SHOCK WAVE MEASUREMENTS 



255 



ever, show the same general features of multiple peaks and irregular 

 decay (Plate VI is an example). 



The presence of multiple peaks is not accounted for by simple 

 acoustic theory, but is rather the result of nonlinear interference of 

 pressure fronts from different parts of the charge. The theory of such 

 effects, as discussed in section 2.10, has been only partially developed, 

 but does give a qualitative description of the phenomenon. Because 

 this explanation involves the consideration of multiple sources, of which 

 asymmetrical charges are hardly a simple example, further discussion is 

 postponed to section 7.7. 



It is interesting to note, in conclusion, that the propagation of the 

 double peaked shock wave outward from the charge offers an illustra- 

 tion of the overtaking effect in shock waves. At distances of a few 



(a) ACOUSTIC 



(b) REGULAR 



Fig. 7.16 Intersection of plane shock waves. 



(C) MACH 



charge radii, the second smaller pressure discontinuity advances more 

 rapidly than the initial shock front and the time interval between them 

 decreases, as is to be expected for finite amplitude waves. As the shock 

 front pressure decreases this overtaking effect is reduced, and the time 

 interval between the peaks changes much less rapidly. For example, 

 the double peaks observed off the end of a 300 pound cylindrical charge 

 have a separation of 150 /xsec. at 20 feet which decreases to 75 ^usec. at 

 30 feet, but are still discernible 500 feet away. 



7.7. Shock Waves from Multiple Charges 



In the acoustic approximation, for which the wave equation for 

 pressure is linear, the resultant pressure at any point in a fluid through 

 which two pressure waves from separate sources pass is simply the sum 

 of the pressures which either disturbance produces when unaffected by 

 the other. Thus, the intersection of two weak pressure fronts of equal 

 strength is represented by Fig. 7.16a in the acoustic case. 



For shock waves, however, the velocity of propagation of the front 



