Structure of Shock Waves in Polyatomic Gases

This paper describes the results of experiments using a shock tube and interferometer to study the role of vibrational relaxation in shock structure. Shocks of Mach number up to 5 have been observed in air, A, ${\mathrm{N}}_2$, C${\mathrm{H}}_4$, C${\mathrm{O}}_2$, ${\mathrm{N}}_2$O, and C${\mathrm{Cl}}_2$ ${\mathrm{F}}_2$. In air and nitrogen below about $M_1=2\mathrm{}$ and in argon the validity of the Rankine-Hugoniot relation using constant specific heat has been established within experimental accuracy. Above $M_1=2\mathrm{}$ in both air and ${\mathrm{N}}_2$, the observed density corresponds to only partial equilibrium, no appreciable excitation of vibrational modes occurring, for example, for at least 150 μsec at 900°K. Both C${\mathrm{H}}_4$ and C${\mathrm{Cl}}_2$ ${\mathrm{F}}_2$ show fast adjustment with relaxation times less than 1 μsec to the expected final state. In C${\mathrm{O}}_2$ and ${\mathrm{N}}_2$O, vibrational relaxation times are observed to be in reasonable agreement with published data. The down-stream state, however, is at a lower density than required for complete equilibrium and the possibility of separate relaxation times for each vibrational mode is suggested; the valence vibrations adjust at least 100 times more slowly than do the bending modes. Added traces of water vapor reduce the visible adjustment greatly but leave the final state unaltered. The catalytic effect of water vapor in speeding equilibration seems therefore to be limited to the bending modes.