  The dynamic evolution of a highly underexpanded transient supersonic jet at
the exit of a pulse detonation engine is investigated via high-resolution
time-resolved schlieren and numerical simulations. Experimental evidence is
provided for the presence of a second triple shock configuration along with a
shocklet between the reflected shock and the slipstream, which has no analogue
in a steady-state underexpanded jet. A pseudo-steady model is developed, which
allows for the determination of the post-shock flow condition for a transient
propagating oblique shock. This model is applied to the numerical simulations
to reveal the mechanism leading to the formation of the second triple point.
Accordingly, the formation of the triple point is initiated by the transient
motion of the reflected shock, which is induced by the convection of the vortex
ring. While the vortex ring embedded shock move essentially as a translating
strong oblique shock, the reflected shock is rotating towards its steady state
position. This results in a pressure discontinuity that must be resolved by the
formation of a shocklet.
