Question

Difference between ideal fluid and real fluid

Answer 1

It is said that an ideal fluid has a zero viscosity, while a real fluid has a non-zero one. However, in reality, there is no such thing as “ideal fluid”, because a fluid with zero viscosity does not exist. Thus, every fluid is real, with no exepction.

At the same time, there is indeed an assumption called ideal flow, where the flow is considered to be inviscid (the viscosity effects on the free flow region* are negligible compared to the relevance of the inertial or pressure effects). The ideal flow is also called inviscid flow, modeled by the Euler equations (the same as the Navier-Stokes equations, where the viscous forces are considered to be zero).

* region away from the surfaces, not influenced by the surface friction.

Even if we might model a flow as inviscid, we should never forget to consider the possible influence of the friction forces over the surfaces in our analysis. Either these will be irrelevant, or they will need to be modeled in another way.