Compressible flow refers to fluid motion in which changes in fluid density are significant, typically occurring when a gas moves at high speeds, especially as it approaches or exceeds the speed of sound.
In contrast to incompressible flow (where density is constant), compressible flow must account for variations in pressure, temperature, and density—all of which become important at Mach numbers (speed relative to sound) above ~0.3.
Key Features:
- Occurs mostly in gases, not liquids (which are nearly incompressible).
- At Mach 1, flow reaches sonic speed, and phenomena like shock waves and expansion fans appear.
- Requires specialized equations (like the compressible Navier–Stokes and energy equations) to analyze.
Examples:
- Supersonic aircraft design must consider compressible flow to account for drag and heating.
- Rocket nozzles and jet engines rely on compressible gas dynamics for efficient propulsion.
- Explosions and shock tubes involve rapidly compressing air and gases.
Understanding compressible flow is essential in aerodynamics, propulsion, and high-speed gas flow applications, where ignoring density changes would lead to incorrect predictions.