1. General equations

In practical fluid flow problems, a perfect fluid flow is assumed when the viscosity of the fluid is negligible or when, in the case of viscous fluids, the velocity gradients are zero. $\mu =\eta =0\text{ ou }\frac{\partial v_i}{\partial x_j}=0$

This is particularly the case when the flow zone under consideration is sufficiently far from any material wall, yet is relatively large in size. This is referred to as external flow or potential flow, since this type of flow is irrotational ( [BE 8151] § 5 ).

Conversely,...