Noun:

1. a decrease in the density of something;

   "a sound wave causes periodic rarefactions in its medium"
   

rare old异乎寻常的, 极度的rareadj.罕见的；珍贵的；煎得嫩的Rare copper plating gold Kwan-yin收来罕见的铜镀金观音rare-earth ores with the scandium含钪稀土矿rare-earth oxide-coated cathode稀土金属氧化物阴极rare earth luminescent microspheres稀土发光微球rare earth-magnesium middle alloy稀土镁中间合金rare earth metal conversion coatings稀土转化膜rare earth fluorescent tracing plane稀土荧光示踪板Rare earth Mg-Si-Fe modification稀土硅镁铁合金变质处理rare and endangered medicinal plants珍稀濒危药用植物Rare earth trifluoromethanesulfonate三氟甲磺酸稀土

1. Simulation results show that rarefaction effect plays an important role in gas damping, which decreases as Kn Number increases.
   数值模拟结果表明，稀薄效应对气体阻尼的影响较大，随着克努森数的不断增大，气体阻尼逐渐减小。
2. Several applications require consideration of rarefaction, which motivates the present thesis to focus on simulation of driven cavity flows in this region.
   因此本文的动机是模拟空穴流场在稀薄区域上的应用。