  Particle flows injected as beams and scattered by an intruder are numerically
studied. We find a crossover of the drag force from Epstein's law to Newton's
law, depending on the ratio of the speed to the thermal speed. These laws can
be reproduced by a simple analysis of a collision model between the intruder
and particle flows. The crossover from Epstein's law to Stokes' law is also
found for the low-speed regime as the time evolution of the drag force caused
by beam particles. We also show the existence of turbulent-like behavior of the
particle flows behind the intruder with the aid of the second invariant of the
velocity gradient tensor and the relative mean square displacement for the
high-speed regime and a large intruder.
