Fluid flow - Perfect fluid dynamics

All real fluids are viscous. However, depending on the practical situation, viscosity forces may be more or less important than the other forces involved in flow, such as inertia, gravity or pressure. This is generally the case in many gas flows, whose viscosity is much lower than that of most liquids. The terms due to viscosity can then be neglected in the general equations. Fluid flow is then treated as that of a perfect fluid, i.e., without viscosity.

Even when a fluid has a high viscosity, it is possible to find flow situations where this viscosity no longer has any influence. These are essentially irrotational flows, or potential flows, of an incompressible fluid and, more particularly, flows far from material walls, outside what are known as boundary layers. In all these cases, despite a viscosity coefficient that may be significant, velocity gradients are such that this viscosity no longer has any influence on the flow. The flow is then treated as if the fluid were a perfect fluid.

Although these may appear to be special cases, they are frequently encountered in practice. So, while the perfect fluid corresponds to a concept devoid of physical reality, perfect fluid dynamics is a truly applicative part of fluid mechanics.

The following article, based on a highly theoretical idea, is therefore of considerable importance for many applications, whether in the field of flow measurements or, for example, in the case of interactions between the flowing fluid and the walls of the pipes containing it.