Funny someone mentioned water, because Hydrodynamic theory (theory of water flows) served as the basis of aerodynamic theory, which is the physics of water flowing around the hulls of ships. It was motivated by the need for faster warships many centuries ago. Basic low speed aerodynamics is really just hydrodynamics up to an including theory pertaining to circulation. Slow-speed air is considered to be incompressible as water is incompressible. Hydrodynamic theory is and was a good fit to aerodynamic theory of low speeds flows, and early theory on the subject was derivative. High speed aerodynamics is really not something most people care much about because there are a lot more variables owing to the entropy losses and thermodynamic effects, and accounting for those losses requires some fancy math. All textbooks therefore start with low speed flow, which should be understood before moving on to high speed flow.

Ok, I have a minute before going back out into the wild blue yonders of America so let me take swag at it.

Inviscid means the flow does not have much thickness to it, for example it has no shear-stress properties like say, motor oil. Inviscid means the flow has easily-flowing properties much like a noble gas. If you stick your hand into a flow of a fast (but not too fast) moving inviscid fluid, say helium, you would not feel very much force on your hand. Inviscid theory does not include the existence of drag because it takes some viscosity to create skin friction drag. We call this skin friction drag or parasite drag. For example, if we drive a truck through pure helium there will be no drag of this type, but if we were to measure the drag driving it down a normal highway there would be many pounds of drag from skin friction or parasite drag, and form drag as well. There would be no form drag for the inviscid case because according to theory the flow would perfectly slow back down to its prior speed when it got to the back side of the truck. In real life we know this is not true, because it take many pounds of force to overcome form and skin friction drag, but as long as the fluid is inviscid the theory is correct. Waxing the truck might help a little bit on the skin friction drag. Reducing pressure gradients by shaping the truck a bullet would reduce the form drag part. Inviscid theory is usually introduced first because it is true, simple, and Bernoulli's relation can be used to determine the pressure-velocity field around an object.


Reynold's number is the ratio of the inertial to viscous forces for a fluid and serves to help predict how flows will behave to a certain extent. For low Reynold's numbers there is a lot of skin friction drag. A paper airplane has a very low Reynold's number and experiences a lot of viscous effects, while a 747 does not experience these effects very strongly. Going back to our truck example, if the air speeding around it encounters a high enough gradient (change) in pressure as it works its way around the back, it may exceed the allowable shear stress of the air itself and on the back side of the truck the flow will separate or burble. No lift is involved in this example, but when there is lift the Reynold's number largely determines whether the air coming down the top side of a lift producing airfoil will stay put or decide to burble off the top of the wing.

Does this help any? I may be able to drone on some more if you like, but you'll have to be patient because I am going back on the road here in a bit.