What happens when M_¥ > M_cr?

The flow accelerates over the front part of the airfoil. Before the point of minimum pressure is reached, it goes supersonic. Once the point of minimum pressure is passed, the flow experiences an adverse pressure gradient (pressure increases downstream). Several things happen.

1. The boundary layer begins to get thicker. Note that information can move upstream through the boundary layer, because the velocity is subsonic (has to reach zero at the wall)
2. The flow is forced to turn because of the thickening boundary layer. Compression waves are formed. These merge into one (or more) oblique shocks (or a normal shock depending on the Mach number and surface curvature, Reynolds number of the boundary layer etc.)

 

 
 





3. The pressure rises suddenly across the shock(s). The boundary layer thickens much more. It may separate!
4. The supersonic region ends in a normal shock and the flow becomes subsonic and decelerates.
5. Drag increases greatly because of the shocks. The wing may stall because of the boundary layer separation.