  Direct numerical simulations are performed for the steady flow normal to a
circular disk at the Reynolds number of 1000. Numerical simulations are
conducted with different levels of simplification procedure by reducing the
azimuthal extension of the disk. The full-disk, the half-disk, the
quarter-disk, the eighth-disk and the two-dimensional (2D) cases with the
identical grid resolution are considered. Intrinsic three-dimensionality is
identified in the wake of the circular disk. Both of the instantaneous and mean
flow quantities are influenced by the simplification level significantly. The
mean drag coefficient obtained from the 2D case is about only 36% of that
obtained from the three-dimensional (3D) simulation for the full-disk.
