1438 
IV. THE CRITICAL ANGLE 
If an infinite plane shockwave is subjected to an oblique 
reflection at an infinite plane free surface, the effect is 
usually steady if we move with a velocity of D/ces @ parallel 
to the surface. Herein D is the speed of the shockwave and @ 
the angle between wavefront and a normal to the free surface. 
INCIDENT WAVE 
SS, fot, o \ 
The undisturbed medium flows from the left to the right side with 
velocity D/cos@. The stream lines in the undisturbed medium 
are deflected through an angle fp when passing the incident 
wavefront. In the pictured diagram the flow velocity along the 
streamline is D/cos @ in the undisturbed medium and vt in 
medium behind the incident wave. v_ usual ly is larger than the 
speed of sound in the medium behind the shockwave as further in- 
vestigations show. Therefore the part of flow to the right of 
the incident wave in the above sketch corresponds to supersonic 
10 
