag force fluctuation can be observed. The Jz number, which
is defined as the ratio between the mean free path {\lambda} of the fluid
molecules and the equilibrium distance of potential energy {\sigma}, is newly
introduced in order to consider the internal size effect of fluid. The St of
the wake flow increases with the Jz until it falls to zero sharply when Jz
{\approx} 1.7. It denotes the discontinuity of the fluid can eventually
eliminate the vortex generation and shedding. Meanwhile, the St decreases with
the Kn because of the intensification of the cavitation.
