SHOCK WAVE MEASUREMENTS 259 



B. Pressure measurements. Although photographs reveal the exist- 

 ence of Mach regions very beautifully, no optical methods have as yet 

 been applied to the measurement of pressure in regions of nonacoustic 

 intersections of shock waves. Some measurements have, however, been 

 made using piezoelectric gauges. ^^ In these experiments, two 4 pound 

 charges at varying separations were detonated simultaneously. Piezo- 

 electric gauges were placed at points 4 feet from each charge on the 

 plane of symmetry, and another gauge was placed on a line through the 

 charges 4 feet from the nearer one, as shown in Fig. 7.18. The pressure- 

 time curves on the plane of symmetry showed peak pressures of 18,000 

 lb./in.2 when the angle between the fronts was 90°, a value approxi- 

 mately double that due to a single charge. This pressure increased to 

 a maximum of about 22,000 Ib./in.^ for an angle of 135°, as shown in 

 Fig. 7.19a. The existence of a Mach region was estabhshed for angles 

 greater than 126° by the fact that gauges placed on, and two inches to 

 one side of, the plane of symmetry both showed a single shock front, 

 indicating a region of finite width with one shock only (stem of the 

 Mach Y). The calculated maximum value of o; = 60° for regular re- 

 flection agrees within experimental uncertainty with the value a = 63° 

 observed. 



The observed increase in peak pressure as a increases is in the direc- 

 tion predicted by theory (dashed line of Fig. 7.19a), but considerably 

 smaller. The initial rate of decay of the curve was found to be much 

 greater, as shown in the plot of initial time constant in Fig. 7.19b (ob- 

 tained by approximating the initial part of the curve with a negative 

 exponential). As the intersection between the two fronts was made 

 increasingly oblique by moving the two charges together, the observed 

 peak pressures fell off rapidly until for angles approaching 180° (coinci- 

 dence of the charges) the pressure was very nearly the same as for a 

 single charge of double the weight, as shown in Fig. 7.19a. At the same 

 time, the time constant and duration parameters increased and ap- 

 proached the value for a single charge of double weight. 



The fact that increases in the observed peak pressures are accom- 

 panied by decreased duration of high pressures means that the strength- 

 ening of two shock fronts by their intersection is not as pronounced as 

 peak pressure values alone might seem to indicate. Measured values 

 of impulse and energy flux density at the plane of intersection in fact 

 decrease continuously, from values corresponding roughly to addition 

 of pressures from the two charges to values comparable with those for a 

 single charge. These estimates are complicated by the fact that, as the 

 charges are brought close together, the later portions of the pressure 

 wave are weakened by rarefaction waves resulting from the reflection 

 of each shock wave off the gas bubble of the other charge! These 



12 A. M. Shanes, UE 21 (114). 



