  The present work investigates the structure of fast supersonic turbulent
flames typically observed as precursors to the onset of detonation. These high
speed deflagrations are obtained after the interaction of a detonation wave
with cylindrical obstacles. Two mixtures having the same propensity for local
hot spot formation were considered, namely hydrogen-oxygen and methane-oxygen.
It was shown that the methane mixture sustained turbulent fast flames, while
the hydrogen mixture did not. Detailed high speed visualizations of nearly
two-dimensional flow fields permitted to identify the key mechanism involved.
The strong vorticity generation associated with shock reflections in methane
permitted to drive jets. These provided local enhancement of mixing rates,
sustenance of pressure waves, organization of the front in stronger fewer modes
and eventually the transition to detonation. In the hydrogen system, for
similar thermo-chemical parameters, the absence of these jets did not permit to
establish such fast flames. This jetting slip line instability in shock
reflections (and lack thereof in hydrogen) was correlated with the value of the
isentropic exponent and its control of Mach shock jetting instability (Mach &
Radulescu).
