Knowra Mach number Mach number The ratio of an object's speed to the local speed of sound in the surrounding medium. It indicates how compressibility and acoustic effects influence the flow around the object.
Speed of sound : The speed at which small pressure disturbances travel through a medium. It is the denominator of the Mach number and varies with the medium’s thermodynamic state.
Transonic flight : Flight near the speed of sound, where subsonic and supersonic regions coexist around an aircraft. Local airflow can become supersonic even while the aircraft's Mach number is below one.
Ernst Mach : An Austrian physicist and philosopher known for work on mechanics, perception, and the physics of motion. The dimensionless speed ratio was named in recognition of his work on supersonic motion.
Reynolds number : A dimensionless ratio comparing inertial forces with viscous forces in fluid flow. It characterizes the balance of inertia and viscosity, not speed relative to sound.
Ideal gas : A gas model whose particles have negligible volume and interact only through elastic collisions. For an ideal gas, temperature and composition help determine the sound speed.
Mach regime : A classification of flow by Mach number, including subsonic, transonic, supersonic, and hypersonic regimes. These regimes translate the ratio into distinct patterns of compressibility and wave behavior.
Supersonic flight : Flight at speeds greater than the local speed of sound. Mach numbers above one describe the regime in which an aircraft outruns sound disturbances.
Jakob Ackeret : A Swiss engineer who advanced theoretical and experimental research in high-speed aerodynamics. His early twentieth-century work helped establish Mach number as a practical aerodynamic parameter.
Knudsen number : The ratio of a gas's molecular mean free path to a characteristic flow length. It indicates whether continuum fluid mechanics applies, a different issue from compressibility.
Adiabatic process : A thermodynamic process in which no heat enters or leaves the system. Small sound waves are approximately adiabatic, shaping the sound-speed relation in gases.
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