Mach-stem

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. 

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... 

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... 

Propagation of quasi-detonation in obstacle array in stoichiometric H2/O2 mixture (a) initial pressure 140 torr, detonation reinitiation via Mach reflection at the bottom wall (b) initial pressure 120 torr, detonation reinitiation by normal Mach stem reflection from the obstacle with subsequent enhancement via reflection from the top wall 6 ps between frames. (From Teodorczyk, A., Lee, J.H.S., and Knystautas, R., Prog. Astr. Aeron., 138,223,1990. With permission.)... 

In the so-called Mach reflection, the incident and reflected waves do not intersect on the ground but at some point above the ground. The lower portions of the incident and reflected waves fuse into one wave which is known as the Mach stem, M, (also known as Mach shock or Mach bridge). 

The generation of strong Mach stem reflections downstream of the diffraction plane of an initiator combustor is a reliable reinitiation mechanism for the transmission of a detonation wave into a larger combustor. The reliable generation... 

As will be shown (e.g., see Fig. 6), major features of the flow, such as the foot of the Mach stem, Rs t) and the moving stagnation point i2sp(t), move at constant velocity. This allows one to define a similarity variable ... 

Figure 6 presents a time-distance diagram for the calculation. It shows that both the foot of the Mach stem i2s(0 moving stagnation point... 

Fig. 6 Location of the Mach stem shock (i2s)moving stagnation point (/ sp)t and triple point (Zip) vs time.

To identify characteristic values of j/, such as the boundary layer thickness, one must consider the flow fleld profiles near the wall. To that end the flow fleld was time-averaged along lines of = constant, using the last 3625 cycles of the calculation t = 16 to 592 ms). Examples of the mean-flow profiles in the wall jet re on are shown in Fig. 7. The mean velocity profiles (Fig. 7a) do indeed resemble those expected for a wall jet the stream-wise velocities reach a peak value of about 5.3 km/s in the jet and decay to a value of about 3.5 km/s far above the wall. Mean density profiles are shown in Fig. 7b. The fluidized bed was compressed somewhat (p 65 mg/cm ) by the Mach stem shock. The profiles converge at a height of z 2.15. As... 

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