Previous experimental work on the diffraction of a detonation wave at a large and abrupt area change in a tube, has shown that every system is characterized by a critical tube diameter at which quenching of the detonation occurs. Zeldovich, Kogarko & Simonov (1956) established that the critical tube diameter, for the oxy-acetylene system with varying dilution of nitrogen, lies between 500 and 700 times the one-dimensional induction zone length. Later, Mitrovanov & Soloukhin (1964) discovered that, for the same system, the critical diameter is 10 or 13 times the transverse wave spacing for a flat channel and cylindrical tube respectively. The two results are shown to be equivalent and are confirmed by further experiments in a 75 × 6 mm channel in which the flow is two-dimensional.Smoked foil and schlieren records show that, for supercritical waves, re-ignition occurs at sites along the wedge formed by the head of the expansion from the diffracting aperture and criticality is attained when the site is located at the apex of the wedge. A universal feature of re-initiation, which is also observed in liquid and solid explosives, is the sudden appearance of a transverse detonation which sweeps through the compressed, but unreacted, gas of the dissociated shock-reaction zone regime; this is signalled by the appearance of fine triple-point writing on smoked-foil records.A criterion for re-initiation is formulated by equating the critical velocity gradient which characterizes the decay of the wavefront in a cell, to that obtaining in the diffracted shock front at the head of the expansion fan; an expression for the latter is derived from Whitham's (1957) theory for non-reactive shocks. The prediction of the criterion is in good agreement with observation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S002211207900135X

The diffraction of a planar detonation wave at an abrupt area change

Unsteady interactions of shock waves

Gaseous detonations—A selective review

The effect of pressure boundary rupture rate on hydrogen spontaneous ignition has been numerically investigated. A mixture-averaged multi-component approach was used for accurate calculation of molecular transport. Spontaneous ignition and combustion chemistry were accounted for using a 21-step kinetic scheme. A 5th-order WENO scheme coupled with ultra fine meshes was employed to reduce false numerical diffusion. The study has demonstrated that the rupturing process of the initial pressure boundary has important influence on the spontaneous ignition of pressurized hydrogen release. When the pressure boundary rupture rate is below a certain threshold value, the predictions showed that there would be no spontaneous ignition. As the rupture rate increases, the shock-heated air temperature drops more quickly due to earlier flow expansion. Once the rupture rate is sufficiently high, spontaneous ignition can still occur. However, the initial flame width would be narrower compared to the sudden release case.

The effect of pressure boundary rupture rate on spontaneous ignition of pressurized hydrogen release

Shock-wave exposure from improvised explosive devices (IEDs) has been implicated as a possible contributing factor to neurological impairment reported in combat veterans However, evidence-based substantiation of this implication, particularly for low-level exposure in the absence of external signs of trauma, remain elusive Accordingly, we constructed an open-ended shock tube producing a short-duration, low-amplitude shockwave Low-level (115 kPa static overpressure) complex shock-wave exposure in rats resulted in no histological evidence of lung injury By contrast, delayed cytoskeletal proteolysis of αII-spectrin was detected in the cortex and hippocampus by 12 h post-injury Cell death was minimal and localized predominantly in the corpus callosum and periventricular regions These regions, with presumably different density interfaces, exhibit biological responses to shockwaves consistent with interface turbulence described by Richtmyer-Meshkov instability Evoked compound action potential (

A model of low-level primary blast brain trauma results in cytoskeletal proteolysis and chronic functional impairment in the absence of lung barotrauma.

A shock-dynamic model based on the symmetrical expansion of the critical shock is used to analyse the progressive decay of an originally planar shock wave through a large and abrupt area change. This is tested against measurements of shock strength made along the axes of area changes where the shock waves are free to expand in two or three dimensions.The critical shock is defined as the configuration when the decaying shock wave at the axis first becomes curved. The axial shock begins to decay less than one diameter from the entrance of the area change. Differences between the experimental onset of decay and the theoretical position of the critical shock are accounted for by the non-ideal behaviour of a practical pressure transducer.The model predicts that, when the shock wave is decaying symmetrically, there is a linear relationship between a derived function e of the decaying shock strength and the distance from the area change. This is confirmed experimentally for all the shocks studied. The quantitative application of the results in three dimensions up to 400 mm enables accurate predictions of experimental results at 1 m for M < 2·0. Also, the model may be applied to three-dimensional results to predict accurately equivalent results in two dimensions.The numerical values of e are based on the equivalence of the ratio of the shock areas and the ratio of their Chisnell (1957) functions. Hence correlations between experimental results and predictions of the model are evidence that Chisnell's theory can be extended to include large and abrupt area changes.

A model for the axial decay of a shock wave in a large abrupt area change

A series of initially planar shock waves was allowed to expand from a shock tube into a near half-space. The strength of the wall shock was measured at two positions on the slightly concave front wall. These measurements are compared with shock strengths predicted by a shock-dynamic model based on the cylindrical expansion of a critical shock. Chisnell's (1957) theory is used to account for the effect of the increasing surface area of the expanding wall shock, and Whitham's (1957) treatment to correct for the curvature of the wall. The critical shock strength is obtained from Skews’ (1967) experimental measurements of the Mach number of the self-similar wall shock following two-dimensional diffraction at a 90° edge.The model predicts the relatively small degree of attenuation observed between the measuring stations, but overestimates the absolute shock strength. The most likely cause is that, in the early stages of expansion, the wall shock experiences further attenuation owing to its interaction with the expanding flow. These effects are shown to be short range and of negligible importance at the first measuring station, 1·86 tube diameters from the axis. Thus, using the experimental results at this station as the starting point, the model predicts accurately the shock strength at 3·76 diameters. It is concluded that Chisnell's theory can be applied to the weakening of the wall shock in ducts with large abrupt changes in cross-section only when the wall shock is some distance from the entrance to the area change.

S. A. Sloan

Papers

A model for the axial decay of a shock wave in a large abrupt area change

A model for the decay of a wall shock in a large abrupt area change