2 Q 



n 



o 



Figure 1. The phases of water entry. 



Although these divisions of the field are rather artificial in character, and do not 

 embrace conditions such as those which hold for ricochet, they do serve the useful 

 purpose of helping towards a specification of the several regimes which are amenable 

 to mathematical analysis. During the last few years most basic research has been con- 

 cerned with phase (d), with a more limited amount of activity in (b), and the follow- 

 ing review is mainly concerned with these two phases. 



III. Shock and Water Entry Phases 



When a missile makes contact with a water surface, the water is compressed 

 near the point of contact and a shock wave is formed which radiates at a velocity a 

 little higher than the speed of sound in water. This initial shock phase, in which the 

 compressibility of the water is important, lasts for a few micro-seconds. During this 

 period, the nose of the body experiences a localised peak pressure, the magnitude of 

 which is approximately pcv where p is the density of water, c the speed of sound in 

 water and v is the vertical component of the velocity of entry. Apart from the possi- 

 bility of damage to the body, the shock phase is relatively unimportant in the study of 

 underwater ballistics because of its very short duration. 



The first stage of importance is the inertial, or flow forming, phase during which 

 the water, which is originally at rest, is accelerated around the nose of the missile. The 

 rapidly changing flow can involve large accelerations and the production of high tran- 

 sient pressures. The determination of the high transient pressure distributions over the 

 surface of the body is of practical importance since the forces produced determine the 



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