1393 
THE REFLECTION OF SMALL-CHARGE SHOCK WAVES 
FROM A FREE SURFACE 
I. INTRODUCTION 
The phenomena accompanying the oblioue incidence of ghock 
waves upon a free surface have already been described. 26} 
However, the theoretical descriptions generally available apply 
only to that part of the pressure field in which the angle of 
incidence is large and the pressure level comparatively low, 
less than 15,000 psi. Under such conditions, the normal 
acoustic laws of propagation offer a satisfactory description 
of the direct shock wave and of the tension wave which is 
reflected from the surface. Experimental evidence previously 
obtained at this laboratory has been seen to indicate that 
for very small angles of incidence and for cases in which 
either the charge or the gauge is very close to the surface, 
the acoustic theory AQ longer offers an adeauate explanation of 
the observed results. ©) 
These earlier results showed that, at points very close 
to the surface, the shock wave duration (the time interval 
between the arrival of the directly propagated wave sat the 
gauge and the subsequent arrivel of the reflected negative wave ) 
is greater than simple acoustic theory predicts. Furthermore, 
pressures measured near the surface were found to be much lower 
than the similitude values obtained for free water, although 
for a given charge fired near the surface this attenuation is 
less pronounced beneath the charge than near the surface. And 
even before the arrival of the negative wave, the decay of 
. pressure with time differs from the exponential decay of free- 
water pressure fields, 
The study here described was undertaken to obtain a more 
nearly complete picture of these surface-reflection phenomena. 
_A program was set up to study shock waves near the surface for 
various charge and gauge depths, as well as for varying. pressure 
levels at a given depth. Experimental results are presented with 
a minimum of interpretation, in the hope that the additional 
information will be of assistance in the development of a satis- 
factory theory for free surface shock-wave phenomena. The 
present foundation for such theoretical development is naturally 
peees on the Meyer Corner theory and the work of, Prandtl and 
Mach i but, except for a brief mention by Cole//the general 
literature does not reveal any extension of this work to cover 
explosive propagation under the conditions here investigated. 
* All such numbers refer to the List of References at the end of 
this report. 
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