  The drag force law acting on a moving circular disk in a two-dimensional
granular medium is analyzed based on the discrete element method (DEM). It is
remarkable that the drag force on the moving disk in moderate dense and pure
two-dimensional granular medium can be well reproduced by a perfect fluid with
separation from the surface of the tracer. A yield force, being independent of
the moving speed of the disk, appears if a dry friction between the granular
disks and the bottom plate exists. The perfect fluidity is violated in this
case. The yield force and the drag force diverge at the jamming point.
