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When Does Air Become Compressible?

Alexander Ross • • 6 min read
When Does Air Become Compressible?

The magnitude of compressibility effect can be judged with flow velocity. For air, when flow velocity is 100 m/s or less, the air is treated as an incompressible fluid

compressible fluid
While all flows are compressible, flows are usually treated as being incompressible when the Mach number (the ratio of the speed of the flow to the speed of sound) is smaller than 0.3 (since the density change due to velocity is about 5% in that case).
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, and when the velocity is greater than 100 m/s, the air is treated as compressible fluid.

What determines whether an airflow is incompressible?

In fluid dynamics, a flow is considered incompressible if the divergence of the flow velocity is zero. ... This can assume either constant density (strict incompressible) or varying density flow.

When can a gas be considered incompressible?

Generally, for theoretical and experimental purposes, gases are assumed to be incompressible when they are moving at low speeds--under approximately 220 miles per hour. The motion of the object traveling through the air at such speed does not affect the density of the air.

Alexander Ross
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Alexander Ross

Alexander Ross has covered the video game industry for a decade, writing deep dives on game design, esports tournaments, VR developments, and gaming culture.