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An inviscid flow is the flow of an ideal fluid that is assumed to have no viscosity. In fluid dynamics there are problems that are easily solved by using the simplifying assumption of an inviscid flow.
Reynolds number 
The assumption of inviscid flow is generally valid where viscous forces are small in comparison to inertial forces. Such flow situations can be identified as flows with a Reynolds number much greater than one. The assumption that viscous forces are negligible can be used to simplify the Navier-Stokes solution to the Euler equations.
The Euler equation governing inviscid flow is:
which is admittedly Newton's second law applied on a flowing infinitesimal volume element. In the steady-state case, combined with the continuity equation of mass, this can be solved using potential flow theory.
Problems with the inviscid-flow model 
While throughout much of a flow-field the effect of viscosity may be very small, a number of factors make the assumption of negligible viscosity invalid in many cases. Viscosity cannot be neglected near fluid boundaries because of the presence of a boundary layer, which enhances the effect of even a small amount of viscosity. Turbulence is also observed in some high-Reynolds-number flows, and is a process through which energy is transferred to increasingly small scales of motion until it is dissipated by viscosity.
- Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0-273-01120-0
- Kundu, P.K., Cohen, I.M., & Hu, H.H. (2004), Fluid Mechanics, 3rd edition, Academic Press. ISBN 0-12-178253-0, ISBN 978-0-12-178253-5
- Clancy, L.J., Aerodynamics, p.xviii
- Kundu, P.K., Cohen, I.M., & Hu, H.H., Fluid Mechanics, Chapter 10, sub-chapter 1
See also 
- Fluid Dynamics
- Stokes Flow, in which the viscous forces are much greater than inertial forces.
- Couette Flow
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