The perfect fluid

The fundamental property which distinguishes a fluid from other continuous media is that it cannot be in equilibrium in the presence of tangential stresses. If any infinitesimal tangential stress acts in the fluid the mechanical equilibrium comes to an end, and some form of fluid flow begins to develop. Thus, in a moving fluid mass tangential forces occur as well as normal forces. But in most engineering problems the tangential forces are relatively small, and may thus be neglected. The most simple description of fluid flow is developed on the hypothesis of purely normal pressure, and this assumption is automatically implied when speaking of a perfect or inviscid fluid. A perfect fluid at rest or in motion is in a hydrostatic state of stress which satisfies the equation

The hypothetic model of the perfect fluid is particularly applicable to fluids which have a low viscosity, such as water, gasoline and the common gases. Solving the majority of fluid flow problems involves the determination of the velocity and pressure distributions as functions of the coordinates and time. This problem can be considerably simplified by assuming that the viscosity of the fluid is zero. Typical cases for which this assumption is valid are flow through orifices, the discharge from large tanks, the flow around aerofoils, etc. In such problems the

flow of the main fluid mass has the greater importance. Viscous effects are significant only in the flow immediately adjacent to the solid boundaries; moving away from the walls the shear stresses decrease rapidly. That region of the flow where fluid friction is important is known as the boundary layer. The main flow, outside of the boundary layer can be described by

the equations derived for perfect fluids.