All tetrahedra meshes can be formed in one of two ways:
An all tetrahedral mesh can consist of only isotropically shaped tetrahedral elements with no special boundary clustering. Such meshes can be used for inviscid Euler flow simulations where there is no need for special elements near the wall surfaces. They can also be used for LES simulations.
Destructured Prisms or Hexahedra
The second type of all tetrahedral mesh is formed by dividing the hexahedra, prisms and pyramids of a typical hybrid mesh into tetrahedral elements. This can be useful for solver development and testing, as well as for solvers that may be limited to only tetrahedral elements.