text: wave also occurs when a wedge or cone angle 
  exceeds some critical value. Whenever the 
  shock wave forms perpendicular to the up- 
  stream flow, 
  is termed a 
  the shock wave 
  “normal” shock wave and the flow immediately 
  behind the wave is subsonic. Any relatively 
  blunt object in a supersonic airstream will form 
  a normal shock wave immediately ahead of the 
  leading edge slowing the airstream to subsonic 
  SO the airstream may feel the presence of the 
  blunt nose and flow around it. Once past the 
  blunt nose the airstream may remain subsonic 
  or accelerate back to supersonic depending on 
  the shape of the nose and the Mach number of 
  the free stream. 
  In addition 
  
  formation of normal 
  shock waves described above, this same type 
  of wave may be formed in an entirely different 
  manner when there is no object in the super- 
  It is particular that whenever 
  sonic airstream. 
  a supersonic airscream is slowed to subsonic 
  without a change in direction a normal shock 
  wave will 
  form as a boundary between the 
  supersonic and subsonic regions. This is an 
  important fact since aircraft usually encounter 
  some “compressibility effects” before the flight 
  speed is sonic. Figure 3.6 illustrates the man- 
  ner in which an airfoil at high subsonic speeds 
  has local flow velocities which are supersonic. 
  As the local supersonic flow moves aft, a 
  the flow 
  normal shock wave forms slowing 
  to subsonic. The transition of 
  from 
  subsonic to supersonic is smooth and is not 
  accompanied by shock waves if the transition 
  is made gradually with a smooth surface. The 
  transition of flow from supersonic to subsonic 
  without 
  direction change always forms a 
  normal shock wave. 
  
  to  the 