Reflected shock

Figure B2.5.6. Temperature as a fiinction of time in a shock-tube experiment. The first r-jump results from the incoming shock wave. The second is caused by the reflection of the shock wave at the wall of the tube. The rise time 8 t typically is less than 1 ps, whereas the time delay between the incoming and reflected shock wave is on tlie order of several hundred microseconds. Adapted from [110].

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

S. Gordon and B. J. McBride, Computer Program for Calculation of Complex Equilibrium Compositions. Rocket Peformance Incident and Reflected Shocks and Chapman-]ouquetDetonations, NASA-Lewis Research Center, NASA, Airport, Md., Mar. 1976. 

An important application of the impedance match method is demonstrated by the pressure-particle velocity curves of Fig. 4.9 for various explosives. Using the above method, the pressure in shock waves in various explosives is inferred from the intersection of the explosive Hugoniot with the explosive product release isentropes and reflected shock-compression Hugoniots (Zel dovich and Kompaneets, 1960). The amplitudes of explosively induced shock waves which can be propagated into nonreacting materials are calculable using results such as those of Fig. 4.9. 

The dotted segments represent the region of two-wave structure for those materials exhibiting transitions the lines have been drawn on the basis of the shock velocity of the first wave. The dashed curves represent reflected shocks and rarefaction release loci from the 2024 A1 Hugoniot at the pressures listed. The three heavy curves are the Hugoniots of 2024 Al, Cu, and U-3 wt.% Mo alloy which were determined independently. These were used as standards to determine the Hugoniots of the other materials. 

Gordon, S. and McBride, B. J. 1976. Computer Program for Calculation of Complex Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman-Jouguet Detonations. Report SP-273. Interim Revision, Report N78-17724. National Aeronautics and Space Administration, Washington, DC. 

Figure 7-44 shows the sequence of events involved in diffraction of a blast vave about a circular cylinder (Bishop and Rowe 1967). In these figures the shock fronts are sho m as thick lines and their direction of movement by arrows normal to the shock front. In Figure 1.13a, the incident shock 1 and reflected shock are joined to the cylinder surface by a Mach stem M. R is now much weaker and is omitted in succeeding figures. 

In general, a reflected shock wave of 55 psi on a human for 400 milliseconds would be just about the tolerance limit [41] (see Table 7-25B). For a more detailed discussion of blast scaling and overpressure, see Ref [40]. 

Mach Stem. A shock wave or front formed above the surface of the earth by the fusion of direct and reflected shock waves resulting from an airburst. Also called mach wave and mach front... 

The development of combustion in PETN by shock was studied by Dubnov et al (Ref 93). Unfortunately, the original article is unavailable to us, but it appears that the effects of incident shock velocity, reflected shock pressure and temp, surrounding gas, and surface roughness of the PETN were examined Deflagra tio n-to -De torn tion Transition (DDT)... 

In the Trauzl-block test the expl is loaded into a Pb block at a loading d near 1.0g/cc and stemmed or sealed in with a screw cap. Theoretically these measurements should correlate well with practical conditions because initial and final states in the measurement and in practice should be similar. Unfortunately the Trauzl method is not very reproducible (in part because of rupture of portions of the block by reflected shocks). Consequently no further discussion of the Trauzl-block measurements will be made in this article. Interested readers are referred to Vol 2, B266—95 and Ref 5 for Trauzl block data for many expls... 

Figure 8.4.10 shows two time sequences of schlieren photographs of quasi-detonation. In Figure 8.4.10a, detonation reinitiation occurs at the Mach stem on the bottom wall. However, prior to complete reinitiation of the decoupled wave by the upward-growing detonation, reflection and, subsequently, diffraction of the detonation occur again by encountering another obstacle. In the sixth frame of Figure 8.4.10a, the curved, diffracted, and reflected shock with a reaction zone close behind is clearly evident. However, as this cylindrical... 

Shock Response Versus Quasi-Static Response for Internal Blast. We noted earlier that internal detonations of high explosives within structures caused both initial and reflected shock loadings, plus longer term gas pressure loads called quasi-static pressures. Figure 11 is a reproduction of a pressure trace showing both phases of the loading. 

The loads from external near-surface burst explosions are based on hemispherical surface burst relationships. Peak pressure (P psi) and scaled. impulse Ci/W psi/lb ) are plotted vs. scaled distance (R/W ft/lb ). Roof and sidewall elements, side-on to the shock wave, see side-on loads (P and i ). The front wall, perpendicular to the shock wave, sees the much higher reflected shock wave loads (P and i ). An approximate triangular pressure-time relationship is shown in Figure 5a. The duration, T, is determined from the peak pressure and impulse by assuming a triangular load. Complete load calculations include dynamic loads on side-on elements, the effect of clearing times on reflected pressure durations, and load variations on structural elements due to their size and varying distance from the explosive source. 

Numerous determinations of the heat of formation of carbon difluoride, a transient intermediate in the production of PTFE, for example, have been made. The most recent one has combined kinetic and equilibrium approaches. The equilibrium C2F4 2CF2 was studied at 1150-1600 K at 0.07-46 bar in dilute argon mixtures using incident and reflected shock waves. The carbene concentration was monitored at 250 nm after a careful study of the extinction coefficient over a wide temperature range. Rate parameters were found for forward and back... 

FIGURE 8.3 Comparison of measured and calculated NO concentration profiles for a CH4—02—N2 mixture behind reflected shocks. Initial post-shock conditions r= 2960K, P = 3.2 atm (from Bowman [12]). 

Gordon, S., and McBride, B. J., computer program for calculation of complex chemical equilibrium compositions, rocket performance, incident and reflected shocks and Chapman-Jougonet detonations, NASA SP-273 (1976). 

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