请翻译以下英文: Fluid flow can be compressible or incompressible, Liquids can usually be considered as flowing incompressible. But even a highly compressible gas may sometimes undergo unimportant changes in density. Its flow is then practically incompressible. In flight at speeds much lower than the speed of sound in air (described by subsonic aerodynamics), the motion of the air relative to the wings is one of nearly incompressible flow.