Unsteady shock propagation in a steady flow nozzle expansion

Abstract

The flow field produced when a strong shock wave propagates into a steady flow expansion has been investigated numerically, experimentally and analytically. The experiments were conducted with a shock tube which was modified to allow steady flow to be established in a hypersonic nozzle prior to arrival of the shock. It has been found that the axial density distribution associated with the prior steady flow allows the unsteady flow following the nozzle primary starting shock to accelerate from supersonic to hypersonic speeds, whereas a uniform density distribution causes it to decelerate to subsonic speeds. The prior steady flow also allows the starting shock system to propagate through the nozzle a t nearly the same velocity as the incident primary shock, and therefore provides a convenient method of ensuring rapid steady flow initiation in shock tunnel nozzles. The analysis shows that the flow behaviour can be understood in terms of two approximate models. The first is applicable to a wide range of flow conditions, and allows calculation of the trajectory of the centre of mass of the starting shock system. The second is applicable to cases involving a prior steady flow, and predicts detailed features of the flow structure.

Characterisation and modeling of a pulsed molecular beam

“…This scenario mimics on a small scale unsteady behaviours observed in shock tubes, for instance the unsteady shock propagation observed by Stalker and Mudford, in a steady flow nozzle expansion. [25] These authors observed that the axial density distribution associated with the prior steady flow allows the unsteady flow following the nozzle primary starting shock to accelerate from supersonic to hypersonic speeds.…”

Modern developments in hypersonic wind tunnels

“…Figure 15(c) displays a time resolved spectrum of the radiation from the shock layer in the stagnation region of a hemisphere at an inertia of any practical diaphragm is too large for it to be removed from the test flow in the few tens of microseconds of test time available in a high performance shock tube. The diaphragm can be rendered unnecessary by arranging that, immediately prior to a test, a steady flow of test gas is established in the nozzle (56,57) . This allows the nozzle starting process to be completed as rapidly as with a pre-evacuated nozzle and a massless diaphragm.…”