Fluid flow which is not subjected to influence of the fluid viscosity.
It may happen for low-viscosity fluids or for the very high Reynolds numbers.
In this case Navier-Stokes fluid flow momentum equation simplifies to Eauler fluid flow momentum equation.
In many practical applications the fluid viscosity can not be neglected even for the low-viscous fluids.
This happens for example, when fluid flow is restricted by solid boundaries which interaction (a friction) with the fluid depends on fluid viscosity.
These effects can be incorporated and can be accounted in inviscid flow model through the external disspative forces.@wikipedia
See also
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Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid flow