ABSTRACT
In many boundary value problems, the equilibrium and constitutive constraints lead to a differential equation formulation in the space and time variables, which often contains all compatibility requirements. However, in general, some equations must be supplemented by additional constraints to satisfy additional compatibility requirements. One of the examples of the situation is the problem of groundwater flows or seepage flows through the soil which, besides the Navier–Stokes equations, requires D'Arcy's law for a complete formulation of the problem. Some aerodynamic problem reduce to potential flow problems. In aircraft aerodynamics, the theory of lift of airfoils produces potential flows. In aerodynamics, the velocity and body dimensions are relatively large. In porous media, since the soil is frequently deposited in layers and is more compact in one direction, the soil grains have a preferred direction. Anisotropic porous media require the approach of orthotropic materials.
