We define Ideal fluid as inviscid (
)
and incompressible
(
). The Reynolds number is defined as the ration between the inertial and viscous forces, so
For `typical' problems we are interested in (for example,
and
)
we have that
In other words, the viscous effect are much smaller than the inertial effects, and under certain circunstances (streamlined bodies), the viscous effects are restricted to a thin layer (boundary layer) around the boundaries of the flow (surfaces of streamlined bodies and their wake, for example). Therefore, outside this thin layer, ideal fluid is a good approximation. (Movie to illustrate the boundary layer along body surfaces and how its thickness depends on the Reynolds number)
Karl P Burr
2003-07-07