Undergrad Thesis

Underexpanded Supersonic Jet in Imposed Oscillating Conditions

Fundamental understanding of the under-expansion process and under-expanded jets is highly beneficial to engineers and scientists of various fields, from aerospace engineering to geophysics. Under-expanded jets are complex high speed flows, which are formed in various engineering applications and devices such as exhaust plumes of aircrafts (rockets and missiles), supersonic combustors, actuators, ejectors and high pressure gaseous injectors. This type of jet can also be observed in geophysical systems (volcanic eruption) and in accidental release of hazardous gases (such as hydrogen) from tiny cracks in high pressure pipelines and reservoirs. Various attempts have been made to know the flow behaviour of under-expanded jets especially the shock train.

Shock diamonds behind a Lockheed SR-71 Blackbird

Thesis: Md. Elius, Md. Mahmudul Hasan

Thesis Supervisor: Dr. A.B.M. Toufique Hasan

Abstract: 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.