  Hubble Space Telescope observations show bright knots of H$\alpha$ emission
within outflowing young stellar jets. Velocity variations in the flow create
secondary bow shocks that may intersect and lead to enhanced emission. When the
bow shocks intersect at or above a certain critical angle, a planar shock
called a Mach stem is formed. These shocks could produce brighter H$\alpha$
emission since the incoming flow to the Mach stem is parallel to the shock
normal. In this paper we report first results of a study using 2-D numerical
simulations designed to explore Mach stem formation at the intersection of bow
shocks formed by hypersonic "bullets" or "clumps". Our 2-D simulations show how
the bow shock shapes and intersection angles change as the adiabatic index
$\gamma$ changes. We show that the formation or lack of a Mach stem in our
simulations is consistent with the steady-state Mach stem formation theory. Our
ultimate goal, which is part of an ongoing research effort, is to characterize
the physical and observational consequences of bow shock intersections
including the formation of Mach stems.
