Rarefied gas dynamics

Japanese: 希薄気体力学 - きはくきたいりきがく（英語表記）rarefied gas dynamics

Also known as superaerodynamics. A field of gas dynamics that deals with gas flows that are so dilute that molecular motion cannot be ignored. In normal gas dynamics, gas is treated as a continuous fluid, but for very dilute gases, when the mean free path of the molecules is on the order of the size of the object in question (for example, the radius of the tube for gas flowing through a tube), the gas must be treated as a group of discrete molecules and gas dynamics must be discussed using molecular kinetic theory. This field began in the 19th century with research on exhaust from high-vacuum devices, but has made great progress in connection with research on high-speed flight in the upper atmosphere and the motion of rockets and artificial satellites. Its scope of application also includes the light emitted by meteors in the upper atmosphere and the heat generated when artificial satellites re-enter the atmosphere. In recent years, it has also been expanded to discuss the fluid motion around tiny objects such as micromachines.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

超空気力学ともいう。分子運動が無視できないほど希薄な気体の流れを扱う気体力学の分野。通常の気体力学では気体を連続流体とみなして扱うが，非常に希薄な気体でその分子の平均自由行程が問題にしている物体の大きさ (たとえば管中を流れる気体では管の半径) の程度になると，気体を離散的な分子の集団とみなし，分子運動論を用いて気体力学を論じなければならない。この分野は，19世紀に高真空装置における排気の研究から始ったが，高層大気中での高速度飛行，ロケットや人工衛星の運動の研究に関連して大いに進歩した。高層大気中での流星の発光，人工衛星の大気圏再突入の際の発熱などもその応用範囲に含まれる。さらに，近年ではマイクロマシンなどの微小な物体のまわりの流体運動を論じる場合にも拡張される。

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