Shock Waves

Formation

When airflow accelerates to supersonic speeds and then decelerates back to subsonic speeds, the pressure change is not gradual but instantaneous. This thin boundary is a Shock Wave.

Normal Shock Wave

Perpendicular to the airflow.

• Before Shock: Supersonic ($M > 1$).
• After Shock: Subsonic ($M < 1$).
• Changes:
  • Velocity: Drops.
  • Static Pressure: Increases (Density increases).
  • Temperature: Increases.
  • Total Pressure: Decreases (Energy is lost as heat).

Oblique Shock Wave

Inclined to the airflow. Occurs at corners/wedges (e.g., nose of a supersonic jet).

• Before Shock: Supersonic.
• After Shock: Still Supersonic (but lower Mach).
• Bow Wave: A detached shock wave formed in front of a blunt nose.

Expansion Wave

Occurs when supersonic airflow follows a convex corner (turns away).

• Velocity: Increases.
• Pressure/Temp/Density: Decrease.
• No energy loss (isentropic).

Wave Drag

The energy dissipated in the shock wave creates a new form of drag called Wave Drag. It increases drastically in the transonic region.