  The present study aims to clarify the effect of the polytropic index (i.e.,
the ratio of specific heats in the context of a perfect gas) on the detonation
structure. This is addressed by two-dimensional numerical simulations. To ease
the clarification of the role of gasdynamics, a simple Arrhenius kinetic law is
used for the chemical model. The activation energy, normalized by the shock
temperature, is kept constant to obtain the same reaction rate sensitivity to
temperature in all considered mixtures. This procedure dissociates the
gasdynamic effects from the chemistry effects. The numerical results reveal
that in mixtures with low polytropic indicies, the convective mixing is
enhanced compared to mixtures with higher polytropic indicies. The mixing is
evaluated using Lagrangian tracers. Moreover, mixtures with low polytropic
indicies are found to have a shorter reaction length than mixtures with high
polytropic indicies. Also, for the range of parameters considered in this study
the results indicate that Mach stem bifurcation in detonations due to jetting
is primarily a gasdynamic driven mechanism.
