SHOCK WAVE MEASUREMENTS 



257 



than twice the pressure of either shock alone. The critical value aextr for 

 which this occurs decreases with the pressure, and for a pressure of 

 40,000 lb./in.2 the value aextr = 40° is obtained, corresponding to an 

 angle of 80° between the two fronts. 



A. Photographic examples of the Mach region. A number of observers 

 have obtained flash photographs of intersecting shock waves. In one 



4 LB CHARGE 



4 LB CHARGE 



GAUGES 



! 



GAUGE 



Fig. 7.18 Gauge arrangement for Mach effect measurements. 



series of experiments reported by MacDougall, Messerly, and Boggs,^° 

 two cylindrical sticks of pentolite inclined at an angle to form a V were 

 simultaneously fired at the point of the V to form shock waves as 

 sketched in Fig. 7.17, illumination being obtained by explosive flash 

 charges. The pressure at the shock front intersections for the condi- 

 tions of the experiment was estimated to be 300,000 Ib./in.^, This 

 corresponds to a pressure ratio of 20,000 ahead of and behind the shock 

 front, and the maximum value of a for regular reflection at this large 

 pressure is calculated from Eq. (2.41) to be 26°. Photographs obtained 

 for a = 22° showed regular reflection, and for a = 36° irregular, Mach 

 reflection as sketched in Fig. 7.17 was observed as predicted. The ex- 

 act positions of the "reflected" portions of the waves travelling in the 

 wake of the original waves are distorted because of refraction of light in 

 passing through the waves (see section 6.3), but narrow lines observed 

 behind the intersection as in Fig. 7.20 were believed to be real. They 

 may thus correspond to the slipstreams predicted by von Neumann: 

 10 Reported in UE 20 (114) . 



