  In the present study, a computation study is performed to investigate the
effect of imposed oscillation of nozzle pressure ratio (NPR) on the flow
structure in a two-dimensional, axisymmetric supersonic converging nozzle. In
this study, the underexpanded flow conditions are considered which are
dominated by diamond shock-cell structure. The computational results are well
validated with the available experimental measurements. The flow is initially
computed to be fully developed and then oscillations are imposed. NPR is
increased from 1.6 to 2.6 and then decreased again to 1.6 and thus completes a
cycle. Results showed that the external flow structure of the nozzle is
dependent on the process of change of pressure ratio during the oscillation.
Distinct flow structures are observed during increasing and decreasing
processes of the change of pressure ratio even when the nozzle is at the same
NPR. Irreversible behaviors in the locations of jet centreline axis and
off-axis as well as expansion, compression and neutral zones, are observed at
the same NPRs during this oscillation. Further, the effect of oscillation
frequency is explored on this irreversible behavior at 100 Hz, 200 Hz, 500 Hz
and 1000 Hz frequencies.
