  High-resolution SiO observations of the NGC 1333 IRAS 4A star-forming region
showed a highly collimated outflow with a substantial deflection. The
deflection was suggested to be caused by the interactions of the outflow and a
dense cloud core. To investigate the deflection process of protostellar
outflows, we have carried out three-dimensional hydrodynamic simulations of the
collision of an outflow with a dense cloud. Assuming a power-law type density
distribution of the obstructing cloud, the numerical experiments show that the
deflection angle is mainly determined by the impact parameter and the density
contrast between the outflow and the cloud. The deflection angle is, however,
relatively insensitive to the velocity of the outflow. Using a numerical model
with physical conditions that are particularly suitable for the IRAS 4A system,
we produce a column-density image and a position-velocity diagram along the
outflow, and they are consistent with the observations. Based on our numerical
simulations, if we assume that the initial density and the velocity of the
outflow are $\sim 10 \cm3$ and $\sim 100 \kms$, the densities of the dense core
and ambient medium in the IRAS 4A system are most likely to be $\sim 10^5 \cm3$
and $\sim 10^2 \cm3$, respectively. We therefore demonstrate through numerical
simulations that the directional variability of the IRAS 4A outflow can be
explained reasonably well using the collision model.
