28 



14 



NON-COMPRESSIVE ACTION ON A TARGET 



At the opposite extreme from local action lies the case of approxi- 

 mately non-compressive action.* The condition for this is that no great 

 change shall occur in the incident pressure during an Interval comparable 

 with the diffraction time, that is, that 



T^ » T, 



where the symbol » means "is much greater than," When this condition holds, 

 the pressures become readjusted by diffraction with such relative rapidity 

 over the face of the targai" that local effects due to compressibility of the 

 water are largely ironed out and the action on the target becomes essentially 

 the same as it would be if the water were incompressible. Viewed in the 

 large, the pressure field results from a compressional wave propagated up to 

 the target, but its local effects are about the same as those due to an equal 



pressure field at the target resulting from or- 

 dinary hydraulic action. 



An important feature of non-compressive 

 action, and one that distinguishes it sharply 

 from the typical local action of waves, is that 

 the energy given to the target may greatly ex- 

 ceed the energy that would fall upon it accord- 

 ing to the laws of wave propagation. In non- 

 compressive action energy is propagated through 

 moving water by the pressure Just as it is in a 

 hydraulic press. 



An excellent example is presented by a 

 Hllliar pressure gage (lU) subjected to the 

 shock wave from a charge of several hundred 

 pounds. The face of the gage, H-H in Figure 

 12, is perhaps U inches across, so that the 

 diffraction time T^ may be l/30 millisecond, 

 whereas the time constant of the wave is of the 

 order of a millisecond. Thus non-compressive 

 theory should give a good account of the effect 

 of a shock wave on a Hllliar gage. The energy 

 acquired by the piston of the gage may greatly 

 exceed that which is propagated in the shock 

 wave across an area equal to that of the face 

 of the piston. The motion of the piston sets 



Figure 12 - Illustration of 

 a Hllliar Pressure Gage 



The steel piston A is projected up- 

 wards by the pressure due to the 

 shock wave, thereby hammering the 

 copper cylinder C against the top 

 of the gage. This diagram is copied 

 from Figure 3A in Reference {^^). 



This is action conditioned by flow, as of an incompressible fluid. 



