vertical to the incident wave; behind the reflected shockwave 
there is 2-73, *™$s and uz *u! vertical to the front 
of the reflected wave. 
INCIDENT 
WAVE 
REFLECTED 
WAVE 
BOUNDARY 
The two conditions, which have to be satisfied, shall be con- 
sidered now. 
(a) The velocity of the intersection between incident wave 
(1) and boundary has to be equal to the velocity of the 
intersection between reflected wave (2) and boundary. 
The reflected wave travels in a medium which flows 
with a velocity + u cos(@+@!) vertical to the re- 
flected wave. The velocity for the observer at rest 
therefore is D' ¢ u cos(@+@!) and therefore 
D'+ u cos(@1Q') D 
cor ©!’ wn @ (20) 
29 
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