26 
Spray dome 
75e The spray dome depends on the pressure pulse being reflected as a 
tensile pulse at the surface with subsequent cavitation of the water. To 
understand the mechanism of the formation of the dome it is first assumed 
that the pulse can be treated as a small-amplitude plane wave. 
(a) 
Fige12 = Curves showing the distribution of pressure with depth 
immediately above the explosion 
In fige12a, the curve AB Q represents the distribution of pressure with 
depth immediately above the explosion at the instant when the front of 
the pulse reaches the surface. At a later time the pulse, which would 
have reached the position 4' B' Q§ in fig.ei2b if there were no surface, 
is instead reflected as a tensile pulse. The tensions in this wave are 
given by the curve AN FD which is the image in AC of the portion 
4&' B' NA of the original pulse which has ceased to exist. The pressure 
in the water (apart from the hydrostatic pressure which is negligible in 
comparison with pulse pressures) is then given by the difference between 
the curve AN Q'representing the remaining original pulse and the curve 
AN FD representing the reflected pulse. Over the depth A D the net 
result is a tension in the water increasing from zero at the surface to a 
magnitude EF at the depth D. Let the instant depicted in fig. 12b be so 
chosen that E F is equal to the greatest tension that the water can 
withstand. At this instant, therefore, the water will cavitate at the 
depth D and the layer of thickness A D will be projected upwards. The 
top D F of the water below will then behave as a new free surface from 
which the remaining pulse D E Q is reflected as a tensile wave and the 
whole process can be repeated. If £ F, representing the tensile 
strength of the water, is small compared with D F, representing the maximum 
pressure in the original pulse, a succession of thin layers is projected 
upwards each of which will break into dropse 
