Fig. 1a: A 2D axisymmetric simulation of a marginally relativistic flow
         (<I>v</I>=0.3<I>c</I>) with Mach number <I>M</I>=6 and adiabatic index <tex2html_image_mark>#tex2html_wrap_inline8#.
         The jet enters from the left and in penetrating a medium in pressure
         balance with it, but with a density <tex2html_image_mark>#tex2html_wrap_inline10# that of the jet,
         drives a bow shock ahead of it. Kelvin-Helmholtz instability at the
         contact surface between the shocked jet and shocked ambient material
         sends pressure perturbations into the jet, driving unstable modes of
         oscillation of the jet, which steepen to form a pattern of incident
         and reflection shocks.

Fig. 1b: A 2D axisymmetric simulation of a relativistic flow (Lorentz factor,
         <tex2html_image_mark>#tex2html_wrap_inline12#) with Mach number <I>M</I>=17 and adiabatic index
         <tex2html_image_mark>#tex2html_wrap_inline16#. The situation is similar to that seen in Fig. 1a, but
         the relativistic flow is far less prone to instability, in part
         because of its high effective mass.

Fig. 1c: A 2D axisymmetric simulation of a relativistic flow (Lorentz factor,
         <tex2html_image_mark>#tex2html_wrap_inline18#) with Mach number <I>M</I>=4 and adiabatic index
         <tex2html_image_mark>#tex2html_wrap_inline22#.  The situation is similar to that seen in Fig. 1b, but
         the hot, shocked ambient medium expands rapidly, to rethermalize its
         energy at the secondary (backward sloping) shocks behind the jet
         head.  Contrary to expectation, in this low Mach number flow
         negligible instability is evident; this is  because of weak coupling
         between the available modes of the jet, and the perturbations
         capable of driving them.

Fig. 2:  A sequence of slices showing the evolution of a jet with <tex2html_image_mark>#tex2html_wrap_inline24#,
         Mach number <I>M</I>=8 and adiabatic index <tex2html_image_mark>#tex2html_wrap_inline28#, with the inflow
         precessing on a cone of semiangle <tex2html_image_mark>#tex2html_wrap_inline30# rad, with rate 0.2885
         rad per light crossing time of the inflow radius. This fully 3D
         simulation shows that the jet flow is substantially disrupted about
         half-way to the bow, but that a core of high momentum flux does
         persist -- the jet has not wholly lost its integrity.