Mach reflection can exist in steady, pseudo-steady and unsteady flows. When a shock wave, which is moving with a constant velocity, propagates over a solid wedge, the flow generated by the shock impinges
on the wedge thus generating a second reflected shock, which ensures that the velocity of
the flow is parallel to the wedge surface. Viewed in the frame of the reflection point, this
flow is locally steady, and the flow is referred to as pseudosteady. When
the angle between the wedge and the primary shock is sufficiently large, a single reflected
shock is not able to turn the flow to a direction parallel to the wall and a transition to Mach
reflection occurs.
In a steady flow situation, if a wedge is placed into a steady supersonic flow in such
a way that its oblique attached shock impinges on a flat wall parallel to the free stream,
the shock turns the flow toward the wall and a reflected shock is required to turn the flow
back to a direction parallel to the wall. When the shock angle exceeds a certain value, the
deflection achievable by a single reflected shock is insufficient to turn the flow back to a
direction parallel to the wall and transition to Mach reflection is observed.
Mach reflection consists of three shocks, namely the incident shock, the reflected shock and a Mach stem, as well as a slip plane. The point where the three shocks meet is known as the triple point in two dimensions, or a shock-shock in three dimensions.