Some engineering problems, especially those involving fluidfree surface, are nonlinear in boundary conditions.
其中不少问题是非线性的，如带自由面的流动区域，边界条件即呈非线性。
2
The free amino acids in the cystic fluid of Cysticercus cellulosae were studied.
研究了猪囊虫囊液中的游离氨基酸。
3
So it's not entirely impossible that free-floating planets have hot cores, cold surfaces, and an intermediate layer, where water could exist in some fluid form.