Originally Posted by
Cubdriver
Not a mistake. Slow moving air <M=0.3 is considered incompressible by the discipline of aerodynamics. The context of this thread is hydrodynamics and by extension, aerodynamics. Wikipedia-
Compressible flow (gas dynamics) ) is the branch of fluid mechanics that deals with flows having significant changes in fluid density. Gases, but not liquids, display such behavior. To distinguish between compressible and incompressible flow in gases, the Mach number (the ratio of the speed of the flow to the speed of sound) must be greater than about 0.3 before significant compressibility occurs. The study of compressible flow is relevant to high-speed aircraft, jet engines, gas pipelines, commercial applications such as abrasive blasting, and many other fields.
You are correct as always, and in addition we should consider Non-Newtonion Fluids. The speed of an object moving through fluids may have some impact on their real viscosity, it may be lowered or raised. We all know gases can be compressed, after all we fill up our tires, but depending on the medium and speed the description and reaction can change quite a bit.
